Ecology and management of Vulpia spp. G.C. Gmelin in perennial pastures of southern AustraliaEcology and management of Vulpia spp. G.C. Gmelin in perennial pastures of southern Australia
Katherine Tozer
December 2004
The University of Melbourne
Abstract
Vulpia species G.C. Gmel. cost the Australian wool industry around $AUS 30
million annually in lost production (Sloane eta/, 1998). Vulpia provides poor quality
forage and replaces other desirable pasture species, thus reducing stock carrying
capacity (Code, 1996). In addition, vulpia seed causes vegetable fault of wool, hides
and carcasses, and vulpia litter can impede the germination and growth of desirable
species because of allelopathic effects (Code, 1996). This weed is prevalent in
pastures and cropping regions throughout southern Australia. Presently vulpia is
controlled by different methods, including combinations of herbicide application,
grazing management, fertiliser application, oversowing competitive pasture species
and mechanical defoliation (Michalk & Dowling, 1996; Matthews eta/., 1998; Taylor &
Sindel, 2000; Dunsford & Morris, 2001).
However, greater understanding is required of vulpia ecology and how it
spreads, to develop cost effective, sustainable control strategies in perennial
pastures of southern Australia. In particular, key questions that need to be addressed
are: 1) how do management strategies influence vulpia populations in perennial
pastures; 2) what is the effect of neighbour competition on vulpia growth; and 3) how
much space is required to sustain a vulpia population? Thus vulpia management
studies incorporating varying levels of disturbance and competition in different
environmental conditions were combined with canopy gap and competition studies in
field and controlled conditions, to investigate factors that influence pasture botanical
composition and vulpia population dynamics.
The effect of disturbance and competition management treatments on pasture
botanical composition was investigated in phalaris (Phalaris aquatica L.)-based
pastures in a 575 mm rainfall zone (Ararat) and a 625 mm rainfall zone (Vasey) in
western Victoria, between 1999 and 2002. At Ararat, different fertiliser, herbicide
(SpraySeed and simazine) and pasture rest treatments were applied. A combination
of all treatments was most effective in enhancing the perennial content and in
reducing vulpia content, tiller density and seed production. At Vasey, the effect of
simazine and ryegrass competition on vulpia content, tiller density and seed
production was investigated in set-stocked, strategic and four-paddock rotationally
grazed pastures. Although vulpia increased over the duration of the study in all
grazing systems, the rate of increase was least in the four-paddock rotation.
Simazine initially reduced vulpia content and tiller density, but vulpia rapidly
increased and simazine was unable to provide effective control of vulpia. Oversowing
ryegrass in simazine treated swards improved vulpia control. These results
demonstrate that competition from perennial pasture species is a key factor in
controlling the growth of vulpia populations.
The dynamics of competition between perennial plants and vulpia were
investigated further in a series of field and controlled experiments. Barley grass
(Hordeum murinum L.) was included in these studies to compare and contrast the
responses of invasive annual grass weed species. In the first of the experiments, the
influence of canopy gap characteristics on vulpia growth and survival was
investigated in rotationally grazed and set stocked pastures at Vasey. Vulpia and
barley grass growth, survival and panicle production were lower under rotational
grazing than set-stocking irrespective of gap size, due to greater perennial
competition and pasture height, and reduced photosynthetically active radiation in
canopy gaps. The estimated critical gap size below which vulpia and barley grass
seed production was prevented was approximately 2 em diameter in both grazing
systems. Timing of gap appearance influenced vulpia and barley grass
establishment, but not tiller production, dry weight, survival or panicle production.
Fewer vulpia and barley grass seedlings established when annual grasses were
sown in June compared to April.
The linear effect of perennial competition on vulpia growth, survival and seed
production was investigated under controlled and field conditions by sowing vulpia
seed at a range of distances from established perennial grass plants. While perennial
neighbour competition reduced vulpia growth and panicle production, it did not
prevent vulpia from producing seed when sown directly adjacent to a row of phalaris
or cocksfoot neighbours (Dactylis glomerata L.). Cocksfoot plants suppressed vulpia
growth and reproduction to a greater extent than phalaris plants, particularly in close
proximity to the perennial neighbour. This was most likely because of cocksfoot
plants being more compact, resulting in greater light competition when annual
grasses grew in close proximity to cocksfoot neighbours. In addition, vulpia survival
was much less under field than controlled conditions, showing that factors other than
competition influenced annual grass survival in the field.
Based on these results, it is unrealistic to expect that vulpia can be eliminated
from pastures in southern Australia because the normal pattern of dry summers
depletes pasture density and creates space that vulpia can exploit when soil moisture
levels increase in autumn/winter. The critical gap size required to prevent vulpia seed
production is small and vulpia populations are predicted to increase even when seed
emerges immediately adjacent to perennial neighbours. However, management
practices such as pasture rest, rotational grazing, fertiliser application and herbicide
ii
application, which increase perennial ground cover and reduce disturbance of
perennial species, do allow some control of this weed. In conclusion, vulpia can best
be managed in perennial pastures by a combination of increasing fertility levels in
rotationally grazed pastures or pastures rested from grazing over summer, applying
herbicide as a winter-cleaning treatment, and over-sowing ryegrass or other
competitive species to fill in canopy gaps.
iii
Declaration
This is to certify that
(i) the thesis comprises only my original work towards the PhD,
(ii) due acknowledgement has been made in the text to all other material
used,
(iii) the thesis is less than 100,000 words in length, exclusive of tables,
maps, bibliographies and appendices.
iv
Acknowledgements
I would like to express my appreciation for the wise advice, encouragement
and kindness of my supervisors Professor David Chapman, Dr. Paul Quigley, Dr.
Peter Dowling and Professor Roger Cousens. I have learnt much from their breadth
of scientific knowledge and excellence in how they conduct their research and
academic responsibilities. Thank-you.
Thanks are also due to the Cooperative Research Centre for Weed
Management Systems, The University of Melbourne and the Department of Primary
Industries for providing funding for this project.
Time spent conducting field work at the Pastoral and Veterinary Institute in
Hamilton in western Victoria was rewarding. Many thanks to the staff of the
Sustainable Agricultural Systems group, including Dian Borg, Emily Borg and John
Byron, who provided advice and assistance in conducting field work. The advice of
Gavin Kearney in design and statistical analysis of experiments was invaluable.
Experimental work could not have been undertaken without the cooperation of the
Lyon and DeFegely families, on whose properties field studies were conducted.
While conducting field work, Peter and Jenny Schroder provided a home away from
home in allowing me to board with them. It was always good to be able to return at
the end of a long day and receive a warm welcome!
Alex Campbell, Nick Osborne and the other nursery staff at the Burnley
College campus of the Institute of Land and Food Resources, The University of
Melbourne, were always so helpful in monitoring my experimental plots between
visits. Thank-you! If not for you, plants would have died on several occasions due to
the irrigation system malfunctioning.
Finally, my parents and friends have been very encouraging, and I am
grateful to them for their moral support: Bruce and Anita Tozer, Felicity Grey, Nicky
De Weerd, Latha Kancherla, and Peter Tribe. Thanks also to the Lord Jesus, for
answering many prayers for help during challenging times in my PhD candidature.
3 "Listen. What do you make of this? A farmer planted seed. 4As he scattered the
seed, some of it fell on the road and birds ate it. 5 Some fell in the gravel; it sprouted
quickly but didn't put down roots, 6so when the sun came up it withered just as
quickly. 7 Some fell in the weeds; as it came up, it was strangled among the weeds
and nothing came of it. 8 Some fell on good earth and came up with a flourish,
producing a harvest exceeding his wildest dreams." Mark 4:3-8.
