Variation among Acacia senegal (L) Willd trees of different genetic origins in relation to climatic adaptation.

Abstract

Predictions of climate change suggest a global increase in CO2 and temperature in Africa, but rainfall patterns are still uncertain. Acacia senegal has received increased interest due to its derived gum arabic, drought tolerance, wide distribution, presence of multiple levels of ploidy and ability to fix nitrogen. To enhance knowledge on degree of genetic variation among populations in A. senegal with a view to its domestication and management in the Sahelian area, common garden trials in Senegal were explored in order to understand the morphological and phenological responses associated with the adaptation of the species to dry conditions. The thesis consists of three manuscripts. The study presented in Manuscript 1 revealed three levels of ploidy namely diploid (2n), triploid (3n) and tetraploid (4n), but at highly different frequencies among populations. Leaf morphology and isotopic discrimination in A. senegal varied according to ploidy level and geographic origin. Triploid trees had a distinct leaf shape, and tetraploids displayed high δ¹³C and low δ¹⁵N values compared to diploids. Manuscript 2 showed that the variation among provenances in survival, height and diameter was significant and differences could be partly explained by the climate at their site of origin. In general, provenances from dry sites survived better at both sites. In addition, provenances from dry sites on average performed relatively poorer in height and diameter at the wetter site (Bambey) compared to the drier site (Dahra), while the opposite was the case for provenances from wetter sites. The study in Manuscript 3 indicated that A. senegal started the development of leaves prior to the beginning of the rainy season with flowering and fruiting initiation occurring during the rainy season. The phenology in A. senegal is under genetic control with significant differences observed among populations and ploidy levels. Variation in leaf phenology corresponded with local adaptation for diploid trees, but the same patterns was not found for tetraploid trees. Flowering phenology differed between diploid and tetraploid trees, but not to an extent that suggests it could provide an effective reproductive barrier between the two ploidy levels. Overall, the present thesis demonstrated that A. senegal across Africa consists of provenances that are genetically differentiated in their leaf morphology, phenology and growth, and these differences most likely reflect adaptation to local climatic conditions.