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Wageningen University (2013)

Surviving and growing amidst others : the effect of environmental factors on germination and establishment of savanna trees

Moribe Barbosa, E.R.

Titre : Surviving and growing amidst others : the effect of environmental factors on germination and establishment of savanna trees

Auteur : Moribe Barbosa, E.R.

Université de soutenance : Wageningen University

Grade : Doctor of Philosophy (PhD) 2013

Résumé partiel
Savanna ecosystems are characterized by a continuous grass layer intermixed with a discontinuous layer of trees and shrubs. A complex set of environmental drivers, such as water, soil nutrients, solar radiance, fire and herbivory, determines vegetation structure and composition in savannas.Such environmental drivers are expected to be strongly affected by future global climatic and land-use changes, potentially modifying savanna vegetation, and consequently savanna fauna. The ability to predict changes in plant community composition is therefore importantfor management and conservation of savannas. However, the mechanisms controlling plant establishment and growth in savannas are still unclear. Germination and seedling establishment are critical recruitment stages in the life cycle of plants and can influence plant community composition. A better understand of the factors influencing plant species recruitment and their ecology is needed. This thesis focuses on seedling recruitment of several savanna tree species.

Water stress is probably the single greatest constraint to tree seedling survival in savanna systems : tree seedling recruitment and survival are hypothesized to be limited by soil moisture availability. Shade by established adult trees may facilitate tree seedling recruitment by maintaining high soil moisture availability. Chapter 2 deals with germination and early seedling establishment of several tree species. I expected that tree species would germinate and establish best under high moisture conditions (high water and shade), while under stress conditions (i.e. low soil moisture due to low water supply and full sun, and in the presence of grasses) plants would suffer. The observed variability of seedling performance among the tree species under stress conditions may be explained by differences in functional traits. Higher soil moisture mostly benefited germination of species with seeds with high calcium concentration and low water content. On the other hand, low soil moisture conditions benefited germination of tree species with seeds with higher magnesium and phosphorus concentration and water content. Furthermore, under low soil moisture availability, grass presence facilitated germination of most tree species but its effect on early survival (positive or negative) differed among species. The findings of this chapter confirm a large difference in the tree species responses to environmental variation during early recruitment, which potentially affect theplant community composition and dynamics under different environmental conditions in savannas.Seed trait differences among the species partly contribute to explain such variability. Therefore, considering inter-specific variation among tree species and information on seed traits can improve the ability to predict and manage the impacts of environment changes.

Mots Clés : trees - savannas - germination - environmental factors - establishment - plant competition - seedlings - plant development - plant ecology


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