The response of riparian vegetation to flood-maintained habitat heterogeneity

The response of riparian vegetation to flood-maintainedhabitat heterogeneity†

BONNIE C. WINTLE* AND J. B. KIRKPATRICKSchool of Geography and Environmental Studies, University of Tasmania, Hobart,Tasmania, Australia

Abstract Riparian environments are subject to the scouring and depositional effects of floods. Riparian vegeta-tion and substrates are scoured during high flows, while litter and sediment is deposited downstream. In the Prosserand Little Swanport River catchments in south-eastTasmania, vascular plant species were surveyed in large riparianrelevés. Within these relevés, 1 ¥ 1 m subplots were placed in both flood-scoured and depositional environments.Species composition was compared between these three datasets, to investigate the importance of floods indetermining species richness and species composition of riparian vegetation. Species richness and diversity werehighest in areas experiencing flood scour. Herbs appear particularly reliant on the creation of gaps for colonization,and some major riparian shrub species may also require disturbance to maintain their abundance.The depositionalenvironment tended to favour shrubs and graminoids. Given that differences in species composition are related toflood-induced features of the riparian environment, the regulation of these rivers might reduce the diversity of theriparian vegetation downstream of dams.

Key words: deposition, riparian vegetation, scour, species richness.

INTRODUCTION

Scouring and deposition are well-recognized phenom-ena in fluvial processes (e.g. Goodson et al. 2001;Benda et al. 2005; de Boer et al. 2005; Steiger et al.2005; Gurnell et al. 2006; Marion et al. 2006). Whilethe ramifications of mechanical disturbance associatedwith floods are relatively well understood for aquaticbiota, there has been comparatively little researchexploring the importance of such physical disturbancein a riparian environment. Riparian vegetation pat-terns and dynamics are considered to be fully inte-grated with fluvial-geomorphic forms and processes(Hupp & Osterkamp 1996). However, the explicitdetails linking the hydrogeomorphological system withvegetation patterns are rarely identified, perhaps dueto difficulties in disentangling the complex range ofinterrelated causal linkages and feedbacks (Bendix &Hupp 2000). Along with other processes such as inun-dation, groundwater recharge and nutrient andpropagule transport, the process of scouring and depo-sition is one component of floods that plays an impor-tant role in shaping riparian vegetation compositionand structure.

Scours are places from which vegetation and/or sedi-ment have been removed by fluvial erosion. Some ofthis material is trapped by vegetation, forming depositsthat are a mixture of litter and sediment.The diversityof plants in riparian vegetation seems highly likely tobe influenced by this habitat heterogeneity, as pre-dicted by the regeneration niche hypothesis (Grubb1977) and the intermediate disturbance hypothesis(Connell 1978).The interplay between succession andboth local and landscape patch dynamics has alsoreceived considerable attention in the past literature(e.g. Baker &Walford 1995; Bornette & Amoros 1996;Ward et al. 1999; Latterell et al. 2006), especially inriverine ecosystems, where dynamic conditionspromote a mosaic of successional stages.

In one of the rare papers that focuses on the directeffect of scouring on community composition,Kimmerer and Allen (1982) found that the periodicremoval of the dominant species by flood disturbanceincreased riparian bryophyte diversity.This was due tothe colonization of gaps by opportunistic species thatwould otherwise be excluded through competition.For vascular plants, the most extensive researchexploring the role of physical disturbance in the ripar-ian zone has been conducted for individual species.Research on cottonwood in the Northern Hemispheredemonstrates that recruitment relies on the scouring ofnew habitat from flood events (Lytle & Merritt 2004;Fierke & Kauffman 2005). Similarly, a study on thepatch dynamics of an endangered riparian plant, Silenetatarica, found that river dynamism created space for

*Corresponding author. Present address: School of Botany,University of Melbourne, Melbourne, Vic. 3010, Australia(Email: [email protected]).

†Taxonomic nomenclature follows Buchanan (1999), andstructural nomenclature follows Specht (1970).

Accepted for publication October 2006.

Austral Ecology (2007) 32, 592–599 doi:10.1111/j.1442-9993.2007.01753.x

© 2007 Ecological Society of Australia

mailto:[email protected]

colonization, but could also cause local extinctions ofsmall patches (Jakalaniemi et al. 2005).We know of nopapers that have directly compared the species andlife-form composition of scours and flood depositswithin the riparian zone.

