Dr Frank van Langevelde Dr Frank van Langevelde Dr Kyle Tomlinson Dr Kyle Tomlinson Prof Steven de Bie Prof Steven de Bie Wageningen University Wageningen University Shell Research Foundation Shell Research Foundation GEST GEST G G lobal lobal E E xperiment on xperiment on S S avanna avanna T T ree ree seedlings seedlings
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Dr Frank van Langevelde Dr Kyle Tomlinson Prof Steven de Bie Wageningen University Shell Research Foundation GEST Global Experiment on Savanna Tree seedlings.
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Dr Frank van LangeveldeDr Frank van LangeveldeDr Kyle TomlinsonDr Kyle TomlinsonProf Steven de BieProf Steven de Bie
Wageningen UniversityWageningen UniversityShell Research FoundationShell Research Foundation
GESTGEST
GGlobal lobal EExperiment on xperiment on SSavanna avanna TTree seedlingsree seedlings
Global Experiment on Savanna Tree seedlingsGlobal Experiment on Savanna Tree seedlings
ContextContext
Research questionsResearch questions
Global experimentGlobal experiment
ContextContext
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!
Theoretical issuesTheoretical issues
How can trees and grasses coexist (“the savanna question”)?
Determinants of savanna tree cover?
…?
However: focus on Africa?
Applied issuesApplied issues
Increase in woody cover (wet regions, “bush encroachment”)
Recruitment limitations of trees (dry regions)
Loss of large savanna trees
…?
““The savanna question”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)
Comments on theoriesComments 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
Savanna tree seedling research to dateSavanna tree seedling research to date(Tomlinson (Tomlinson et al.et al. in prep) 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
But if we look at savannas more broadly… 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
Are different traits required for each of these environments?
What can we predict?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?)
Research questionsResearch 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?
Global experimentGlobal 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
Location of sitesLocation of sites
Tree species choiceTree species choice
In wet and dry savannas, we identified eutrophic and dystrophic savannas (total 4 savanna types)
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
Site characteristicsSite characteristics
1. Fenced or no macroherbivores
2. Shallow gradient
3. Soils sandy-textured, well-drained
4. No trees
Preferably5. Access to tapped water
Proposed treatmentsProposed treatments
1. 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
2. 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)
3. Shade: full sun & 80% shading
4. Grass: no grass & grass• local abundant species
5. Defoliation: no defoliation & defoliation• Seedling (above 2nd internode) and grass (3 cm height) clipped twice over 6 months (wet season)
Incomplete factorial design of 16 treatment combinations
Experimental designExperimental design
L0 = full sun
W0 = natural rain
N0 = no nutrients added
G0 = no grass
D = defoliation
All 8 species grown together in ±2.5 m2 treatment plots
Experimental designExperimental design
4 seedlings of each species per plot: 2 harvested after 1st year, 2 replanted after 1st year, 2+2 harvested after 2nd year
Experimental designExperimental design
5 block reps, giving a total of 10 seedlings per species per treatment
Experimental designExperimental design
MeasurementsMeasurements
Monitored development1. stem length & stem height (2 weeks)2. basal stem diameter (1 cm above ground, 2 weeks)3. number of leaves (bimonthly) & leaf area (bimonthly)4. mark 1 leaf per seedling per month5. number of live seedlings at begin of wet season6. 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
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
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)
Variation in life-history traitsVariation 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
ProductsProducts
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)
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