Correlates of inter-specific variation in germination response to water stress in a semi-arid savannah

Within arid plant communities species vary considerably in the ability to germinate under water stress. Attempts to correlate this variation with environmental gradients have remained largely inconclusive. Germinating only at high water potentials can be seen as a form of predictive germination. Predictive germination provides a fitness variance reducing mechanism and is therefore expected to show negative correlations with other variance reducing life-history attributes such as large seed size or dormancy. We predicted that differences in life-history attributes rather than edaphic gradients could explain the variation in germination responses to water stress found in arid plant communities. To test our hypothesis we determined the germination response of 28 species from the arid Kalahari savannah to a gradient of osmotic stress, expressed as the water potential needed to reduce germination by 50%. In addition, we determined the life-history variables seed mass and germination fraction and the habitat variables soil texture preference and association with acacias. The data were analysed using phylogenetically independent contrasts in a multiple regression model. Contrary to our hypothesis we found no increase in the capacity to germinate under osmotic stress with increasing seed mass and an increase with increasing germination fraction. However, we also found no significant effect of the habitat variables. This result may be explained by variation in seedling drought tolerance. Drought tolerance will also have a variance-reducing effect and can be expected to trade-off with fractional germination. Our results suggest that in arid plant communities most variation in the capacity to germinate under water stress expresses different ways to make a living under similar conditions rather than adaptations to environmental gradients. Innerhalb arider Pflanzengesellschaften gibt es große zwischenartliche Unterschiede in der Fähigkeit unter Wasserstress zu keimen. Versuche, diese Variation mit Umweltgradienten zu korrelieren, bleiben meistens ergebnislos. Die Keimung nur bei hohen Wasserpotenzialen ist eine Art prädiktiver Keimung. Prädiktive Keimung ist ein Mechanismus zur Reduzierung der Varianz der Fitness, weshalb man eine negative Korrelation mit anderen Varianz-reduzierenden Life-history-Attributen wie große Samen oder Dormanz erwarten kann. Wir nehmen an dass statt edaphischer Gradienten unterschiedliche Life-history-Merkmale die Variation im Keimungsverhalt