Table of contents
2.1. Pasture botanical composition ........................................................... 5
2.1.1. What determines pasture botanical composition? .............................. 5
2.1.2. Succession in pastures ...................................................................... 9
2.1.3. Invasions of plant communities .......................................................... 9
2.2. Gap dynamics ................................................................................. 10
2.2.2. How plants colonise canopy gaps .................................................... 11
2.2.3. Effect of gap architecture on colonisation ........................................ 12
2.2.4. Influence of grazing on gap ecology and pasture botanical
composition ................................................................................... 15
2.3.3. Present day botanical composition .................................................. 21
2.4. Vulpia ecology and weed potential .................................................. 23
2.4.1. What is vulpia? ................................................................................ 23
2.4.2. Why is vulpia a problem? ................................................................. 24
2.4.3. What does vulpia look like? ............................................................. 26
2.4.4. Where does vulpia occur? ............................................................... 27
2.4.5. What factors influence vulpia population dynamics? ........................ 28
1 . Dormancy ........................................................................................ 28
3. Vegetative growth and survival ........................................................ 32
4. Flowering and seed production ........................................................ 34
vi
2.4.6. How does vulpia respond to nutrients? ............................................ 37
2.4.7. How do vulpia populations behave in pastures? .............................. 39
2.5. Management of vulpia in pastures .................................................. .43
2.5.1. What methods are used to control vulpia? ...................................... .43
2.5.2. Mechanical defoliation ..................................................................... 44
2.5.5. Oversowing ..................................................................................... 47
2.5.7. Grazing management ...................................................................... 48
Chapter 3. Effects of grazing method, ryegrass oversowing and herbicide
application on vulpia content, tiller density and reproduction in phalaris/subterranean
clover pastures ...................................................................................................... 53
3.1. Introduction ........................................................................................ 53
3.2. Methods ............................................................................................. 55
3.2.2. Grazing treatments ............................................................................. 57
treatments ...................................................................................... 58
3.2.6. Seed production ................................................................................. 60
3.2.8. Phalaris clump density and leaf area .................................................. 61
3.2.9. Statistical analysis .............................................................................. 63
3.3.3. Phalaris clump density and leaf area .................................................. 66
3.3.4. Subterranean clover content and density ............................................ 67
3.3.5. Rye grass content ............................................................................... 69
3.3.6. Volunteer annual content and density ................................................. 70
3.3. 7. Vulpia content ..................................................................................... 72
3.3.8. Vulpia tiller and plant density .............................................................. 74
vii
3.3.1 0. Vulpia panicle production ................................................................. 75
3.3.11. Effect of grazing method and management treatment on seed
production ...................................................................................... 76
3.4. Discussion ......................................................................................... 77
Effects on perennial species ............................................................... 77
Effects on subterranean clover ........................................................... 80
Effects on vulpia and other volunteer annual grasses ......................... 81
3.4.2. Impacts of ryegrass oversowing and herbicide application ................. 84
3.5. Conclusion ......................................................................................... 86
Chapter 4. Effects of pasture rest, fertiliser and herbicide application on vulpia
content, tiller density and reproduction in phalaris/subterranean clover pastures ..... 87
4.1. Introduction ........................................................................................ 87
4.2. Methods ............................................................................................. 88
4.2.2. Treatments and experimental design .................................................. 89
4.2.3. Botanical composition ......................................................................... 91
4.2.5. Seed production ................................................................................. 92
4.2. 7. Phalaris basal cover ........................................................................... 93
4.2.8. Statistical analysis .............................................................................. 93
4.3.3. Subterranean clover content. .............................................................. 96
4.3.4. Volunteer grass content. ..................................................................... 98
4.3.5. Vulpia content. .................................................................................... 99
4.3.8. Vulpia residual seed population ........................................................ 103
4.4. Discussion ........................................................................................ 1 04
viii
4.4.4. Differences between Vu!pia species ................................................. 1 08
4.4.5. Differences between field sites ......................................................... 109
4.5. Conclusion ....................................................................................... 1 09
Chapter 5. Effect of grazing method, gap size and species sown in the gap on
vulpia and barley grass growth and reproduction ................................................... 111
5.1. Introduction ...................................................................................... 111
5.2. Methods ........................................................................................... 112
5.2.2. Measurements .................................................................................. 113
5.3.1. Cumulative establishment ................................................................ 118
5.3.2. Total establishment .......................................................................... 119
Effect of grazing, gap size and species sown on total establishment 119
Effect of time of sowing on total establishment ................................. 120
Summary of treatment effects on total establishment ....................... 120
5.3.3. Survival ............................................................................................ 121
Effect of grazing, gap size and species on survival ........................... 121
Effect of time of sowing on survival ................................................... 127
Summary of treatment effects on survival ......................................... 128
5.3.4. Tiller production ................................................................................ 128
Effect of grazing, gap size and species on tiller production ............... 128
Effect of time of sowing on tiller production ....................................... 130
Summary of treatment effects on tiller production ............................. 130
5.3.5. Dry weight ........................................................................................ 130
Effect of grazing, gap size and species on plant dry weight.. ............ 130
Effect of time of sowing on plant dry weight.. .................................... 131
Summary of treatment effects on plant dry weight ............................ 131
5.3.6. Panicle production ............................................................................ 131
Effect of grazing, gap size and species on panicle production .......... 131
Effect of time of sowing on panicle production .................................. 133
Summary of treatment effects on panicle production ........................ 133
5.3. 7. Summary of significance of treatment effects on all parameters ....... 134
5.3.8. Environmental variables ................................................................... 135
Photosynthetically active radiation .................................................... 135
5.4.2. Influence of grazing .......................................................................... 145
5.4.3. Influence of competition in the gap ................................................... 147
5.4.4. Influence of timing of gap appearance .............................................. 149
5.5. Conclusion ....................................................................................... 151
Chapter 6. Effect of neighbour competition on vulpia and barley grass survival
and reproduction under field conditions ................................................................. 152
6.1. Introduction ...................................................................................... 152
6.2. Methods ........................................................................................... 153
6.2.2. Single and double neighbour treatments .......................................... 154
6.2.3. Sowing distance treatments .............................................................. 154
6.2.4. Measurements .................................................................................. 159
6.3.4. Plant dry weight (22 weeks after sowing) .......................................... 163
6.3.5. Tiller production (22 weeks after sowing) .......................................... 165
6.3.6. Panicle production (22 weeks after sowing) ...................................... 166
6.4. Discussion ........................................................................................ 167
6.4.1. Effects of perennial neighbour proximity on growth, survival and
reproduction ................................................................................. 167
6.4.3. Effects of perennial neighbour identity .............................................. 171
6.4.4. Effects of competition with annual neighbours .................................. 172
6.5. Conclusion ....................................................................................... 174
Chapter 7. Effect of neighbour competition on vulpia and barley grass survival
and reproduction under controlled conditions ......................................................... 175
7.1. Introduction ...................................................................................... 175
7.2.2. Single and double neighbour treatments .......................................... 177
7.2.3. Sowing distance treatment ............................................................... 178
7.2.4. Measurements .................................................................................. 178
7.3.2. Survival ............................................................................................ 181
7.3.4. Tiller production ................................................................................ 185
7.3.5. Panicle production ............................................................................ 187
7.4.2. Effects of perennial neighbour proximity on growth, survival and
reproduction ................................................................................. 190
7.4.4. Effect of perennial neighbour identity ................................................ 193
7.4.5. Differences between annual species ................................................ 194
7.5. Conclusion ....................................................................................... 195
Chapter 8. General discussion ...................................................................... 196
8.1. Factors affecting the spread of vulpia in perennial pastures ................ 196
8.2. Vulpia and barley grass life history strategies ...................................... 198
8.3. Further research .................................................................................. 199
Figure 3-1. The Vasey field site ............................................................................... 58
Figure 3-2. Quadrat used to measure phalaris clump density and leaf area ............. 61
Figure 3-3. Phalaris leaf area ................................................................................... 62
