Bush encroachment in African savannas

David Ward

How do we go from this ?

to this ?

Namibia

India

Bush encroachment affects between 12- 20 million hectares of

South Africa

This is a biodiversity problem that is also

an agricultural problem

A multi-species grass sward is transformed into an impenetrable and

unpalatable thicket dominated by a single species of thorn tree

Heavy Grazing is often considered to be the cause

of bush encroachment

• Walter’s (1939) two-layer model – – grasses outcompete trees in open savannas by

growing fast and intercepting moisture from the upper soil layers,

– trees are thereby prevented from gaining access to moisture in the lower soil layers where their roots are mostly found.

– when heavy grazing occurs, grasses are removed and soil moisture then becomes available to the trees, allowing them to recruit en masse.

Post hoc ergo propter hoc

• The fact that many bush-encroached areas are heavily grazed means neither that grazing causes encroachment nor that Walter’s model is correct

• Bush encroachment is widespread in areas where there is a single soil layer and where grazing is infrequent and light

Magersfontein battlefield in 1899 and 2001 – it is now bush encroached in spite of an

absence of heavy grazing

Distribution of A. mellifera

Pniel study site (nr. Kimberley)

Acacia mellifera

Resource allocation models of plant community structure

David Tilman

Univ. of Minnesota

In order to predict the outcome of competition for a single limiting

resource, it is necessary to know:

• The resource level (=R*) at which the net rate of population change for a species is zero

• This occurs when vegetative growth and reproduction balance the loss rate the species experiences in a given habitat

R* and loss or disturbance rates

• The loss rate of a population is caused by numerous components, including disturbance, seed predation, fire and herbivory

• Independent of the causes of losses, the number of species competing, or competitive abilities of species in a habitat, average (equilibrial) resource levels (R*) will increase with the loss rate

R, Resource level

R*

Growth

Loss

Gro

wth

or

Lo

ss r

ate,

dB

/Bd

t

A population can only be maintained in a habitat if its growth rate > loss rate

R* will increase with the loss rate

Species C will exclude the other 2 species in competition because it has the lowest R*

R, Resource levelR*B

Growth

LossA

Gro

wth

or

Lo

ss r

ate,

dB

/Bd

tSpecies A

Species B

Species C

LossB

LossC

R*AR*C

Resource-dependent Growth Isoclines

• When a species consumes 2 or more resources, it is necessary to know the total effects of the resources on the growth rate of the species

• These effects can be summarized by the zero net growth isocline (ZNGI)

• This isocline shows the levels of 2 or more resources at which the growth rate per unit biomass of a species balances its loss rate

Perfectly essential resources

Population size decreases for resource levels in the white region and increases in

the green region

If a habitat is at point x, an increase in R1 will not affect population size. However, any increase in R2 will cause an increase in population size (& vice versa for habitat at y).

R2

R1

x

y

00

R2

When the ZNGI cross, each species will have a range of R* for the 2 resources where it will dominate

Species A dominant

Species B dominant

R1

A

B

Bc1

R2

R1

Bc2Bc

The consumption vector, Bc, has 2 components: c1 = amount of resource 1 consumed per unit biomass per unit time and c2 (~ for R2)

•Thus far, we have considered resource availability•Consumption also needs to be considered because it affects subsequent availability

The consumption vectors are

determined in large part by the plasticity

of plant growth

e.g. If R1 = a nutrient and R2 = light, the plant must allocate resources to above-ground growth (towards the light) and to below-ground growth (towards the nutrients)

Bc1R2 (l

igh

t)

R1 (nutrient)

Bc2Bc

When there are perfectly essential resources, the optimal strategy for a plant is to growso that the 2 resources are consumed in a way that they equally limit growth

R2

R1

cA

cB

A wins

B wins

A + BStably coexist

B

A

So

il W

ater

Soil Nitrogen

+N

+H2O

In South African savannas

Treeswin

Grasses win

Stablycoexist

Gra

sses

Grasses

Aca

cia

Acacia

How do grazing or fire affect the isoclines ?

• Grazing/Fire increase the loss rate for grasses

• Thus, R* for grasses is raised relative to that of the Acacia trees

So

il W

ater

Soil Nitrogen

Grasses

+N+H2O

Either of these scenarios is possible

Gra

sses

Aca

cia

Acacia

When ZNGIs do not cross, Acacias always outcompete grasses

So

il W

ater

Soil Nitrogen

GrassesAca

cia

Global climate change models predict that C3 trees will grow faster following climate

change than C4 grasses

C3 (trees)

C4 (grass)30

20

10

CO2 (ppm)200 600 1000

Ph

ot o

syn

thes

i s (m

ol. m

-2.s

-1)

Now Predicted

Increased atmospheric CO2 levels will mean that:

•Net photosynthetic rates of C3 trees will increase more than those of grasses

•Consequently, growth rates of trees will increase, and…….

Because more carbon will be available:

• Acacia trees will be able to invest more in carbon-based defences, such as condensed tannins (see e.g. Lawler et al. 1997, Kanowski 2001, Mattson et al. 2004)

•Consequently, loss rates of Acacias are likely to decline

Increased growth and decreased loss for Acacias results in a lower R*

R, Resource levelR*now

Growth

Gro

wth

or

Lo

ss r

ate,

dB

/Bd

t

Growth – after climate change

Growthnow

Lossnow

Loss – after climate changeR*predicted

This resource allocation model predicts that this will lead to bush encroachment because the ZNGI of Acacias will be lower (closer to the origin) than that of grasses on both axes

So

il W

ater

Soil Nitrogen

Aca

cia

Grasses

Do we have any empirical support for this model ?