In increasing the spatial and temporal heterogeneityof the riparian environment and creating distinctregeneration niches, scouring and deposition couldreasonably be expected to increase overall species rich-ness and diversity. Environmental differences betweenscours and flood deposits could also be expected tobe reflected in differences in species and life-formcomposition.

The present paper investigates whether abundanceand compositional differences exist between scours,depositional patches and the overall site, and whetherspecies with different growth forms are favoured bycertain patch environments. Comparisons of the veg-etation attributes of scoured sites, depositional sitesand the riparian vegetation as a whole allow us to testwhether either or both of these flood-related environ-ments are important for any elements of the vegetationand, thus, for maintaining riparian biodiversity.

STUDY AREA

The research was conducted in the Prosser and LittleSwanport Catchments, located on the east coast ofTasmania, Australia (Fig. 1).This region has a subhu-mid climate. Rainfall is relatively evenly distributedthroughout the year.The average annual rainfall varieswith altitude, ranging from 500 to 750 mm in thelower basins to 750–1000 mm in the more elevatedheadwaters.The geology of the study area largely con-sists of Jurassic dolerite and Triassic sandstone.

Rivers in the study area display low mean annualrunoff, comparable to those of semiarid regions(Hughes 1987). They also exhibit high coefficient ofvariation of annual flows, indicating their susceptibilityto significant flooding (Hughes 1987).

Many of the watercourses in both catchments runthrough highly modified agricultural land, and weedssuch as gorse and blackberry are abundant, particu-larly in the Prosser. The Little Swanport Catchmentcontains sections of relatively pristine land, but islargely comparable to the Prosser in terms of land use,floristics and hydrology.

Fig. 1. Location of the study area.

FLOOD DISTURBANCE IN RIPARIAN VEGETATION 593


The native vegetation of the two catchments islargely composed of eucalypt-dominated dry sclero-phyll forest and woodland, graduating into wet sclero-phyll forest where the environment is moist andshaded. Both catchments also contain patches ofsedgeland, heathland, grassland, scrub and wetland.

METHODS

A total of 85 sites were subjectively selected with nopreconceived bias (Mueller-Dombois & Ellenberg1974), within a general strategy of attempting to covergeographical, geological and hydrological variationwithin the study area. The floristic sampling of siteswas based on the relevé approach (Mueller-Dombois& Ellenberg 1974).The size of the relevés was approxi-mately 60 ¥ 15 m, although sometimes smallerdepending on the magnitude of the watercourse. Thiswas to ensure that dry land vegetation was notincluded in the survey. At each site, the abundances ofvascular plant taxa were estimated for relevés thatextended from the river to non-riparian vegetation,taking care not to cross within-riparian vegetationboundaries (Daley & Kirkpatrick 2004). Species coverwas noted within the following classes: 1 = <1%;2 = 1–5%; 3 = 6–25%; 4 = 26–50%; 5 = 51–75%;6 = 76–100%.The percentage covers of leaf litter, finesoil (<1 mm particles), gravel (1–60 mm particles),cobble (6–30 cm particles) and boulders or bedrock(>30 cm) comprising the riparian substrate were esti-mated, together with surface soil texture, using therelative proportions of sand, silt and clay as describedby Gordon et al. (1992).

A 1 ¥ 1 m subplot was placed in the centre of thefirst sighted scour larger than 1 m2 within the riparianrelevé. The scour subplots were usually located in themidst of the relevé, to minimize the influence of edgeeffects from the river and adjoining land use. A second1 ¥ 1 m subplot was placed in the centre of a nearbyarea of deposition of litter and sediment, usuallylocated immediately downstream of the scour zone.The cover of all identifiable vascular plant taxa in eachof these subplots was estimated using the above scale.Most of the sites contained scours and/or deposits,which usually occupied approximately 5–10% of thetotal area of the relevé. All of the area within the siterelevé was suitable for the occurrence of scour anddeposition subplots. Sixty of the 85 sites containedboth scour and deposition subplots, and only thesesites were used in the analysis that compares theirattributes with those of the site as a whole.

This study compared the floristics of the overallrelevés with those of the subplots. Future investiga-tions of this type should consider also sampling undis-turbed patches in 1-m2 subplots, to allow directcomparison of patch types at the same scale. This

would prevent any potentially confounding effects dueto inequality of scale. While the comparison of datafrom the site as a whole with data from plots that formpart of the site violates the assumption of indepen-dence of samples, this violation is conservative, in thatit would mitigate against recognition of differences.