Figure 3-4. Content of phalaris ................................................................................. 65
Figure 3-5. Content of subterranean clover .............................................................. 68
Figure 3-6. Content of ryegrass ............................................................................... 70
Figure 3-7. Content of volunteer grasses (barley, brome and winter grass) ............. 71
Figure 3-8. Content of vulpia .................................................................................... 73
Figure 3-9. Vulpia tiller density ................................................................................. 74
Figure 3-10. Vulpia seed production ........................................................................ 77
Figure 4-1. Ararat field site ....................................................................................... 91
Figure 4-2. Content of phalaris ................................................................................. 95
Figure 4-3. Content of subterranean clover. ............................................................. 97
Figure 4-4. Content of volunteer grasses (ryegrass, barley, brome, onion and
winter grass) ......................................................................................... 99
Figure 4-6. Vulpia tiller density in 2000 .................................................................. 102
Figure 4-7. Vulpia tiller density in 2001 .................................................................. 102
Figure 5-1. Sowing vulpia and barley grass seed ................................................... 115
Figure 5-2. Vulpia and barley grass establishment under rotational grazing ........... 118
Figure 5-3. Vulpia and barley grass establishment under set-stocking ................... 118
Figure 5-4. Survivorship in Cohort 1 ....................................................................... 122
Figure 5-5. Survivorship in Cohort 2 ....................................................................... 123
Figure 5-6. Survivorship in Cohort 3 ....................................................................... 124
Figure 5-7. Survivorship in Cohort 4 ....................................................................... 125
Figure 5-8. Survivorship in Cohort 5 ....................................................................... 126
Figure 5-9. Mean daily photosynthetically active radiation ..................................... 135
Figure 5-10. Daily photosynthetically active radiation ............................................. 136
Figure 5-11. Mean daily temperature ..................................................................... 137
Figure 5-12. Daily temperature .............................................................................. 138
Figure 5-13. Volumetric soil moisture content ........................................................ 139
Figure 5-14. Mean pasture height. ......................................................................... 140
xii
Figure 5-15. Relationship between gap diameter and panicle production .............. 142
Figure 6-1. Neighbour competition studies under field conditions .......................... 155
Figure 6-2. Neighbour treatments in a cocksfoot plot. ............................................ 155
Figure 6-3. Diagram of a perennial species plot. .................................................... 156
Figure 6-4. Sowing vulpia and barley grass ........................................................... 157
Figure 6-5. A vulpia and barley grass target plant. ................................................. 157
Figure 6-6. Survival of annual grass species 4 weeks after sowing ........................ 161
Figure 6-7. Survival of annual grass species 12 weeks after sowing ...................... 162
Figure 6-8. Survival of annual grass species 22 weeks after sowing ...................... 163
Figure 6-9. Dry-weight of annual grass species ..................................................... 165
Figure 6-10. Tiller production of annual species
(interaction between sowing distance and annual species) ............... 165
Figure 6-11. Tiller production of annual species
(interaction between sowing distance and perennial species) ............ 166
Figure 6-12. Panicle production of annual grass species ....................................... 167
Figure 7-1. Diagram of a perennial species plot. .................................................... 176
Figure 7-2. Vulpia and barley grass target plants 21 weeks after sowing ............... 177
Figure 7-3. Vulpia and barley grass target plants 33 weeks after sowing ............... 177
Figure 7-4. Seedling number 3 weeks after sowing ................................................ 180
Figure 7-5. Survival of annual grass species .......................................................... 181
Figure 7-6. Dry weight of annual grass species
(interacton between sowing distance and the number of neighbours) .. 183
Figure 7-7. Dry weight of annual grass species
(interacton between sowing distance and annual species) .................. 184
Figure 7-8. Tiller production of annual grass species
(interacton between sowing distance and the number of neighbours) .. 185
Figure 7-9. Tiller production of annual grass species
(interacton between sowing distance and annual species) .................. 186
Figure 7-10. Panicle production of annual grass species
(interacton between sowing distance and the number of neighbours). 187
Figure 7-11. Panicle production of annual grass species
(interacton between sowing distance and annual species) ................. 188
yj ii
Table 3-3. Fertiliser application and Olsen P test value ............................................ 57
Table 3-4. Stocking rate ........................................................................................... 58
Table 3-5. Plant, tiller and meristem density measurements .................................... 60
Table 3-6. Significant treatment effects on the content of phalaris ........................... 64
Table 3-7. Effect of management treatments on the content of phalaris ................... 66
Table 3-8. Phalaris clump number and leaf area ...................................................... 66
Table 3-9. Significant treatment effects on the content of subterranean clover ........ 67
Table 3-10. Effect of management treatments on the content of subterranean clover
......................................................................................................................... 69
Table 3-11. Effect of management treatments on the content of ryegrass ................ 69
Table 3-12. Significant treatment effects on the content of volunteer grasses .......... 70
Table 3-13. Significant treatment effects on the content of vulpia ............................ 72
Table 3-14. Effect of management treatments on the content of vulpia .................... 74
Table 3-15. Effect of management treatments on vulpia plant density ..................... 75
Table 3-16. Effect of Vulpia species on seed production .......................................... 75
Table 3-17. Effect of management treatments on vulpia panicle production ............. 76
Table 3-18. Net herbage production ......................................................................... 78
Table 4-1. Monthly rainfall totals .............................................................................. 88
Table 4-2. Species content and density ................................................................... 89
Table 4-3. Herbicide, fertiliser and pasture rest treatment combinations .................. 90
Table 4-4. Significant treatment effects on the content of phalaris ........................... 94
Table 4-5. Significant treatment effects on the content of subterranean clover ........ 96
Table 4-6. Effect of fertiliser and simazine on the content of volunteer grasses ....... 98
Table 4-7. Significant treatment effects on the content of volunteer grasses ............ 98
Table 4-8. Effect of fertiliser and simazine on the content of vulpia ........................ 100
Table 4-9. Significant treatment effects on the content of vulpia ............................ 100
Table 4-10. Effect of species and SpraySeed on vulpia seed production ............. 103
Table 5-1. Degrees of freedom for analysis of variance ......................................... 117
Table 5-2. Effect of gap size on establishment of vulpia and barley grass .............. 119
Table 5-3. Differences between species in establishment in Cohorts 1 and 4 ........ 119
Table 5-4. Differences between all cohorts in establishment.. ................................ 120
xiv
Table 5-5. Differences between annual species in tiller production ........................ 128
Table 5-6. Interaction between gap size and annual species in tiller production .... 129
Table 5-7. Interaction between grazing method and gap size in tiller production .... 129
Table 5-8. Differences between species in plant dry weight.. ................................. 130
Table 5-9. Effect of gap size on annual species dry weight.. .................................. 131
Table 5-10. Differences between species in panicle production ............................. 132
Table 5-11. Interaction between grazing method and gap size in panicle
production .......................................................................................... 132
Table 5-13. Estimated vulpia population growth (Cohort 1) .................................... 141
Table 5-14. Estimated vulpia population growth (all cohorts) ................................. 141
Table 5-15. Differences between annual species in estimated population growth .. 149
Table 6-1. Number of treatment replicates ............................................................. 159
Table 6-2. Annual grass survival ............................................................................ 163
Table 6-3. Annual grass dry weight.. ...................................................................... 164
Table 6-4. Parameter values describing annual species growth ............................. 164
Table 6-5. Annual species panicle production ....................................................... 166
Table 6-6. Estimated vulpia population growth ....................................................... 169
Table 7-1. Parameter values describing annual species growth ............................. 180
Table 7-2. Vulpia and barley grass survival 3 to 33 weeks after sowing ................. 182
Table 7-3. Vulpia and barley grass survival
(interaction between neighbour number and annual species) ................ 182
Table 7-4. Annual species dry weight. ................................................................... 184
Table 7-5. Annual species tiller production
(interacton between perennial species and the number of neighbours). 186
Table 7-6. Annual species tiller production
(interacton between perennial species and annual species) .................. 187
Table 7-7. Annual species panicle production ........................................................ 188
Table 7-8. Estimated vulpia population growth ....................................................... 192
Chapter 1. Introduction
'Silvergrass' or 'vulpia', is the common name given to winter annual grasses
from the genus Vutpia (Walsh & Entwisle, 1994). Five Vutpia species are prevalent in
temperate southern Australian pastures, the most prominent being V. bromoides, V.
myuros and V. fascicutata (Lamp et at., 1990; Mcintyre & Whalley, 1990). Vulpia
provides poor quality feed in comparison with many other desirable pasture species
and the seed causes damage to hides, fleeces and carcasses of stock. In addition,
vulpia residues contain allelopathic chemicals which impede the germination of other
desirable species (Leigh et at., 1995a; Wallace, 1998). It is estimated that vulpia
costs the Australian wool industry at least $AUS 30 million annually in lost production
(Sloane et at, 1998).
A considerable amount of research on vulpia has been undertaken in
Australia and elsewhere. Beginning in the 1950's, research investigated the
nutritional content of vulpia (Dick et at., 1953; Loneragan et at., 1967; Gladstones &
Loneragan, 1975a). Later work has, among other issues, focused on vulpia ecology,
phylogeny, herbicide resistance, economic impact, allelopathy and integrated
management, using combinations of grazing management, fertiliser addition and
herbicide application (Jones et at., 1992; Jones & Whalley, 1993; Dowling et at.,
1997; Freckleton, 1997; Bolger, 1998; An et at., 1999; Vere et at., 2002; Vere et at.,
2003; Torrecilla et at., 2004; Yu et at., 2004; Burton & Dowling, 2004). However, to
enhance vulpia control strategies, further knowledge is required concerning how
vulpia spreads in pastures and how its spread is influenced by current management
practices. Key questions that need addressing are: 1) how do management decisions
influence vulpia populations in perennial pastures; 2) what is the effect of neighbour
competition on vulpia growth; and 3) how much space is required to sustain a vulpia
population? Answering these questions will give new insight into critical factors that
control competitive interactions and vulpia population dynamics in perennial
pastures. This knowledge can then be used to increase the proportion of perennial
species, thus altering the botanical composition to a more desirable state and
increasing the level of vulpia control.
