Responses of Herbaceous Savanna Vegetation to Land Use and Habitat Conditions: Investigations along a Climatic Gradient in West Africa
Zusammenfassung der Projektergebnisse
Savanna ecosystems cover one eighth of the earth's land. They are characterised by the co-dominance of herbaceous vegetation and less-abundant woody species. In West Africa, savanna ecosystems are thought to undergo dramatic alterations in species composition in response to changes in climate and land use. However, little is known about how these factors interact to affect vegetation. Previous studies have focused primarily on woody species; studies of herbaceous species are lacking, despite the fact that these species comprise 75–90% of the biomass of the savanna. The main objective of this project was to analyse the responses of herbaceous savanna vegetation to land use, climate, habitat and their interactions. As we assume that vegetation responds differently to land use depending on climate and habitat conditions, we studied four habitats types, two land use types (unused vs. used sites) within seven research sites that represent a climatic gradient ranging from the Sahelian to the Sahelian zones. We investigated the influence of climate, habitat and land use on (i) species composition and diversity, (ii) the competitive performance of single species within one ecosystem, (iii) and single species populations of different life history types (grass/herb, annual/perennial). Concerning the first question, our results clearly showed a dependency of vegetation composition and richness on climatic, land use, habitat type and related soil parameters. We were able to stress the importance of interactions between these factors. Using a functional trait approach, we were able to identify some general pattern of vegetation adaptation to differences in the abovementioned environmental factors. It is important to distinguish between functional groups (like grasses vs. trees vs. herbs vs. Fabaceae; annuals vs. perennials), as these sometimes react differently on the same environmental factor. Answering the second question, we expected that the chlorophyll fluorescence parameters Fv/Fm and the performance index PI are closely related to photosynthetic activity and, thus, may be used as proxies for the potential to quickly build up new plant tissues, e.g. the competitive performance. However, we observed that Fv/Fm does not seem to be closely related to gas-exchange measurements, thus, the question came up, if chlorophyll fluorescence parameters may be useful to establish such relationship. This question was analyzed more in depth, and we observed that in ecological field studies Fv/Fm showed only a weak relationship to maximum carboxylation rates, but PI is suitable to characterize the photosynthetic activity. More in detail analyses will follow, but first applications to the data from Burkina Faso also stress the potential of PI to characterize competitive performances. In regard to the third question concerning single species populations of different life history types the results underline that climate is the most determining factor for the population structure. Climate showed also the highest impact on within-trait variability for the annual herb and grass species indicating a better performance in the dryer study areas. Land use affected particularly the annual species indicating more favourable growing conditions in the parks compared to fallow sites. Contrary, habitat had almost no effect on the within-trait variability of all species. Overall, based on these results we presume that the investigated species would be able to adapt to predicted climatic changes but might react sensitive to higher land use pressure. Concluding, we were able to show a dependency of vegetation composition and richness on climatic, land use, habitat type and related soil parameters. Furthermore, the interactions between environmental parameters are very important, thus, vegetation reactions to potential climatic changes will be context dependent on habitat type and land use. Future projects should investigate these more in detail, especially the influence of land use (using more detailed information on the land use history) need more attention.
Projektbezogene Publikationen (Auswahl)
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(2016): Effects of Climate and Land Use on Herbaceous Species Richness and Vegetation Composition in West African Savanna Ecosystems. Journal of Botany
Zerbo, I., Bernhardt-Römermann, M., Ouédraogo, O., Hahn, K., Thiombiano, A.
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(2017): Dispersal potential of herbaceous species in West African savannas in relation with climate, land use and habitat conditions. Bois et Forêts Tropiques, 332: 69-87
Zerbo, I., Hahn, K, Bernhardt-Römermann, M., Ouédraogo, O., Thiombiano, A.
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(2017): Effects of climate, habitat and land use on the cover and diversity of savanna herbaceous layer in Burkina Faso, West Africa. Folia Geobotanica, 52: 129–142
Jakubka, D., Lessmeister, A., Hahn, K., Traore, S., Schumann, K., Thiombiano, A., Bernhardt-Römermann, M.
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(2018): Diversity and occurrence of herbaceous communities in West African savannas in relation to climate, land use and habitat. Folia Geobotanica, 53: 17-39
Zerbo, I., Bernhardt-Römermann, M., Ouédraogo, O., Hahn, K., Thiombiano, A.
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Chlorophyll fluorescence and gas exchange measurements in field research: an ecological case study, Photosynthetica
Bucher, S.F., Bernhardt-Römermann, M., Römermann, C.
