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

Assemblage structure (rainfall, soil type) Plant characteristics (selection pressure, phylogeny)

There are major biogeographic patterns in tree distribution between savannas around the world

AfricaAfrica

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

AustraliaAustralia

Local dominance by single genus or family

Eucalyptus (Myrtaceae) Mimosoideae Proteaceae

Soil type and rainfall generally define dominant genera abundances

South AmericaSouth America In Cerrado, co-dominance by species of numerous

families some of which have low representation in other continental savannas

Asia 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

North AmericaNorth America

Warm temperate to tropical savannas

Small tree family pool, but some dominant families Fagaceae, Pinaceae, Mimosoideae

What do we observe?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?

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)

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

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

Greenhouse experimentGreenhouse experiment

Common-garden & transplant experimentsCommon-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)

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

to learn about statistics (EU-funding?)