Perennial ryegrass (Lolium perenne L.), phalaris (Phataris aquatica L.),
cocksfoot (Oactylis gtomerata L.) and tall fescue (Festuca arundacea Schreb.) are
the predominant perennial species sown to improve productivity in the perennial
pasture zone of southern Australian. However, the perennial grass content of these
pastures varies. In the New South Wales perennial pasture zone perennial species
comprised approximately a third of the total pasture biomass and 'improved'
perennial species about a quarter of the total biomass, while the annual content
comprised greater than 20 to 40% (Dowling & Kemp, 1997; Dellow et a!., 2002).
Similarly in western Victoria, sown perennial grasses comprised 20% and annual
grasses 22% of the total dry matter (Quigley eta/., 1992).
Increasing the perennial content increases pasture productivity and
sustainability by: a) providing a more even and sustained feed supply (Chapman et
a/., 2003); b) providing better quality forage, particularly during late spring and early
summer when annual species senesce (Chapman et a!., 2003); c) maintaining
pastures in a more stable state which will reduce weed invasion, (by perennial
species pre-empting and outcompeting annual species for space, water, nutrients
and light (Smith, 1965; Garden & Bolger, 2001 )); d) enabling pastures to better
tolerate drought (Kemp eta!., 2000); e) increasing soil water use and thus reducing
dryland salinity (Ridley et a!., 1997; White et a/., 2003); f) increasing ground cover
and soil stability over summer and thus reducing erosion (Chapman eta!., 2003); and
g) reducing nitrate leaching and soil acidity (Ridley et a/., 1990; Kemp & Dowling,
2000).
Appropriate grazing management and high soil fertility are crucial to maintain
stability and achieve maximum productivity from perennial pastures (Sanford et a/.,
2003). Productive perennial species such as phalaris have higher nutrient
requirements than unproductive, less desirable species such as vulpia (Hill, 2003).
They require high fertility levels to maintain a competitive advantage over these less
desirable species that often grow well in nutrient poor conditions. In addition, grazing
management needs to manipulate competitive interactions between the desirable
perennial species and less desirable annual and broadleaved species (Kemp & King,
2001 ). By preventing grazing defoliation of perennial species at critical times,
appropriate management enhances seed production, new tiller production and
recruitment of perennial species, while reducing the seed production and recruitment
of less desirable species (Dowling et a!., 1998; Waller et a!., 2001; Cullen, 2002).
Perennial pastures are best grazed at a high stocking rate of 15-23 dry sheep
equivalents/ha, in combination with rotational grazing and resting pastures from
grazing (Sanford eta!., 2003).
To investigate the effect of management treatments on the botanical
composition and vulpia content, tiller density and seed production, two sites were
established in western Victoria in phalaris pastures. The first site was in a higher
rainfall zone at Vasey (average annual rainfall = 623 mm), and the second site in a
lower rainfall zone near Ararat (average annual rainfall = 565 mm). These treatments
2
conditions and included grazing method, ryegrass oversowing, herbicide application,
fertiliser addition and pasture rest. Increasing perennial competition reduces vulpia
growth and survival (Peart 1970, Watkinson 1978c, Watkinson 1990). Thus it was
hypothesised that management treatments that enhanced the perennial component
would reduce vulpia population growth while treatments which increased disturbance
of the perennial component of pastures would increase vulpia population growth.
Management practices have a large impact on gap dynamics, through
creating gaps, modifying gap characteristics and affecting competitive interactions
occurring within gaps (Panetta & Wardle, 1992; Bullock eta!., 1994; Deregibus eta/.,
1994). The characteristics of canopy gaps influence germination, establishment and
reproductive success of a colonising species, the spread of this species, and
ultimately the botanical composition of a plant community (Silvertown & Smith, 1988;
Bullock et a/., 1995; Burke & Grime, 1996; Bullock, 2000). However, there is little
information available on how vulpia populations are influenced by gap dynamics in
perennial pastures, or the critical canopy gap size that prevents vulpia plants from
producing seed. Thus canopy gap studies were established in set-stocked and
rotationally grazed pastures at the Vasey field site to determine how gap
characteristics influence vulpia population growth. It was hypothesised: 1) that vulpia
growth and reproduction would increase with gap size, but to a lesser extent under
rotational grazing than under set-stocking due to greater competition for resources;
and that 2) there is a critical gap size below which vulpia population growth is
severely reduced or prevented.
To further investigate the role of perennial neighbours on vulpia survival and
reproduction in perennial pastures, perennial neighbour competition studies were
established in the field and under controlled conditions. These studies investigated
the effect of perennial neighbour competition on vulpia establishment, survival and
reproduction and aimed to determine the critical distance from a perennial neighbour
below which vulpia seed production is severely reduced. It was hypothesised: 1) that
sowing vulpia immediately adjacent to a perennial neighbour would prevent vulpia
from producing seed and reduce its survival; and 2) that increasing the number of
perennial neighbours would reduce vulpia survival, with the extent of the reduction
being dependent on the perennial neighbour species.
This thesis is presented in eight chapters. Chapter two reviews literature on
competition and succession in pastures, and on the ecology and management of
vulpia. Chapters 3 and 4 report experiments investigating the influence of
management practices on vulpia content, tiller density and reproduction at two sites
3
in western Victoria. Chapter 5 reports a study into the influence of canopy gaps on
vulpia and barley grass growth, survival and reproduction in rotationally grazed and
set-stocked pastures, and Chapter 6 and 7 report on the influence of perennial
neighbour competition on vulpia and barley grass growth, survival and reproduction
under controlled and field conditions respectively. Chapter 8 presents a general
discussion of the results and considers implications of this research for vulpia
management practices in grazed pasture systems in southern Australia.
Chapter 2. Competition and succession in pastures
2.1. Pasture botanical composition
2.1.1. What determines pasture botanical composition?
To develop an effective weed management strategy for vulpia in perennial
pastures, it is necessary to understand the ecology of vulpia, and how it interacts with
other species in the pasture. This involves understanding processes that determine
pasture botanical composition, and what factors make pastures prone to invasion by
vulpia, with particular reference to southern Australian pastures.
Many factors influence the botanical composition of pastures. These include
competitive interactions, invasibility of resident vegetation and characteristics of
invading species, disturbance caused by management decisions such as grazing,
herbicide application, burning and cultivation, availability and architecture of canopy
gaps, nutrient enrichment and global and local climate change (Peart, 1970; Grubb,
1977; Werner, 1977; Mack, 1981; Gross & Werner, 1982; Goldberg, 1987; Huston &
Smith, 1987; Goldberg & Gross, 1988; Collins, 1989; Huenneke et a/., 1990;
McConnaughay & Bazzaz, 1990; Thorhallsdottir, 1990; Mesleard eta!., 1991; Hill et
a!., 1992; Bullock eta!., 1994; Campbell & Hunt, 2001; Garden & Bolger, 2001 ).
Different models have been proposed to better understand interactions that
occur between plants and the factors determining plant community structure. Grime's
(1977, 1979) competition-stress-disturbance model classifies plants into different
functional groups based on their life histories. In this model, there are four possible
combinations of high or low stress and high or low disturbance that plants
experience. Disturbance refers to total or partial destruction of a plant by various
causes, including desiccation, grazing, pathogen attack and mechanical defoliation.
Stress is defined as 'the phenomena which restrict photosynthetic production such as
shortage of light, water and nutrients, or suboptimal temperatures' (Grime, 1979).
While a plant would not recover from severe disturbance when greatly stressed, a
plant could survive in habitats that incorporated the other three combinations of
stress and disturbance. Therefore plants develop traits to live in these different
conditions and can be defined as competitors, stress tolerators or ruderal species, as
shown in Table 2-1.
Table 2-1. Plant functional groups developed under different levels of disturbance and stress.