Scour and deposition classes were ordered from themost scoured to that with the most deposition. Scour-ing was visually estimated as (1) severe (all the originalvegetation removed, roots exposed or removedentirely); (2) moderate to severe (most of the originalvegetation removed, roots exposed); (3) gentle to mod-erate (over half of the original vegetation still intact);and (4) gentle (most of the original vegetation stillintact). Given that all sites were sampled within onegeographical area and season, and were all locatedwithin the immediate flood zone, the scour subplotsappeared to be approximately the same age.The degreeto which they were scoured is likely to be a product ofseveral factors, including flood severity, substrate type,position above water line and anchoring of vegetation.Deposition was noted in the following classes:(5) <10 cm3; (6) 10–30 cm3; (7) 30–100 cm3; (8)>100 cm3. This classification was created for the pur-poses of the present study, and the categories reflect areasonable representation of the range of volumes ofdepositional features contained in both catchments.Similarly, the scour classification represents the rangeof scour types observed in the study area.

The mean cover and percentage frequency of taxawas calculated for each of the scour and depositionsubplots, the sites as a whole (relevés), and for each ofthe scour/deposition classes. Overall species richness,native richness, exotic richness, the richness of lifeforms, the Shannon-Weiner diversity index, exoticplant abundance, the ratio of exotic and native abun-dance, total abundance and the abundances of lifeforms were calculated for each subplot and entire site.The abundances were the sum of the mid-points of thecover classes. One-way anova and the paired T-testwere used to compare the abundances of life-forms,natives and exotics between the scour plots, depositionsubplots and the sites as a whole (relevés).The normalsize of relevés (approx. 900 m2) was also such that the1-m2 subplots would have very little influence on theestimates of cover in the relevé. One-way analysis ofvariance was used to determine whether the degree ofscouring or deposition affected any of the aboveattributes, or overall species composition. A globalnon-metric multidimensional scaling ordination, fol-lowing the default options in decoda (Minchin 1990),was undertaken for scour and deposition plots. Scoreswere obtained for the vector related to the scour/deposition index (P = 0.005) and for a single axissolution (stress = 0.11), as indicators of speciescomposition. The Kruskal–Wallis H-test was used todetermine if there were any significant differences

594 B. C. WINTLE AND J. B. KIRKPATRICK


between medians of scour/deposition class variablesand non-parametrically distributed continuousvariables. The c2-test was used to determine whetherthe number of species in each life-form class that hadtheir highest cover in each of scours, deposits andrelevés varied significantly from equality.

RESULTS

Herbs were significantly more abundant in scours andthe general site environments than in deposits(Table 1). Scour and deposition plots have less overallplant cover than the relevés due to the large cover ofbare ground in scours, and the large cover of litter indeposits.Thirty-nine out of 45 herb species were moreabundant in scours than deposits (c2 = 11.61, d.f. = 1,P < 0.001), and 29 were more abundant in scours thanin the site in general (c2 = 21.07, d.f. = 1, P < 0.001).All other cover variables were greatest in the site as awhole, with the depositional subplots having signifi-cantly higher values than the scour subplots for tree,shrub and graminoid covers, and significantly less forgrass cover (Table 1).

Forty species had their highest mean cover in thescour subplots, 10 in the depositional subplots and 60in the site as a whole. The most abundant taxa in thescour subplots were Bryophyta, a grass, Ehrharta sti-poides, and two herbs, Oxalis perennans and Hydrocotylehirta. Shrub seedlings were observed to be common inthe scour subplots. The species that were most abun-dant in the depositional subplots were the shrubs andlarge tussocks that initially trapped the litter andsediment. These were Acacia mucronata, Callistemonpallidus, Gahnia grandis, Lomandra longifolia, Leptosper-mum lanigerum and Lepidosperma ensiforme.

There was significant floristic variation on both axis1 of the ordination and the vector scores relating to

scour and deposition classes.The highest scores on thefirst axis of the ordination were where most materialhad been deposited, while the lowest scores werewhere deposition was least (Table 2). The mostextreme vector scores occurred in the most scouredsubplots, and those containing the most deposition(Table 2). The proportion of the ground’s surfacewithin the site that was covered by sand, silt or clay(fine soil) was highest where the subplots were theleast scoured and contained the least amount of depo-sition (Fig. 2: H = 26.72; d.f. = 7; P = 0.000 adjustedfor ties).

