GEST G lobal E xperiment on S avanna T ree seedlings

1. GESTGlobal Experiment on Savanna Tree seedlings Dr Frank van Langevelde Dr Kyle Tomlinson Prof Steven de Bie Wageningen University Shell Research Foundation

2. Global Experiment on Savanna Tree seedlings • Context • Research questions • Global experiment

3. Context • Savanna is vegetation where trees and grasses coexist • we focus on savannas where seasonality is controlled by rainfall rather than temperature • However: large variation in vegetation structure and composition around the world • Environmental characteristics: • water (resource: limited in dry season) • nutrients (resource: limited in wet season) • fire (disturbance) • herbivory (disturbance) • Heavy human interventions!

4. Theoretical issues • How can trees and grasses coexist (“the savanna question”)? • Determinants of savanna tree cover? • …? However: focus on Africa?

5. Applied issues • Increase in woody cover (wet regions, “bush encroachment”) • Recruitment limitations of trees (dry regions) • Loss of large savanna trees • …?

6. “The savanna question” • Existing theories (Sankaran et al. 2004) • Equilibrium: stable co-existence independent of rainfall variability and disturbance (Walter’s 2-layer hypothesis, Walker et al. 1982) • Non-equilibrium: co-existence depends on recruitment bottlenecks mediated by disturbance; independent of competitive interactions (Higgins et al. 2000) • Disequilibrium: unstable co-existence depends on disturbance; without disturbance → grassland or woodland (Jeltsch et al. 2000)

7. Comments on theories • Problem: no unified theory of how trees and grasses coexist in savannas • 2 types of arguments for coexistence: • Competitive interactions (equilibrium, disequilibrium) • Demographic bottlenecks on tree recruitment (climate, disturbances) (non-equilibrium, disequilibrium) • Disequilibrium and non-equilibrium: tree seedling recruitment is critical to long-term dynamics & disagree over relevance of tree-grass competition • Research on tree seedling growth could help to resolve this debate

8. Savanna tree seedling research to date(Tomlinson et al. in prep) • Research on tree seedling growth and survival in savannas is fragmentary and continentally-biased • Preponderance of demographic studies of recruitment • effects of fire, herbivory, drought • Scarcity of formal experiments to elucidate relative importance of competitive suppression versus resource limitation for seedling growth • mostly North American • single species considered

9. But if we look at savannas more broadly… • Not all responses of trees are adaptations to current environment, they may also be explained by evolutionary or biogeographic constraints • Major physiognomic differences within savannas around the world • Assemblage structure (rainfall, soil type) • Plant characteristics (selection pressure, phylogeny) • There are major biogeographic patterns in tree distribution between savannas around the world

10. Africa • Numerous genera present, but local dominance by single genus or family • Mimosoideae (eutrophic) • Combretaceae (dystrophic) • Caesalpinioideae (dystrophic) • Soil type and rainfall generally define dominant species abundance • High tree species diversity

11. Australia • Local dominance by single genus or family • Eucalyptus (Myrtaceae) • Mimosoideae • Proteaceae • Soil type and rainfall generally define dominant genera abundances

12. South America • In Cerrado, co-dominance by species of numerous families some of which have low representation in other continental savannas

13. Asia • More extreme soil fertility (both poor and rich) • Phylogenetic commonality with Africa, but also Asian genera and codominance by genera of multiple families? • The most important: • Combretaceae • Mimosoideae • Dipterocarpaceae

14. North America • Warm temperate to tropical savannas • Small tree family pool, but some dominant families • Fagaceae, Pinaceae, Mimosoideae

15. What do we observe? • Tree characteristics of savannas are varied, both within continents and across continents … Why? • There are differences in environmental limitations constraining savanna tree seedling recruitment • rainfall, nutrients, temperature, light, fire, herbivory • Are different traits required for each of these environments?

16. What can we predict? • Current environmental factors determine tree seedling growth and survival in the presence and absence of grass in savannas around the world, irrespective of phylogeny. • Environmental adaptation • Abiotic: water & variability in water, nutrients, light • Biotic: competition with grasses (local species), defoliation (local herbivores), N-fixation, VAM • Role of fire? • Phylogenetic origin • Evolutionary selection from local species pool • Invasion from external source? (Mimosoideae?)