Intensity of disturbance
Stress tolerator No plants survive
Grime proposed that competitors developed high relative growth rates, high
net assimilation rates, large leaf area ratios, and other characteristics that maximize
vegetative growth and confer high competitive ability in productive, low stress and
low disturbance conditions (Grime, 1977, 1979). In this model, competition is defined
as 'the tendency of a neighbouring plant to utilize the same quantum of light, ion of a
mineral nutrient, molecule of water or volume of space' (Grime, 1973a). Stress
tolerators are less vigorous during vegetative and reproductive phases of their life­
cycle, but tolerate unproductive, low disturbance, environmentally stressful conditions
and/or conditions of resource depletion by surrounding vegetation. Stress tolerators
have also developed defences against grazing. In contrast, ruderal species, such as
annual herbs, grow rapidly, have a short life-span, produce prolific quantities of seed
and inhabit productive, unstressed, but highly disturbed environments such as
pastures. Thus botanical composition and succession within a plant community can
be described and predicted in terms of these three functional groups. Grime (1977,
1979) illustrates this model by a triangle showing relationships between these groups
(Figure 2-1 ).
0% R-S s 100%
100% 0%
Figure 2-1. A model describing relationships between competitors (C), stress tolerators (S) and ruderals (R), (after Grime 1977, 1979). lc: relative importance of competition, ld: relative importance of disturbance and 15 : relative importance of stress.
This model incorporates plants that are adapted to intermediate intensities of
competition, stress and disturbance, which fall in between the three apexes of the
triangle. For example, V. bromoides (L) S.F. Gray has been described as a 'stress­
tolerant ruderal', because it inhabits unproductive but highly disturbed environments
(Grime eta/., 1988). Competitive index scores have been proposed for many British
plant species based on their CSR functional grouping (Grime, 1973a; Grime eta!.,
1988).
Tilman (1985) proposed an alternative model, known as the resource-ratio
hypothesis. This model is based on the ability of a plant to compete for resources
and on the long-term supply of limiting resources. A superior competitor is defined as
one that has the lowest resource requirement, R*, for a particular resource, such as
water, nutrients, light or space. R* can be determined by measuring the level of
resource that remains when a monoculture of a single species reaches its equilibrium
density. The superior competitor can reduce the ratio of resources to a level at which
it could survive, but at which the inferior competitor will not. Tilman illustrated this
theory by using zero net growth isoclines for different species (Tilman, 1985, 1988),
as shown in Figure 2-2.
> .. S Oniy g /\ a survi 0 ...., ~
N
B onh·ompdes .\
Isocline for Species A
Isocline for Species B
Figure 2-2. The resource-ratio hypothesis explaining interactions between species (after Tilman 1985). The diagram shows zero net growth isoclines for two species: A and B. R1 :
supply of resource one: R2: supply of resource two. CA and Cs: Resource capture rate for species A and B. This diagram shows the outcome of competition between two species for different regions of resource supply or capture.
The zero net growth isocline is the line showing the ratio of resources below
which a species will not survive. Resource supply or resource capture rate for a given
resource, such as nitrogen, is shown on the x-axis. Resource supply or resource
capture rate of another resource, such as light, is shown on the y-axis. Isoclines
shown on this graph illustrate resource levels at which Species A will survive,
7
increase, decrease or die as a result of competition with Species B. The origin of
these isoclines indicates the lowest level of each resource that is necessary for the
species to survive. The point at which these lines intersect ( o) is the ratio of
resources where both species can exist in a stable equilibrium. In this diagram,
species A has a lower requirement for the resource on the x-axis than species B,
while Species B has a lower requirement for the resource on the y-axis than Species
A. The outcome of competition between species A and B in the different regions is
shown in Figure 2-2 and depends on combinations of resources supplied and the
rates of resource consumption for Species A (CA) and Species B (CB). Coexistence
occurs when two different species are limited by different resources and each
species consumes the resource that most limits its own growth. Competitive
exclusion occurs when one species reduces the ratio of resources to a level below
which the other species cannot survive. Tilman's theory can be used to predict the
outcome of competition between one or more plants for multiple resources.
In both Grime's and Tilman's theories, competition for resources is crucial in
determining which species dominate plant communities. Competition may occur for
above or below ground resources to varying degrees. The relative importance of
above and below ground competition has been investigated in many studies, with
seemingly conflicting results. However, Wilson (1988) concluded from a review of 23
studies on relative importance of shoot and root competition that root competition is
usually more intense and usually affects the balance between components more than
shoot competition. Grime (1977) commented that although competition in productive
environments may be for space and light, 'the outcome may be strongly influenced or
even predetermined by earlier competition below ground'.
Although Grime's and Tilman's theories allow for initial resource pre-emption
or asymmetric competition by one species (Wedin & Tilman, 1993), their theories
differ on how competition determines the ultimate outcome. Grime focuses on the
ability of a plant to capture resources, while Tilman focuses on the ability of a species
to extract resources down to low concentrations (Grace, 1991 ). Tow and Lazenby
(2001) commented that Grime's theory may be better applied to lightly grazed, low­
input pastures rather than intensively grazed, well-fertilized pastures. In contrast,
Kemp and King (2001) suggested that Grime's theory may be best applied in
productive environments, while Tilman's theory may be better applied over the
longer-term in environments with low soil nutrient levels.
Other competition models have been developed and incorporate
characteristics such as leaf area, plant height and seed mass (Westoby, 1998) or
plant height, specific leaf area, leaf turnover and interspecific differences (Schippers
8
& Kropff, 2001 ). Although these models provide useful information on plant
competition, it is thought that more complex models are required to explain the many
competitive interactions that occur in plant communities (Grubb, 1992; Kemp & King,
2001 ).
2. 1.2. Succession in pastures
Succession is the sequential change that occurs in the botanical composition
of a plant community. During this process, the number of species present and/or the
abundance of existing species, changes (Sarmiento et a!., 2003). Grazing greatly
influences the rate of succession, biodiversity and abundance of species present (Hill
eta!., 1992; Bullock eta!., 1994; Chapman, 2001 ).
In the early twentieth century an American ecologist F. E. Clements proposed
a linear model of succession, in which plant communities sequentially changed as a
result of environmental variables, management and competition (AIIen-Diaz &
Bartolome, 1998). These communities eventually formed a stable, climax community
which is controlled predominantly by the climate and other abiotic factors such as
resource levels. The climax community is often prone to nutrient and shade stress
(Chapin, 1980). Clements further proposed that climax communities could shift to
disclimax (or unstable) stages, where the stability of the system was maintained by
management decisions as occur in grazed pastures (Wolfe & Dear, 2001 ).
More recent models such as the 'state and transition' model incorporate
multiple pathways and vegetation states that occur in response to different factors
and can be better used to explain and predict non-linear and non-equilibrium
community dynamics (Westoby et a!., 1989). In the state and transition model,
botanical composition may be stable within states, but may move quickly between
states (Kemp, 1994; Allen-Diaz & Bartolome, 1998). There may also be transient
states that are changed by climatic and management factors to more stable states.
This model better incorporates the ability of a system to recover from disturbance
(resilience), and has been used to explain pasture succession in southern Australia
(Wolfe & Dear, 2001).
2. 1.3. Invasions of plant communities
Invasion of plant communities by exotic species can lead to dramatic changes
in their botanical composition. The annual grass Bromus tectorum L. (cheat grass)
9
invaded western North America in the late 19th century (Mack, 1981 ). Within 40 years
this grass replaced native species such as Vulpia microstachys (Nutt.) Benth. and
Vulpia octoflora (Walter) Rydb., and dominated native perennial grasslands,
particularly wt1ere overgrazing and cultivation occurred.
Lonsdale (1999) analysed data from invasion studies from 184 sites around
the world. Successful invasion of a plant community depended on the number and
characteristics of the invading species and invasibility of a plant community, which is
the susceptibility of a community to invasion. lnvasibility depends on climatic factors,
level of disturbance, resistance to disturbance, the ecosystem's resistance to
invasion and competitive abilities of species initially present in the plant community
(Lonsdale, 1999).
Davis et a!. (2000) proposed that the key factor controlling invasibility is
resource fluctuation. They hypothesised that as the amount of unused resources
increased, the plant community became more susceptible to invasion. Resource
availability may increase as a result of reduction in resource use by existing
vegetation or by increased rate of resource supply. Therefore, environments would
be more susceptible to invasion when there was increased resource supply, a
reduction in the rate of resource use by resident vegetation, fluctuation in resource
supply (including nutrient enrichment), introduction of grazing animals, disease
outbreaks, pest outbreaks and disturbance (Davis eta!., 2000).