So

il W

ater

Soil Nitrogen

+N

+H2O

Treeswin

Grasses win

Stablycoexist

Gra

sses

Grasses

Aca

cia

Acacia

• Treatments: rain, nutrients, grazing

• Completely crossed design

Pot ExperimentPot Experiment

Rainfall frequency overwhelmingly more important

than other factors

0

20

40

60

80

RN_ RO_ RNG ROG DN_ DO_ DNG DOG

Me

an

# s

urv

ivin

g p

lan

ts (

+S

E)

R = rain

D = dry

N = nitrogen

O = no nitrogen

G = grazing

_ = no grazing

Field experiment - randomized block designTreatments: rain, fire, nutrients, grazing

Rainfall addition increased Acacia

germination & survival

01234567

NitrogenAdded

Control

No.

Tre

e S

eedl

ings

0

1

2

3

4

5

Rain Added Control

No

. Tre

e S

eed

lings

Nitrogen addition decreased Acacia germination & survival

Grass No Grass

15N

Nat

ura

l Ab

un

dan

ce

-1

0

1

2

3

4

5

6

F(2, 165) = 93.9, p < 0.001

Competition

Jack Kambatuku, a PhD student of mine, has shown that Δ15N is related to competition with grass

Jack has shown that dry matter production is affected by competition

with grass

No Grass Grass

Dry

Mat

ter

Pro

du

ctio

n (

g)

0

2

4

6

8 = Total D.M. Production= AboveGround D.M. Prod.= BelowGround D.M. Prod.

Jack has also shown that free-growing trees have higher nitrogen content than trees growing with grasses

Interaction effect (Rain*Seeds): F=7.961, p=0.006

Vertical bars denote 0.95 confidence intervals

Added ControlSeeds

-1

0

1

2

3 130 year Max. Rainfall Natural Rainfall

See

dlin

g s

urv

ival

per

Plo

t

Experimental results thus far

• Grazing and fire not important• Rainfall far more important than other

factors• Rainfall frequency more important than

rainfall amount • Nutrients = second-most important factor• More nutrients = competitive advantage to

grasses = tree suppression• Thus, the resource allocation model

seems appropriate

The relationship between grass/tree biomass and rainfall

Annual Rainfall

Bio

mas

s

Trees

Grass

Without grazingOpen Savanna

In areas prone to bush encroachment, farmers should limit stock in WET years

Annual Rainfall

Bio

mas

s With heavy grazing

GrassTrees

We are also using Spatially-explicit Patch Dynamic

Models of Savanna Dynamics

Experiments show that mature trees are competitively superior to grasses while

grasses tend to outcompete immature trees

• This asymmetry in competitive effects implies instability

• However, weakening the suppressive effect of the grass layer on young trees in a patch of a few hectares can lead to an open savanna patch being converted to a tree-dominated thicket (bush encroachment)

• Once established, the thicket may take decades to revert to an open savanna

Figures show a time series of hexagonal subsets of a larger patch. Each (small) hexagonal represents a bush, the relative sizes of the hexagonals represent

relative bush sizes

A B C

D E F

Honeycomb rippling model

The predictions of the honeycomb rippling model are consistent with field data that

show that:

• Distances between trees increase with age

• Trees become more evenly spaced as they age

Distances betweentrees increase as they age

Variabilityin distances betweentrees decreases as they age

c.v.

Nea

rest

Nei

gh

bo

ur

Dis

tan

ce N

eare

st

Nei

gh

bo

ur

Dis

tan

ce

We showed experimentally that there is significant competition

between trees

0

5

10

15

20

25

30

% Size Increase

Neighboursremoved

Control

We have shown that:

•Any process that weakens the suppressive effect of grasses on young trees can convert an open savanna patch into a tree-dominated thicket (= bush encroachment)

•Thicket may eventually revert to an open savanna as a result of intra-specific competition between trees (= cyclical succession)

Viewed this way, bush encroachment may be a natural stage in savanna dynamics

Summary of patch dynamic model results

Another South African example of cyclical succession – Karen Esler

One of our students, Jana Förster, has shown that there may be strong

competition between two encroaching species, Acacia mellifera and Tarchonanthus camphoratus

Uncut plots

Cut plots

a

b

A. mellifera

T. camphoratus

With A. mellifera removed, T. camphoratus gets larger and has recruitment

>

>

Rel

ativ

e fr

equ

ency

Rel

ativ

e fr

equ

ency

Canopy diameter, cm

2 44 86 120 168 210 260 292 >

2 44 86 120 168 210 260 292 >

Overall Conclusions• Heavy grazing is only one of several sources of

loss to plants that affect R* and consequently competitive ability of trees against grasses

• Rainfall frequency and nutrient availability are important in initiating encroachment

• Resource allocation models are useful for predicting changes in savanna dynamics

• Patch dynamic models can explain bush encroachment as a natural stage in savanna dynamics