Overall taxon richness, native taxon richness, exotictaxon richness, native grass richness and the Shannon-Weiner diversity index were all consistently higher inthe scour classes than in the depositional classes(Table 2). Tree taxon richness, shrub taxon richnessand shrub cover were least in the least scoured andleast deposited subplots (Table 2). Herb coverdecreased with increasing severity of scouring and wasnegligible in all depositional classes (Table 2). Grami-noid cover was relatively constant within the scourclasses, while decreasing from high to very low valueswith increasing deposition (Table 2). Grass coverdeclined with increasing severity of both scouring anddeposition (Table 2).

Ehrharta stipoides, O. perennans and Dichondra repenswere most frequent in the severest scour class(Table 3). Bryophyta, H. hirta, Schoenus spp., Pomad-erris apetala, Agrostis spp. and Austrostipa spp. weremost frequent in the second most severe scour class.Acaena novae-zelandiae, Euchiton spp., Geranium poten-tilloides and Viola hederacea were most frequent in thethird most severe scour class. The least severe scourclass had the highest frequencies of several exoticherbs and grasses. Only two species, Poa labillardierei inclass 5 and A. mucronata in class 8, had their highestfrequency within depositional classes (Table 3).

Table 1. Means and standard errors (in brackets) of percentage cover of life forms for scour, deposition and general siteenvironments

Life formScour mean

% coverDeposition mean

% coverSite mean% cover P-value

Herb 13.61 (1.76)B 0.96 (0.30)A 10.17 (1.23)B 0.000Tree 1.27 (0.44)A 6.38 (2.45)B 32.67 (3.09)C 0.000Shrub 2.96 (1.08)A 17.93 (3.69)B 37.18 (2.84)C 0.000Graminoid 3.00 (0.78)A 13.68 (3.32)B 25.71 (2.04)C 0.000Grass 8.84 (1.32)B 3.00 (0.98)A 25.13 (3.25)C 0.000Fern 0.09 (0.08)A 0.00 (0.00)A 7.58 (2.11)B 0.000Climber 0.37 (0.26)A 0.04 (0.04)A 1.71 (0.79)B 0.036Exotic 3.18 (0.73)A 2.88 (1.54)A 17.35 (4.08)B 0.000Native 44.33 (3.91)A 43.71 (4.11)A 136.55 (6.08)B 0.000Total 47.52 (4.09)A 46.59 (4.07)A 153.90 (6.07)B 0.000

Significance (P) obtained using anova. Figures that share a letter within rows are statistically identical at a confidence level ofone in 20 (paired T-test), n = 60.



DISCUSSION

Our data suggest that the scouring and depositioncaused by floods favours a substantial set of speciesfound in riparian vegetation, particularly herbs(Table 3). Previous research has found herb richnessto be greatest after disturbance (Roberts & Gilliam1995; Kobayashi et al. 1997; Pabst & Spies 1998), asherbs are common feature of the early stages of suc-cession (Rooney & Dress 1997). Mild, rather thansevere scouring seems most effective in promotingherb cover, with severe scouring leading to the loss offine soil particles (Fig. 2), leaving a lag deposit ofgravel, cobbles and rocks more suited to the coloniza-tion of shrubs (Walter 1971).

In one of the few Australian studies that explores thechanges in composition and life-form structure due tofloods, Pettit et al. (2001) found a strong relationshipbetween floristics, life-form structure and populationdynamics with stream hydrology. Although they didnot detect a correlation between species richness andflooding regime for the Ord River, they noted thatcover of perennial grasses increased with flooding fre-quency while cover of shrubs decreased. Their resultsfor the Blackwood River, which exhibits relatively lowvariability of flows, showed a reduction in species rich-ness with increased duration and frequency offlooding. This suggests that high variability of flowregime (as experienced in the present study area) maybe an important feature in the role of floods in pro-moting species richness. This may be particularlyimportant for Australian native riparian vegetation,which is naturally adapted to the highly variable flowregimes of Australian rivers, compared with the morepredictable flows exhibited in Northern Hemisphererivers (Lake et al. 1985).