2.2. Gap dynamics
fluctuations and destructive events, including fire, drought and flash flooding (White &
Pickett, 1985). In pastures, disturbances such as herbicide application, grazing,
trampling and defecation by stock, cultivation, insect attack of plants, disease,
predation and burrowing animals have a more localised impact and create gaps in
the canopy (White & Pickett, 1985; Parish & Turkington, 1989; Peart, 1989c; Leys &
Dowling, 1992; Panetta & Wardle, 1992; Bullock eta!., 1994; Marriott eta!., 1997;
Moretto & Distel, 1998). Bullock (2000) defines a gap as a competitor-free space,
ranging in size from part of a plant to many plants. When a gap is created, resources
such as light, nutrients and water are released and can be used by invading
propagules (McConnaughay & Bazzaz, 1987).
10
Gap ecology is a key to understanding plant regeneration strategies and
changes that occur in plant communities (Grubb, 1977; Silvertown & Smith, 1988;
Peart, 1989c; Bullock et a/., 1995). Gaps are particularly important for competitive
ruderals and stress tolerant ruderals, which undergo seasonal regeneration (Grime et
a/., 1988). Gaps enhance both population growth and weed invasion because it is
difficult for most species to establish under a closed canopy (Goldberg & Gross,
1988; Silvertown & Smith, 1989). In one study, colonisation success of
Anthoxanthum odoratum L. (sweet vernal grass) and Holcus /anatus L. (Yorkshire fog
grass) increased by 6 to 2500 fold in canopy gaps in comparison to germinating
under a closed canopy (Peart, 1989c). However, there are some species which grow
better under a closed canopy and do not require canopy gaps for establishment
(Ryser, 1993).
To investigate the importance of gap dynamics in pastures, gaps have been
artificially created by applying herbicides such as glyphosate, although spray-drift
and translocation to surrounding vegetation can be a problem. To reduce damage to
surrounding vegetation, Arnth6rst6ttir (1994) severed roots around patch edges and
applied herbicide to a vegetation patch surrounded by a drainage pipe segment.
Gaps have also been created by clipping vegetation to the soil surface (Marriott et
a/., 1997) or by taking soil cores and replacing them with sterile soil (Panetta &
Wardle, 1992; Bolger, 1998).
2.2.2. How plants colonise canopy gaps
Colonisation of a gap occurs through either expansion of existing pasture
plants or germination of seed. Expansion of existing plants occurs through
stoloniferous or rhizomatous growth, increased tillering or through expansion of leaf
size and reorientation of leaves (Parish & Turkington, 1989; O'Conner, 1991; Marriott
eta/., 1997). Generally, species colonise the centre of small gaps much faster than
the centre of large gaps, which are often colonised from the edge first (Peart, 1989b;
Arnth6rst6ttir, 1994; Bullock eta/., 1995). Small gaps are predominantly colonised by
vegetative growth of existing plants neighbouring the gap, or from reorientation of
leaves of surrounding vegetation, while larger gaps are often colonised
predominantly by seed (Arnth6rst6ttir, 1994; Marriott eta/., 1997).
Small-seeded species are more dependent than large-seeded species on
canopy gaps for successful colonisation as they are more likely to possess a light
requirement for germination (Thompson & Grime, 1983). In contrast, large-seeded
11
- -
---
species often display more rapid germination, growth and development, are less
susceptible to desiccation, are less dependent on gaps for colonisation, are less
affected by gap size and can more readily establish under a closed canopy
(McConnaughay & Bazzaz, 1990; Ryser, 1993; Burke & Grime, 1996).
In addition to seed size, other characteristics such as seed production
capacity, competitive ability, biomass, and other unique characteristics of the
invading propagule influence colonisation success (Howe & Snaydon, 1986; Peart,
1989b; McConnaughay & Bazzaz, 1990; Bullock et a!., 2001; Silvertown & Bullock,
2003). Phenotypic plasticity also enhances light interception and colonisation
success. When there is an adequate supply of resources, plants respond by
producing larger leaves and more extensive roots, thus shading and depleting a
wider area of resources. However, under these circumstances, if a plant can increase
its branch number and internode length, the probability that newer portions of the
plant will be placed outside the depleted zone into new resource patches, is
increased (Sutherland & Stillman, 1988). Further, reorientation of leaves can
enhance light interception. The ability of Vu!pia species to position their narrow
leaves in a vertical orientation enhances light interception and increases their survival
when growing at high densities in gaps (Watkinson, 1984).
2.2.3. Effect of gap architecture on colonisation
In addition to characteristics of the invading propagule, gap characteristics,
such as gap size, impact upon colonisation success. There is great variation in gap
size in grasslands, corresponding to smaller gaps created by defoliation, plant death,
dung patches and urine patches, and larger disturbances of greater than 50 em in
diameter caused by burrowing animals or camp areas (Marriott et a!., 1997).
However, small gaps on the scale of a few centimetres are often more numerous in
grasslands than larger gaps and affect resource availability for plant establishment
and growth (McConnaughay & Bazzaz, 1990; Thorhallsdottir, 1990). In a British
study undertaken in grazed pastures, the majority of gaps were in the 1 - 2 em in
diameter range (Silvertown & Smith, 1988) and in a New Zealand dairy pasture 95%
of gaps were within the range of 2, 6 or 10 em in diameter (Panetta & Wardle, 1992).
In southern Australia, pastures contain many winter annual species such as Trifolium
subterraneum L. (subterranean clover) and Lo!ium rigidum Gaudin (Wimmera
ryegrass). Thus there are numerous and large gaps created in pastures over
summer. In this situation, invasion of canopy gaps by annual weedy species, such as
vulpia, occurs readily (Wolfe & Dear, 2001 ). However, there are no published data on
the distribution and size of canopy gaps and their influence on the spread of annual
grass weeds in southern Australian pastures.
Increasing gap size generally enhances survival, growth and fecundity of
invading species, as has been demonstrated in numerous grassland studies (Peart,
1989c; McConnaughay & Bazzaz, 1990; Panetta & Wardle, 1992; Bullock et a!.,
1995; Burke & Grime, 1996). Below a critical gap size, performance of an invading
species is severely restricted. The threshold gap size for the probability of the
invading species surviving to reproductive maturity and producing seed varies among
species (McConnaughay & Bazzaz, 1987); these threshold differences between
species are crucial in determining colonisation success of plants competing for
available gaps. At present, no data are available on critical gap sizes of annual grass
weeds found in southern Australian pastures.
Gap size also influences competitive interactions between propagules
invading a gap and neighbouring vegetation. Silvertown and Bullock (2003) showed
that density dependent mortality of plants occurred especially in small gaps of 3 em
diameter, but was less in larger gaps of 6 em and 9 em diameter. In larger gaps,
competition from vegetation surrounding the gap and from conspecific neighbours
was not as important. Competition was influenced to a greater extent by the
frequency of occurrence of species within the gap. Survival of a species increased as
its frequency in the gap increased.
In addition to gap size, the arrangement of gaps in a pasture canopy
influences the extent of weed invasion. Burke and Grime (1996) created a number of
small gaps, large gaps and both small and large gaps in different plots, forming a
gradient of between 0 and 100% ground cover. Seeds of introduced weed species
were sown in gaps. More weed species were present where there was a greater
number of small gaps than large gaps for a given percentage of bare ground. This
was consistent for different percentages of bare ground. Burke and Grime concluded
that the dominant grass, Festuca ovina L. (sheep fescue), a tussock grass, was
subject to greater damage when creating numerous, small gaps than when creating
fewer, larger gaps. In the larger gap treatment, more tussocks were left intact and
this impeded invasion of weed species (Burke & Grime, 1996). Thus the influence of
gap size on gap colonisation and weed invasion depends on the identity of resident
vegetation, as well as on the distribution of gaps within the sward.
Gap size and surrounding vegetation influence the microclimate experienced
by propagules invading a gap and thus how successfully they colonise the gap.
Firstly, gap size and architecture influence light quality and availability. Tall
13
vegetation reduces light penetration and quality in the gap, hindering growth of the
invading species (Bourd6t, 1996; Marriott eta!., 1997). Leaves selectively absorb red
light; thus shading reduces the ratio of red: far red light experienced by propagules
colonising a gap (Balian§ & Casal, 2000). This explains why Latium multiflarum
(Lam.) (Italian ryegrass) germination was reduced by canopy filtered light, and
increased by exposure to sunlight and high red: far red light ratios (Deregibus eta!.,
1994). Further, the response of Latium multiflarum to light prevents it from
germinating in unfavourable conditions under a thick canopy, but enhances its
germination in a canopy gap. Likewise, Carduus acanthaides L. (plumeless thistle)
and Carduus nutans establishment in gaps is enhanced by increasing gap size, as
this increases the amount of available red light (Feldman eta!., 1994; Bourd6t, 1996).
