GEST
 

Methods

In the GEST Project, we are testing the effect of resource availability, competition from grasses, and defoliation, on the growth of seedlings of tree species that are dominant in dry and moist environments in tropical to sub-tropical savanna types on all the vegetated continents of the world. We hope to distinguish whether the growth traits and growth responses of the seedlings indicate different selection pressures in different savanna types (dry versus wet), whether these growth response differences reflect plant family lineages (phylogeny), and whether there are differences between species across the continents, indicating responses both to biogeography and different historical selection pressures (e.g. defoliation by macroherbivores versus the lack thereof; effects of phylogenetic species pools). The results will also provide quantitative information on tree seedling growth rates in different savanna systems, and modes of grassland invasion employed by different tree species, and hence indicate the likely spatial dynamics of trees and grasses in different savannas.

In order to answer the research questions we have adopted a two-pronged approach, in that GEST consists of two types of experiments, namely a comparative greenhouse experiment and in-field growth experiments.

1. Greenhouse experiment

A controlled-environment greenhouse experiment, designed to compare traits of species’ seedlings from all continental savannas directly, will provide a data set to elucidate whether there is evidence for co-evolution for similar environmental conditions, whether there is evidence for differences between the continental savannas, and will allow us to evaluate the importance of phylogenetic effects. The greenhouse experiment is being conducted in the climate control greenhouses at Wageningen University. In so doing, we seek not only to provide answers to our own questions, but also seek to provide data that can be added to the accumulating data base of life-history traits possessed by different plant species.

2. Field experiments

A collection of on-site field experiments are being established on each continent in a humid and a semi-arid savanna type  (Figure 1). These have been selected using the  Köppen-Geiger Climate Classification. The field experiments are designed to quantify seedling growth rates of within-continent species at both field sites in response to resource supply, grass competition and seedling defoliation, providing information on the probable modes and rates of grassland invasion employed by different species, the limitations imposed by local conditions on species from the two sites (hence explaining their differences in location), and the importance of grass competition for restricting the growth rate of tree seedlings. At each site we will observe the growth of seedlings of at least eight (8) continental savanna tree species in response to treatments composed of resource levels, grass competitors, and defoliation.