Variation in phenology of Acacia senegal (L.) Wild. in relation to origin and ploidy level: Implications for climatic adaptation

Abstract

Correct timing of phenology is crucial for the survival and growth of species in arid areas with long dry seasons. Nevertheless, knowledge on genetic variation and adaptive patterns in phenology in deciduous African dryland species is limited. Here we study the variation in phenology of diploid and polypoid A. senegal trees from rangewide populations growing in a common garden trial in Senegal and test correlations between population phenology and climate at the site of origin. The leafing, flowering and fruiting phenology was monitored during 17 months and compared to detailed observations of the rainfall in the common garden during the period. We found that A. senegal trees in general started development of leaves prior to the beginning of the rainy season with flowering and fruiting initiation occurring during the rainy season. The results lead us to conclude that is was not the rain per se that initiated leaf development. We also conclude that phenology in A. senegal is under genetic control, because significant differences could be observed among populations and ploidy levels when grown at the same site. In general, early leaf flushing trees had a longer growing period and performed better in terms of growth at the tested site and the results thus support that leaf phenology influence fitness. We further found that differences among trees in phenology seem to be associated with differences in climate at their site of origin, because the timing of leaf development in the common garden and the timing of the rainy season at the site of origin was significantly correlated for the diploid trees (not for tetraploids). However, it was diploid trees from sites with a late arriving rainy season that developed leaves earliest in the year. The environmental cues that control leafing phenology and the associated physiological mechanisms therefore still need to be identified in order to understand how the variation among populations has evolved, its relationship to local adaptation and the implication for smart transfer of seed sources as mean to mitigate changing growing conditions related to global warming.