Bush encroachment in African savannas David Ward

How do we go from this ?

to this ?

India Namibia

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* will increase with the loss rate

Loss

R*

R, Resource level

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

Species A

Loss

A Species B

Loss

B Species C

Loss

C R* C R* A R* B

R, Resource level

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

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 R 2

0 0

y R 1 x If a habitat is at point x, an increase in R 1 will not affect population size. However, any increase in R 2 will cause an increase in population size (& vice versa for habitat at y).

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

Species A dominant

R 2 A B

Species B dominant

R 1 When the ZNGI cross, each species will have a range of R* for the 2 resources where it will dominate

R 2 Bc 1 Bc Bc 2 R 1

•

Thus far, we have considered resource availability

•

Consumption also needs to be considered because it affects subsequent availability The consumption vector, Bc, has 2 components: c 1 = amount of resource 1 consumed per unit biomass per unit time and c 2 (~ for R 2 )

Bc 1 Bc The consumption vectors are determined in large part by the plasticity of plant growth Bc 2 R 1 (nutrient) e.g. If R 1 = a nutrient and R 2 = light, the plant must allocate resources to above ground growth (towards the light) and to below-ground growth (towards the nutrients)

A A wins A + B Stably coexist R 2 B B wins c B c A R 1 When there are perfectly essential resources, the optimal strategy for a plant is to grow so that the 2 resources are consumed in a way that they equally limit growth

In South African savannas Trees win Stably coexist +H 2 O +N Grasses win Acacia Grasses Soil Nitrogen

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

+H 2 O +N Grasses Acacia Soil Nitrogen When ZNGIs do not cross, Acacias always outcompete grasses Either of these scenarios is possible Soil Nitrogen

Global climate change models predict that C 3 trees will grow faster following climate change than C 4 grasses 30 C 3 (trees) C 4 (grass) 20 10 Now Predicted 200 600 CO 2 (ppm) 1000

Increased atmospheric CO 2 mean that: levels will

•

Net photosynthetic rates of C 3 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* Growth – after climate change R* predicted R* now

R, Resource level

Growth now

Loss

now Loss – after climate change

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 Soil Nitrogen

Do we have any empirical support for this model ?

Trees win Stably coexist +H 2 O +N Grasses win Acacia Grasses Soil Nitrogen

Pot Experiment

• Treatments: rain, nutrients, grazing • Completely crossed design

Rainfall frequency overwhelmingly more important than other factors

80

R = rain D = dry

60 40

N = nitrogen O = no nitrogen

20

G = grazing _ = no grazing

0

RN_ RO_ RNG ROG DN_ DO_ DNG DOG

Field experiment - randomized block design Treatments: rain, fire, nutrients, grazing

5 4 1 0 3 2 Rain Added Control Nitrogen addition decreased Acacia germination & survival Rainfall addition increased Acacia germination & survival 2 1 0 7 6 5 4 3 Nitrogen Added Control

2 1 0 -1 Jack Kambatuku, a PhD student of mine, has shown that Δ 15 N is related to competition with grass 6 5 F (2, 165) = 93.9, p < 0.001

4 3 Grass Competition No Grass

8 6 4 2 0 Jack has shown that dry matter production is affected by competition with grass = Total D.M. Production = AboveGround D.M. Prod.

= BelowGround D.M. Prod.

No Grass Grass

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

1 0 -1 3 Interaction effect (Rain*Seeds): F=7.961, p=0.006

Vertical bars denote 0.95 confidence intervals 2 130 year Max. Rainfall Natural Rainfall Added Control Seeds

Experimental results thus far

• • • • • •

Grazing and fire not important Rainfall far more important than other factors Rainfall frequency more important rainfall amount than 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 Without grazing Open Savanna

Grass

Trees Annual Rainfall

In areas prone to bush encroachment, farmers should limit stock in WET years With heavy grazing Grass Trees Annual Rainfall

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 instability implies 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

Honeycomb rippling model A B C D E F 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

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

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Distances between trees increase with age

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Trees become more evenly spaced as they age

Distances between trees increase as they age Variability in distances between trees decreases as they age

We showed experimentally that there is significant competition between trees

30 25 20 % Size Increase 15 10 5 0 Neighbours removed Control

Summary of patch dynamic model results We have shown that:

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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

Another South African example of cyclical succession – Karen Esler

One of our students, Jana F örster, has shown that there may be strong competition between two encroaching species, Acacia mellifera and

Tarchonanthus camphoratus

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

A. mellifera T. camphoratus

a Cut plots Canopy diameter, cm b

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