17. Research questions • Do tree seedlings in savannas around the world differ in their • response to resources (water, nutrients, light)? • ability to compete with grasses for different resources? • ability to tolerate defoliation? • Do the species’ seedlings show convergent traits for environmental conditions, or are the differences related to continent of origin, or to phylogeny?

18. Global experiment • Experiment comparing seedling growth of dominant (“matrix”) tree species of tropical and warm temperate savannas around the world • Runs for 2 years • Experiment will be conducted at two sites on each continent • high rainfall: MAP > 800 mm • low rainfall: MAP < 600 mm

19. Location of sites

20. Tree species choice • In wet and dry savannas, we identified eutrophic and dystrophic savannas (total 4 savanna types) • dry+eutrophic, dry+dystrophic, wet+eutrophic, wet+dystrophic • We selected 3 tree species from each type • In the field experiment, we shall grow at least 2 species from each type (total 8 species) = transplant experiment • 12 species from each continent grown in a comparative pot trial in the Netherlands (total 60 species) = common-garden experiment

21. Site characteristics • Fenced or no macroherbivores • Shallow gradient • Soils sandy-textured, well-drained • No trees Preferably • Access to tapped water

22. Proposed treatments • Water: even rainfall & natural rainfall • dry site: 400 mm/year, per week over 6 months • wet site: 1000 mm/year, per week over 6 months • Nutrients: no fertilizer added & fertilizer added • NPK start + every month for first 3 months (4 applications), so that N = 4 g N m-2 per application (Kraaij & Ward 2006) • Shade: full sun & 80% shading • Grass: no grass & grass • local abundant species • Defoliation: no defoliation & defoliation • Seedling (above 2nd internode) and grass (3 cm height) clipped twice over 6 months (wet season)

23. Experimental design • Incomplete factorial design of 16 treatment combinations L0 = full sun W0 = natural rain N0 = no nutrients added G0 = no grass D = defoliation

24. Experimental design • All 8 species grown together in ±2.5 m2 treatment plots

25. Experimental design • 4 seedlings of each species per plot: 2 harvested after 1st year, 2 replanted after 1st year, 2+2 harvested after 2nd year

26. Experimental design • 5 block reps, giving a total of 10 seedlings per species per treatment

27. Measurements • Monitored development • stem length & stem height (2 weeks) • basal stem diameter (1 cm above ground, 2 weeks) • number of leaves (bimonthly) & leaf area (bimonthly) • mark 1 leaf per seedling per month • number of live seedlings at begin of wet season • Grass biomass (disc meter, monthly) • Harvested measurements (even: 1 week after last water supply) • aboveground dry mass of each seedling (separate leaves and stem) • aboveground dry mass of grasses • seedling and grass leaves N and P and K and PSC • Harvested measurements (natural: same moment as even) • aboveground dry mass of stem of each seedling • aboveground dry mass of grasses

28. Greenhouse experiment • Rigorous comparison of tree species to test effect environmental adaptation and phylogenetic constraints • Why? • Common-garden experiment & transplant experiment • Ecological significance of variation in life-history traits

29. Common-garden & transplant experiments • Transplant experiment: estimate local performance by measuring individuals that have been moved between environments • there is an effect of environmental variation • Common-garden experiment: estimate performance by measuring species drawn from local environments within common-garden (greenhouse) experiment • minimizes contribution of environmental variation • Careful design to make quantitative statements about the importance of environmental variation (Nuismer & Gandon 2008)

30. Variation in life-history traits • Cause for variation: coping with disturbance and adapting to fast versus slow growth conditions • Life-history invariants: traits that occur in same combination • for example: leaf mass per area – leaf life span • Greenhouse experiment allows to measure this variation

31. Products Several papers • Synthesis paper: “Evolution and ecology of savannas around the world: differences and similarities” (co-authored by all participants) • Worldwide comparison of seedling growth under different environmental conditions: results of field experiment (co-authored by all participants) • Traits adapted to environment or constrained due to phylogeny: results of greenhouse experiment (co-authored by all participants) • Experiments per continents (co-authored by local participants and WUR team) • Possibly: a subsequent synthesis paper on strategies used by trees in different savannas (co-authored by all participants)

32. Miscellaneous • GEST website • All presentations on the site (accessible for GEST members) • All data on the site • Suggestions or material (papers, pictures)? • GEST is opportunities for training • For example, Wageningen MSc students could join experiments • (Regular) GEST meeting for participating students? For example to learn about statistics (EU-funding?)