Secondly, gap architecture influences temperature fluctuations at the soil
surface. Forty-six of 112 herbaceous plants collected from the Sheffield area in
Britain showed a positive germination response to fluctuating temperatures, which
are greater on bare soil than under a closed canopy (Thompson & Grime, 1983). This
response may enable seeds that are buried in the soil to detect a gap in the canopy
of foliage and litter, and increase their chance of successful germination and
establishment (Thompson & Grime, 1983). Likewise, it was concluded that
temperature fluctuations in gaps reduced dormancy and promoted rapid germination
of Stipa spp. L., thus enhancing seedling establishment of these species (Moretto &
Distel, 1998). Further, temperature fluctuations are often greater in larger gaps,
which may be one reason for increased establishment in these gaps. In a New
Zealand dairy pasture, greater seed germination of thistles Carduus nutans and
Cirsium vulgare in 6 em rather than in 2 em diameter gaps, and in the closed canopy,
was attributed to greater temperature fluctuations in the larger gap (Panetta &
Wardle, 1992).
Thirdly, gap size and neighbouring vegetation influence competition for water
within gaps; larger gaps are particularly prone to desiccation (Milton, 1995).
However, surface litter can modify the gap environment and reduce desiccation
because of increased water holding capacity of litter and reduced speed of drainage
through the subsoil (Evans & Young, 1987; Feldman et a!., 1994). Carduus
acanthaides and Carduus nutans germination and survival were reduced in larger
gaps because of desiccation but litter presence in the gap enhanced Carduus
acanthaides establishment (Feldman et a!., 1994). Litter similarly enhanced
germination of Themeda triandra Forsk. (syn. Themeda australis (R. Br.) Stapf.,
kangaroo grass) in a South African savanna (O'Conner, 1991 ). Litter reduces
14
moisture stress and enhances survival of shallow rooted species, which are
especially vulnerable to desiccation (Platt & Weis, 1977).
In addition to gap microclimate, gap fertility influences colonisation of invading
propagules. Burke and Grime (1996) found that application of fertiliser containing
nitrogen, phosphorus and potassium enhanced seedling establishment, especially
where much bare ground was present. Fertility was less important where little bare
ground was present. However, increased fertility levels also enhance persistence of
desirable perennial pasture species, making it more difficult for weedy species to
establish in gaps between the perennial plants (Bullock et at., 1994).
2.2.4. Influence of grazing on gap ecology and pasture botanical composition
Grazing greatly influences gap dynamics in pastures and thus changes the
pasture botanical composition in a number of ways. Firstly, one of the greatest
effects of grazing is to create gaps in the pasture canopy throughout the year by total
or partial plant death, through defoliation, stock camps, dung deposition, urine
deposition and trampling (Noy-Meir et at., 1989; Thorhallsdottir, 1990; Haynes &
Williams, 1993; Marriott et at., 1997; Bullock, 2000). Overgrazing, particularly during
summer, creates numerous gaps and enhances invasion of annual weedy species
that germinate when the summer drought ends and autumn rains begin (the 'autumn
break'). This changes the botanical composition (Noy-Meir et at., 1989; Panetta &
Wardle, 1992; Bourd6t, 1996). Thus timing of gap appearance influences what
species invade gaps, and ultimately the botanical composition of a pasture (Bullock
et at., 1994; Marriott et at., 1997). Grazing management can be used to increase
perennial pasture cover and impede germination of weedy species (Jones & Whalley,
1992; Kemp, 1994).
Secondly, grazing influences the gap microclimate, by altering the
temperature regime, light regime and moisture content, as well as making nutrients
from dung and urine available for utilisation by invading species (Panetta & Wardle,
1992; Haynes & Williams, 1993; Deregibus et at., 1994; Bourd6t, 1996). Grazing also
alters gap micro-topography, which in turn influences seedling establishment and
plant growth (Titus & del Moral, 1998). Seeds are more firmly anchored and
establishing seedlings experience less water stress in gaps with rough surfaces than
with smooth surfaces. Gaps with rough surfaces contain many cracks and hold more
water (Evans & Young, 1987; Feldman et at., 1994).
15
of species colonising gaps and alters competitive interactions occurring in pastures.
Intense defoliation generally reduces above ground biomass, culm production, seed
production, leaf height, leaf length, root biomass and length, total non-structural
carbohydrate storage and plant vigour (Heady & Child, 1994b; Tow & Lazenby, 2001;
Noy-Meir & Briske, 2002). Reduced competition for light, nutrients and moisture from
neighbouring plants in grazed swards may offset some of these effects and increase
seedling establishment (Heady & Child, 1994b; Hutchings & Booth, 1996; Dear eta!.,
1998; Van Der Wal eta/., 2000; Tow & Lazenby, 2001 ). Defoliating phalaris swards
(Phalaris aquatica L.) doubled the amount of light reaching germinating Trifolium
subterraneum seedlings, increased surface moisture retention and increased
subterranean clover seedling size, weight and turgor (Dear et a!., 1998). Likewise,
Triticum dicoccoides (K6rn.) K6rn. (wild wheat) growth and reproduction increased by
50% when neighbouring plants were defoliated (Noy-Meir & Briske, 2002).
As grazing intensity is increased, shorter, more prostrate species are
advantaged while taller growing species and species less able to colonise gaps are
disadvantaged (Rosiere, 1987; Bullock eta!., 2001 ). For example, increasing grazing
intensity increased the abundance of the more prostrate Trifolium subterraneum,
Erodium cicutarium (L.) L'Her. (common storksbill) and Erodium botrys (Cav.)Bertol.
(long storksbill) while the abundance of Bromus hordeaceus L. (soft brome) declined
(Rosiere, 1987).
The extent to which grazing defoliation affects a plant depends on its ability to
avoid or tolerate grazing (Briske, 1996). Plants avoid grazing, by mechanical and
biochemical mechanisms such as awns, alkaloids and phenolic compounds. Further,
plants with prostrate growth and those producing a greater number of smaller tillers
with reduced leaf number, avoid grazing damage (Briske, 1996). Plant tolerance
mechanisms include positioning meristems close to the ground, increasing the
number of meristems, asynchronous tiller development, reallocating resources,
minimising nutrient loss, maximising nutrient uptake, undergoing rapid compensatory
growth, possessing a large root mass, rapid or prolonged root growth, rapid growth
rate and maintaining a large leaf area (Briske, 1996; Nurjaya & Tow, 2001; Skarpe,
2001 ). As a plant is dependent on the residual leaf area from which to produce
photosynthetic products, maintaining a large residual leaf area will enable a plant to
respond rapidly to defoliation. Many Australian native species are grazing intolerant
and are easily replaced by the more grazing tolerant, introduced species (Garden &
Bolger, 2001 ).
16
A plant's ability to tolerate or avoid grazing also depends on its requirements
for nutrients and other resources. This interaction between grazing and nutrient
requirements is demonstrated by the change in the abundance of Erica tetralix L.
(crossleaf heath) and Molinia caerulea (L.) Moench (purple moor grass) when
grazing was discontinued. Erica tetralix was much more prevalent than Molinia
caerulea in grazed, infertile pastures as Erica tetralix had a lower nutrient
requirement and was better able to withstand the pressure of grazing in the nutrient
poor regime. However, after grazing ceased, Mo/inia caeru/ea outcompeted Erica
tetralix and dominated pastures (Berendse, 1985). Similarly, requirements for
moisture influence a species' response to grazing. While Festuca arundinacea
Schreb. (tall fescue) dominated irrigated pastures, discontinuing irrigation changed
the pasture botanical composition; Festuca arundinacea abundance declined and the
abundance of Bromus hordeaceus and Hordeum marinum Huds. (sea barley grass)
increased, as these annual species could better survive grazing under conditions of
greater water stress than Festuca arundinacea (Mesleard eta/., 1991 ).
Grime's CSR model predicts that productive pastures are dominated by
competitive or competitive ruderal species, which have a high relative growth rate
and compete well for light. These species can live under conditions of grazing
disturbance, where they can avoid or tolerate grazing and trampling (Grime, 2001 ).
Species such as these can rapidly establish a canopy and shade out more prostrate,
slower growing species, thereby reducing biodiversity (Chapman, 2001 ). In contrast,
drier, less fertile pastures would be dominated by slower growing species better able
to tolerate stress. These species may be efficient at extracting nutrients in these
infertile environments or have a reduced demand for nutrients (Chapman, 2001 ).