Herbaceous and early colonizing weeds are oftenpromoted by regular disturbance (Groves 1986).Pettit et al. (2001) found exotic species cover andannual herbs to increase with increased floodingalong the Blackwood River. In contrast, this was notthe case with flood disturbance in the present studyarea, exotics being most abundant where disturbancewas least (Tables 2,3) and less abundant in the scourand deposition subplots than in the vegetation ingeneral (Table 1). Scouring is equivalent to the scalp-ing that is often used in Australian restoration worksto remove the exotic seed store and reduce the fertil-ity that favours many exotics. It has been previouslynoted in Tasmania that subtractive disturbancefavours natives over exotics, and that additive distur-bances tend to promote exotics (Kirkpatrick &Gilfedder 1995; Gilfedder & Kirkpatrick 1998). Theadditive disturbance of flood deposition smothers allbut the tallest graminoids and shrubs, among whichonly gorse (Ulex europaeus) is a frequent exotic in thetwo catchments.T

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Only two of 26 woody species were solely found indepositional subplots, and these had very lowfrequencies. Some riparian shrubs, such as Pomaderrisapetala, Micrantheum hexandrum and L. lanigerum,were commonly found as seedlings in scours, while notbeing observed as seedlings outside this environment.This observation suggests that flooding may be impor-tant in their persistence in the riparian zone. Severescouring appears more effective than mild scouring in

promoting shrub regeneration (Table 2). In the depo-sitional subplots, the increase in shrub cover withincreasing deposition (Table 2) may reflect the effec-tiveness of shrubs in detritus capture. The ability oflarge riparian shrubs and tussocks to survive partialburial is apparent (Table 2).

If herbs and grasses could potentially be excludeddue to a lack of gaps for colonization (created throughfloods), then the biodiversity of the riparian vegetation

Fig. 2. Percentage of fine soil cover across the eight scour/deposition classes (1 = most scoured, 8 = heaviest deposit),displaying means (points), medians (lines), interquartile ranges (boxes) and 95th percentiles (whiskers).

Table 3. Percentage frequency of species in scour and deposition classes organized from the most scoured (1) to the mostburied (8)

Taxa

Scour/Deposition class

1 2 3 4 5 6 7 8

Ehrharta stipoides (g) 64.52 53.85 58.82 33.33 15.38 13.79 18.18 18.18Oxalis perennans (h) 51.61 30.77 35.29 22.22 7.69 6.90 4.55 –Dichondra repens (h) 22.58 3.85 5.88 11.11 7.69 – – –Bryophyta spp. (cr) 80.65 84.62 76.47 66.67 – 34.48 31.82 36.36Hydrocotyle hirta (h) 25.81 26.92 11.76 22.22 – – – –Schoenus spp. (gr) 22.58 23.08 11.76 22.22 – – – –Pomaderris apetala (s) 6.45 23.08 – – – 3.45 9.09 –Agrostis spp. (g) 3.23 19.23 – 11.11 – – – –Austrostipa spp. (g) 3.23 15.38 – – – 6.90 – –Acaena novae-zelandiae (h) 9.68 15.38 47.06 33.33 7.69 6.90 – 9.09Euchiton spp. (h) 9.68 3.85 17.65 – – – – –Geranium potentilloides (h) 9.68 15.38 17.65 – – 3.45 4.55 –Viola hederacea (h) 12.90 11.54 17.65 11.11 – – – –Holcus lanatus (g)† – – 17.65 33.33 – 10.34 4.55 –Leontodon taraxacoides (h)† – 7.69 17.65 22.22 – 3.45 – –Trifolium spp. (h)† – 11.54 17.65 22.22 – – – –Cardamine spp. (h) 9.68 11.54 5.88 22.22 – – – –Prunella vulgaris (h)† 3.23 – 5.88 22.22 – – – –Ehrharta acuminata (g) 3.23 – – 22.22 7.69 3.45 – –Poa labillardierei (g) 9.68 – – – 15.38 13.79 – –Acacia mucronata (s) 3.23 3.85 – – – 6.90 9.09 27.27

Species that do not occur in at least 15% of the subplots in at least one class are excluded.†Exotic species. cr, cryptogam; f, fern; g, grass; gr, graminoid; h, herb; o, orchid; s, shrub; t, tree.



would be decreased. As the successional trajectoryadvances to a stable state, a likely shift towards closedscrub would influence species interactions and alterthe environmental conditions of the riparian zone.While our results cannot be taken to prove thatincreased regulation of the Prosser and Little Swan-port Rivers would result in the loss of any species fromthe regional riparian vegetation, they do indicate thatthe relative abundances of species downstream of newdams could be expected to change, with the potentialloss of rarer riparian species.

ACKNOWLEDGEMENTS

The handling editor gave several helpful commentsthat substantially improved the manuscript. Thanksalso to those who provided assistance with data collec-tion, particularly Alasdair Macdonald, John Hannaganand Paul Wintle. This work was partly supported bythe Australian Centre of Excellence for Risk Analysis.

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The response of riparian vegetation to flood-maintained habitat heterogeneity