Bush encroachment in African savannas

1. Bush encroachment in African savannas David Ward

2. How do we go from this ? to this ?

3. Namibia India

4. 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

5. 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.

6. 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

7. Magersfontein battlefield in 1899 and 2001 – it is now bush encroached in spite of an absence of heavy grazing

8. Distribution of A. mellifera Pniel study site (nr. Kimberley)

9. Acacia mellifera

10. Resource allocation models of plant community structure David Tilman Univ. of Minnesota

11. 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

12. 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

13. R* will increase with the loss rate Growth Loss Growth or Loss rate, dB/Bdt R* R, Resource level A population can only be maintained in a habitat if its growth rate > loss rate

14. Species A Growth LossA Species B LossB Growth or Loss rate, dB/Bdt Species C LossC 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*

15. 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

16. Perfectly essential resources 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). y R2 x 0 R1 0 Population size decreases for resource levels in the white region and increases in the green region

17. Species A dominant A R2 B Species B dominant R1 When the ZNGI cross, each species will have a range of R* for the 2 resources where it will dominate

18. Thus far, we have • considered resource • availability • Consumption also • needs to be considered • because it affects • subsequent availability R2 Bc1 Bc Bc2 R1 The consumption vector, Bc, has 2 components: c1 = amount of resource 1 consumed per unit biomass per unit time and c2 (~ for R2)

19. The consumption vectors are determined in large part by the plasticity of plant growth R2 (light) Bc1 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) Bc Bc2 R1 (nutrient)

20. A wins A + B Stably coexist A R2 B B wins cB cA R1 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

21. In South African savannas Stably coexist Trees win Grasses +H2O Acacia Soil Water +N Grasses win Acacia Grasses Soil Nitrogen

22. 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

23. Acacia Soil Water Grasses Soil Nitrogen Either of these scenarios is possible Grasses Acacia +H2O Soil Water +N Acacia Grasses Soil Nitrogen When ZNGIs do not cross, Acacias always outcompete grasses

24. Global climate change models predict that C3 trees will grow faster following climate change than C4 grasses C3 (trees) 30 C4 (grass) 20 Photosynthesis (mol.m-2.s-1) 10 Now Predicted 200 600 1000 CO2 (ppm)

25. 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…….

26. 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

27. Increased growth and decreased loss for Acacias results in a lower R* Growth – after climate change Growth Growthnow Growth or Loss rate, dB/Bdt Lossnow Loss– after climate change R*predicted R*now R, Resource level

28. Acacia Soil Water Grasses Soil Nitrogen 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

29. Stably coexist Trees win Grasses +H2O Acacia Soil Water +N Grasses win Acacia Grasses Soil Nitrogen Do we have any empirical support for this model ?

30. Pot Experiment • Treatments: rain, nutrients, grazing • Completely crossed design

31. Rainfall frequency overwhelmingly more important than other factors R = rain D = dry N = nitrogen O = no nitrogen G = grazing _ = no grazing

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

33. Rainfall addition increased Acacia germination & survival Nitrogen addition decreased Acacia germination & survival

34. Jack Kambatuku, a PhD student of mine, has shown that Δ15N is related to competition with grass 6 5 F(2, 165) = 93.9, p < 0.001 4 3 15N Natural Abundance 2 1 0 -1 Grass No Grass Competition

35. Jack has shown that dry matter production is affected by competition with grass = Total D.M. Production 8 = AboveGround D.M. Prod. = BelowGround D.M. Prod. 6 4 Dry Matter Production (g) 2 0 No Grass Grass

36. Free-growing trees Trees with grasses 3.4 F = 4.529, p = 0.0003 3.0 N content (%N/g) 2.6 2.2 1 2 4 0 0.05 Treatment (mM N) Jack has also shown that free-growing trees have higher nitrogen content than trees growing with grasses

37. Seedling survival per Plot

38. 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

39. Trees The relationship between grass/tree biomass and rainfall Without grazing Open Savanna Grass Biomass Annual Rainfall

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

41. We are also using Spatially-explicitPatch Dynamic Models of Savanna Dynamics

42. 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

43. A B C Honeycomb rippling model 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

44. 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

45. Distances between trees increase as they age Nearest Neighbour Distance Variability in distances between trees decreases as they age c.v. Nearest Neighbour Distance

46. We showed experimentally that there is significant competition between trees

47. Summary of patch dynamic model results • 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

48. Another South African example of cyclical succession – Karen Esler

49. One of our students, Jana Förster, has shown that there may be strong competition between two encroaching species, Acacia mellifera and Tarchonanthus camphoratus

50. With A. mellifera removed, T. camphoratus gets larger and has recruitment A. mellifera T. camphoratus a Uncut plots Relative frequency 2 44 86 120 168 210 260 292 > > b Cut plots Relative frequency 2 44 86 120 168 210 260 292 > > Canopy diameter, cm