Climate change effects in a semiarid grassland: Physiological responses to shifts in rain patterns

Abstract

Several studies have predicted changes in precipitation patterns affecting both rain quantity and its temporal distribution for the Mediterranean Basin. This three-year study was performed to determine the physiological response of Macrochloa tenacissima, a dominant species in the western Mediterranean grasslands, to these changes. A rainfall manipulation experiment was therefore conducted to test the interaction of two factors: quantity (Q) and frequency (F) of rainfall, both at three levels (100%, 75% and 50% of natural rainfall). A mobile transparent polycarbonate rain-out shelter was designed to cover experimental plots of M. tenacissima when it rained, and then treatments were implemented by watering. Reductions in Q and F caused seasonal down-regulation of net photosynthesis (A) and stomatal conductance (gs), but the first variable showed greater resistance to change. At the annual scale, only the reductions of F had negative effects on A rates, but without causing significant changes to gs. The decrease in Q and F had opposite effects on intrinsic water-use efficiency (IWUE), enhancing and diminishing it, respectively. However, the response to Q was stronger, even exceeding the range of natural interannual variability. Rainfall Q and F reduction did not decrease Fv/Fm, as compared to ambient conditions. In conclusion, although the responses to the simulated rainfall patterns did not surpass the current seasonal oscillations of M. tenacissima's physiological parameters, they caused a down-regulation of its gas exchange and increased its water-use efficiency.