Relationships Between Flowering Time and Rainfall Gradients Across Mediterranean-Desert Transects

There is growing evidence for rapid adaptive evolution in response to climate change, including phenological transitions such as earlier flowering with climate warming. The consequences of these evolutionary changes for population dynamics and shifts in species ranges remain, however, quite unexplored. Here, we propose that inter-population differences in patterns of flowering across geographic precipitation gradients can be considered a proxy for changes in flowering time due to variation in rainfall resulting from climate change. To this end, we analyze trends of variation in flowering time across rainfall gradients in the eastern Mediterranean region in three main plant life-forms present in the local vegetation: winter annuals, geophytes, and perennial grasses. These life-forms cope with the hot and dry summer via a drought escape strategy. The analysis is based on published and unpublished data from common-garden experiments in which plants from populations sampled along rainfall gradients were grown under similar conditions, thus allowing detection of genetic differences in flowering time along the gradient. The data clearly indicate that decreasing rainfall across a Mediterranean-desert transect is associated with earlier flowering in winter annual species. In contrast, the limited available data shows no consistent trend of change in flowering time with decreasing rainfall in geophytes and perennial grasses. The phenological shift to early flowering in winter annuals coping with terminal drought appears to be a widespread method for adaptation to arid environments by stress avoidance, diminishing the risk of early death before seed production. However, changes in flowering time associated with the reduction in precipitation predicted by climate change models are relatively small, suggesting that additional traits are involved in the adaptation to increasing aridity. The hypothesis that low water availability is an environmental signal inducing earlier flowering of annual plants under drought conditions is not supported by experimental data.