Resilience of semi-arid forests to precipitation patterns: Lessons learned from a dry forest in Israel and a ground-water controlled forest in California

Background/Question/Methods

Current and to a larger extent future climate change is expected to affect the hydrological cycle, having further implication on vegetation extent, biomass and production. For arid and semi-arid regions of the world, changes in precipitation are expected to concurrently lead to a decrease in total precipitation amounts and an increase in storm intensity. An additional decrease from already low water availability might have a damaging effect on vegetation, but on the other hand, vegetation of water-limited climates has developed mechanisms to increase survival during drought seasons and years. We therefor ask: how sensitive are semi-arid forests to precipitation changes associated with climate change? To answer this question we investigated linkages between forest productivity and the water budget at two sites: grass-oak savanna in California (‘precipitation = 565 mm y^-1, Quercus douglasii), and afforestation in Israel (precipitation = 285 mm y^-1, Pinus halepensis). In both sites carbon and water fluxes were measured, in addition to a large suite of meteorological measurements.

Results/Conclusions

In the Israeli site, soil evaporation was twice as high in between tree patches than below tree canopies. These results implied that while current tree density can be supported by current precipitation amounts, an increase in tree density or a decrease in precipitation will limit tree water availability. However, measurements showed that during large rain events (storm amounts > 30 mm), deeper infiltration decreased evaporation losses, stored deep soil moisture for longer periods, and increased tree water uptake and productivity. At the Californian site, grasses reacted to variability in precipitation amounts, but carbon and water fluxes from the trees were relatively constant throughout the ten year measurement period, although precipitation ranged between 400 to 925 mm yr^-1. Trees in this ecosystem are winter-deciduous, and in a sense not synchronized with the wet winter season, but rather relay on wet soil conditions during spring and groundwater uptake during the dry summer. Both conditions (spring wet soils and summer groundwater) occured even during dry years, leading to relatively constant annual fluxes from the trees. We therefore conclude that forests in these semi-arid ecosystems are not highly sensitive to the expected decrease in total precipitation amounts. Other factors, such as deep water uptake sources produced by high intensity storm events (Israeli site) and groundwater uptake (Californian site) are more important to the resilience of these semi-arid forests.