Variation of shoot mass of annual plants with rain pattern

Köchy, M., F. Jeltsch

We are using a spatially explicit, individual- and rule-based model for studying the consequences of climate change on the production and viability of annual plant communities in the Near East. The model comprises modules for the dynamics of soil water, seeds, and plants. We parameterized the model with germination requirements and drought tolerance for ten species occuring along a climatic gradient from arid to mesic Mediterranean. We tested first whether rain volume or rain pattern (number of days with rain >5, 10, 15, or 20 mm) better predicted shoot production in the ten species under current conditions. Second, using rainfall scenarios varying volume (100 - 800 mm mean annual precipitation) and pattern (arid to mesic Mediterranean) in a factorial way, we examined how the two factors affect shoot production of an average annual. Shoot production of eight species was predicted better by rain pattern than volume than volume. The two exceptions were species that are most abundant in arid climate. Shoot production of the average annual increased significantly with rain volume, as expected. In addition, ANOVA on ln-transformed data indicated that shoot production increased significantly with rain pattern from arid to mesic Mediterranean independent of rain volume. The interaction between both factors was significant because any pattern for less than 400 mm rain resulted in low production (<20 g/m2). The results show that the production of annuals is likely to decrease not only if global warming decreases mean annual precipitation but also if rains fall more rarely. The greatest difference observed was for regions currently receiving 600 mm rain (arid pattern: 41 g/m2 vs mesic Mediterranean pattern: 131 g/m2). In the Near East, these regions are densely populated comprising major parts of Israel and the Palestinian Westbank.

Fig. 1

Fig. 1. Effect of rain pattern (as characterized by climate) on the aboveƂground production of an average annual plant (mean, SE = 5.4). The means presented have been corrected for rain volume. ANOVA: rain volume ***, rain pattern ***, volume × pattern ***. ***: P<0.001.