2.3. Pasture succession in southern Australia
2.3. 1. Climax community before European settlement
Having discussed factors that are crucial in determining pasture botanical
composition and succession in pastures, it is important to understand how these
factors have influenced pasture succession in southern Australia and why pastures
are so prone to invasion by annual grass weeds. This requires knowledge of the
original community before the introduction of exotic species, the main factors leading
to change in temperate Australian grasslands, and knowledge of the present pasture
botanical composition.
17
At the time of European settlement, it is thought that warm season perennial
tussock grasses dominated grasslands. Themeda triandra was prevalent at lower
altitudes and Poa sieberiana Spreng (Tussocky poa) was prevalent at higher
altitudes. Aristida ramosa R.Br. (purple wire grass) and Sorghum leiocladum (Hack)
C.E. Hubbard (wild sorghum) along with native legumes and a small number of
annual species, were present to a lesser extent (Whalley et a!., 1978; Hartridge,
1979; Stylinski & Allen, 1999; Wolfe & Dear, 2001 ). These perennial species dried-off
at the end of summer and left a large layer of litter over the ground, which inhibited
germination and establishment of annual species (Whalley eta!., 1978). These native
species generally competed well for nitrogen and kept soil nitrogen at low levels.
Under these conditions C4 native perennial grasses were able to outcompete C3
grasses, which were poorer competitors for nitrogen and required higher nitrogen
levels (Garden & Bolger, 2001 ).
With burning, land clearing, grazing and cultivation, these tall perennial
species were replaced by short, more prostrate warm and cool season perennial
species such as Austrodanthonia spp. H.P. Linder (wallaby grass), Bothriochloa
macra (Steud.) S.T.Biake (red grass), Chloris truncata R.Br. (windmill grass) and
Microlaena stipoides (Labill.) R.Br. (weeping grass) which were resistant to heavy
grazing (Whalley eta!., 1978; Stylinski & Allen, 1999; Garden & Bolger, 2001; Wolfe
& Dear, 2001 ). Themeda triandra disappeared (Hartridge, 1979). Species such as
Bothrioch!oa macra, that were better adapted to nutrient poor environments, were
disadvantaged, while Austrodanthonia spp. was able to exploit nutrients that
accumulated from dung and urine deposition and persisted when stock were
introduced (Whalley eta!., 1978; Hartridge, 1979).
Throughout southern Australia, continued clearing of land by settlers and
grazing led to soil compaction, salinity, erosion and increased mortality of native
species. Grazing prevented dry matter accumulation and opened up pastures,
facilitating invasion of exotic species. Overstocking, rabbit infestations, droughts and
economic hardship during the 191 h century resulted in further pasture degradation
and invasion by cool season Mediterranean annual and cool season dwarf perennials
species, including Vu/pia spp., Bromus spp. L., Hordeum murinum L. (barley grass)
Erodium spp. L'Her. ex Ait., Echium plantagineum L. (Pattersons curse) and
Arctotheca calendula (L.) Levyns (capeweed) (Whalley eta!., 1978; Hartridge, 1979;
18
Stylinski & Allen, 1999; Garden & Bolger, 2001; Wolfe & Dear, 2001 ). Recent
research has shown how grazing in remnant eucalypt woodlands altered soil nutrient
levels, changed the soil microclimate, increased soil erosion, reduced water
infiltration, reduced native perennial cover and increased exotic annual cover (Yates
eta!., 2000).
An excerpt of a letter from John Robertson to Governor La Trobe in
September 1853 is quoted below and gives an indication of the landscape at that
time and the changes that were occurring (Lloyd 1862, in Conley 1983, p 17).
'The few sheep at first made little impression on the face of the country for
three or four years; the first great change was a severe frost, 11th November
1844, which killed nearly all the beautiful blackwood trees that studded the hills in
every sheltered nook some of them really noble, 20 or 30 years old; nearly all
were killed in one night; the same night a beautiful shrub that was interspersed
among the blackwoods (Sir Thomas Mitchell calls it Acacia glutinosa) was also
killed ... before this catastrophe all the landscape looked like a park with shade
for sheep and cattle.
Many of the herbaceous plants began to disappear from the pasture land;
the silk-grass began to show itself in the edge of the bush track, and in patches
here and there on the hill. The patches have grown larger every year;
herbaceous plants and grasses give way for the silk-grass and the little annuals,
beneath which are annual peas, and die in our deep clay soil with a few hot days
in spring, and nothing returns to supply their place until later in the winter
following. The consequence is that the long deep-rooted grasses that held our
strong clay hill together have died out; the ground is now exposed to the sun, and
it has cracked in all directions, and the clay hills are slipping in all directions; also
the sides of precipitous creeks - long slips, taking trees and all with them.'
Another letter from the 1860's (Lloyd 1862, in Conley 1983, p 17) tells of soil
compaction that occurred with increasing stock numbers.
'When the colony was first settled, great difficulty was experienced in riding
over the country at any pace exceeding that known as the farmer's jog-trot. The
untrodden sward, at the early period, during the winter season particularly, was
literally comparable to a bed of sponge; our horses sank to the fetlock at almost
every step. The soil upon the summits of the highest hills, also, was so
remarkably 'tender' throughout many months of the year, as to render fast riding
19
over them absolutely dangerous, if not impracticable ... No sooner had the rich
native pastures been well fed down, and, as a consequence, every square inch of
land continually impressed with the weight of thousands upon thousands of the
sharp little hoofs of sheep, than the whole of the occupied country began to
assume a totally different aspect. A two years' occupation, in most instances,
rendered a station so 'firm', that horseracing, kangaroo, emu, and dingo (native
fox hunting), with or without hounds, formed one of the principal sources of
amusement to the lighthearted squatters.'
Lloyd (Lloyd 1862, in Conley 1983, p 18) also commented that 'kangaroo
grass, the most succulent of the Australian Herbage will soon be exterminated .... '
and that it should be protected to prevent this. By this time, pastures were sown to
introduced species; there had been a dramatic increase in cultivation and continuous
cropping and introduction of large mobs of sheep and cattle (Conley, 1983).
By the end of the nineteenth century, many fields were cultivated and fully
stocked with sheep, cattle and horses (Conley, 1983). Bride (Bride 1898, in Conley
1983, p 17) also commented on the effects of salinity and soil compaction in
increasing the mortality of perennial tussock grasses:
:4 rather strange thing is going on now. One day all the creeks and little
watercourses were covered with a large tussocky grass, with other grasses and
plants, to the middle of every watercourse but the Glenelg and Wannon, and in
many places of these rivers; now that the only soil is getting trodden hard with
stock, springs of salt water are bursting out in every hollow or watercourse, and
as it trickles down the watercourse in summer, the strong tussocky grasses die
before it, with all others'.
Native perennial tussock grass swards were vulnerable to invasion by exotic
species for several reasons. Emerging tillers remained erect inside the leaf sheath
and were exposed to grazing, more so than tillers of exotic stoloniferous or
rhizomatous species. These native grasses relied on sexual reproduction, so grazing
of reproductive tillers and seedlings greatly reduced their abundance. In addition,
these grasses were not adapted to selective grazing and trampling of hard hoofed
species. Grazing disturbance therefore destroyed native plants and created
conditions that enhanced the dispersal and invasion of exotic annual species, which
were well adapted to grazed conditions (Garden & Bolger, 2001 ).
20
The introduction of fertilizer at the turn of the 201 h century led to further
ingress of Mediterranean cool season annuals, dwarf cool season perennials and
short, warm season perennials (Whalley eta!., 1978; Stylinski & Allen, 1999). Many
of the introduced species were better able to respond to added phosphorus and
displaced native species from pastures (Hartridge, 1979). Recent research in
remnant white box woodlands, demonstrated that exotic species Vulpia bromoides
(L.) S.F. Gray (squirrel's tail fescue) and Echium plantagineum showed a greater
response to phosphorus addition and depressed the growth of the native species
Themeda triandra, Bothriochloa macra and Austrodanthonia racemosa (R. Br.) H. P.
Linder (Allcock, 2002). Introduced species included Trifolium repens, Phalaris
aquatica L. (phalaris), Festuca arundinacea, Oactylis glomerata L. (cocksfoot),
Lolium perenne L. and Lolium rigidum (Whalley et a!., 1978; Hartridge, 1979;
Stylinski & Allen, 1999).
Even if fertiliser, grazing o