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Freie Universität Berlin (2017)

The role of diversity in savannas : modelling plant functional diversity and its effects on ecosystem functioning

Guo,Tong

Titre : The role of diversity in savannas : modelling plant functional diversity and its effects on ecosystem functioning

Die Rolle von Diversität in Savannen : Modellierung von funktioneller Pflanzendiversität und ihren Effekten auf Ökoszstemfunktionen

Auteur : Guo,Tong

Université de soutenance : Freie Universität Berlin

Grade : Doctor of Philosophy (Ph.D.) in Plant Sciences 2017

Résumé partiel
Savanna ecosystems cover approximately 20% of the global land surface and account for 30% of the terrestrial net primary production. Thus, they form integral parts of global cycles such as those for water and carbon. The savanna biome comprises a heterogeneous suite of local soil conditions, as well as a diverse vegetation composition, both of which are key determinants of savanna ecosystem functioning. Its vegetation is characterized by a mixture of woody and herbaceous plants. The relative share of each functional type in the total vegetation cover is thereby determined by a set of environmental and land use factors such as precipitation or grazing. Due to global change, savanna ecosystems are faced with increasing levels of drought and heavy degradation, both of which can alter vegetation composition and biodiversity and ultimately ecosystem functioning. The so-called shrub encroachment represents a particular form of degradation which is common to many savanna ecosystems and manifests in a gradual replacement of the grass components by shrubs and woody plants in general. To which degree the drivers of change such as precipitation regime and grazing intensity affect a given savanna ecosystem, however, depends on local soil conditions as well. Hence, a holistic understanding of savanna ecosystems is required to qualify and quantify the causes and consequences of biodiversity and environmental variations. This will ultimately help to assess variations of ecosystem functioning in semi-arid savannas. In my thesis, I aim at enhancing this understanding by using a spatially explicit savanna simulation model. I extended an ecohydrological savanna model to simulate vegetation diversity and its response to different precipitation and grazing scenarios, as well as its effect on ecosystem functioning. The model uses the plant functional type (PFT) concept instead of discrete plant species to represent plant assemblies. Each PFT is characterized by its unique combination of functional traits. Thus, vegetation diversity is explicitly modelled as functional diversity of the broad herbaceous and woody plant types. Hence, the aim of this work is to find relationships between functional diversity and ecosystem functioning in semi-arid savannas. Environmental conditions are simulated by different scenarios of mean annual precipitation, grazing intensity, and soil properties. I assess the effects of simulated environmental conditions on the functional composition of savanna vegetation and ecosystem functioning, as well as their interrelations. Based on the simulation results, I identify the response of savanna vegetation to environmental drivers at the community and the trait level, as well as the hierarchical levels of the environmental effects in semi-arid savannas. In the general introduction to this thesis, environmental conditions and vegetation functional diversity in savanna ecosystems are introduced. With a focus on simulating vegetation diversity, I address the role of plant functional traits in describing the vegetation response to different environmental conditions. In particular, I introduce concepts by which the functional composition and diversity of the plant community can be used as a means for assessing environmental effects on ecosystem functioning. The simulations in chapter 1 reveal that trait diversity within the broad PFT level strongly affects vegetation composition and ecosystem functioning. As a starting point, I analyzed the community level effects of trait variability within the broad PFT perennial grass on ecosystem functioning. The results show that the functional composition of perennial grasses is strongly affected by grazing intensity while mean annual precipitation moderates those effects. Increasing the functional diversity of perennial grasses generally increases total vegetation cover and water use efficiency of the plant community. This result underlines the positive effect of functional diversity on ecosystem functioning, which is in line with observations of empirical studies. Increasing the perennial grass diversity proves to have a negative effect on shrub cover, nevertheless, the patterns of land degradation associated with encroachment of woody plants still remain observable.

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Page publiée le 4 novembre 2017, mise à jour le 28 décembre 2018