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Accueil du site → Doctorat → Allemagne → 2005 → Stable carbon isotopes and plant water relations in the acacia savanna woodlands of Ethiopia : implications for reforestation and paleoclimatic reconstructions

Göttingen, Universität Allemagne (2005)

Stable carbon isotopes and plant water relations in the acacia savanna woodlands of Ethiopia : implications for reforestation and paleoclimatic reconstructions

Aster Gebrekirstos

Titre : Stable carbon isotopes and plant water relations in the acacia savanna woodlands of Ethiopia : implications for reforestation and paleoclimatic reconstructions

Auteur : Aster Gebrekirstos

Université de soutenance : Göttingen, Universität

Grade : Doktorgrade 2005

Résumé
The thesis focuses on plant ecophysiological, dendroclimatological and stable carbon isotope aspects of co-occurring dryland species in Ethiopia, where the environment is strongly associated with its climate and sustainable management and restoration of natural resources is critical. In a changing environment it is not clear to what extent climate change disrupts ecophysiological stability and leads to poor adaptations. Hence, understanding the physiological response of the species to water stress and their tolerance scope under sever stress are needed to explain differences among species in survival and distribution and determine their relative suitability for restoration. I made plant water and osmotic potential measurements, in the field at midday and predawn at sites of different biophysical settings to asses the effect of deforestation and climate change on water status of both plants and their respective sites. Based on these measurements I was able to characterize co-occurring species as drought tolerant and drought sensitive (avoider) species. For example, Acacia tortilis, Balanities aegyptiaca and Dichrostachys cinerea, which exhibited wide tolerance limits to water changes, were considered as drought tolerant and suitable candidates for reforestation in drought prone areas. Identifying potential paleoclimate proxies that could provide empirical data regarding past climatic events is indispensable to understanding natural and anthropogenic climate change and to reconstruct climate history. The ongoing changes in climate are likely to significantly affect plant-atmospheric interactions. Therefore plant responses to their environment would offer a potential to trace the course of past environmental and climatological fluctuations. I have used tree rings and stable carbon isotopes in tree rings to explore climate- growth dynamics and investigate their potential as climate proxies. I constructed long term tree ring width chronology (1930-2003), which shows strong correlation coefficients with precipitation. Spectral analysis of the master tree ring analysis indicated occurrences of periodic drought events, which falls within the spectral peak equivalent to 2-8 years. Periodic droughts in Ethiopia are found to associate with El Niño Southern Oscillation. By taking co-occurring species with contrasting physiological performances (deciduous and evergreen species) I tried to enhance environmental information from stable carbon isotope ratios (delta13C) in tree rings. The results suggests that the interplay between climatic conditions and species behaviour determines the inter-annual delta13C patterns of the co-occurring species uniformly. However, the long term trend observed in the delta13C (of the evergreen) seems to depend on the life strategy of the particular species. In fact, a part of this trend can be attributed to the lowering of the delta13C value of the global atmosphere by about 1.5 ‰ since industrialization suggesting that it reflects response of this species to rising CO2. However, since the trend was species- specific the cause could also be due to other physiological process peculiar to this species. Hence, understanding of these strategies is important for selection of species to reconstruct climate data using delta13C ratio variations in tree rings. Tree ring delta13C values of all the species revealed significant negative correlation (up to r = -0.82) with precipitation but not with temperature and relative humidity. The strong relationship between precipitation and delta13C in this study indicates that in semiarid areas moisture stress may have a direct impact on the stomatal conductance and explain the strong negative relationship between delta13C and precipitation. The synchronicity and consistency of inter-annual isotopic patterns are very promising with respect to the potential for annual drought or paleoclimatic reconstructions from tree rings of Acacia species. The outcomes of this study have important implications for paleoclimatic reconstructions and in restoration of degraded lands in Africa.

Mots clés : Äthiopien ; Savanne ; Akazie ; Anpassung ; Messung

Présentation

Page publiée le 12 mars 2006, mise à jour le 2 janvier 2019