Effects of precipitation variability on carbon and water fluxes in the understorey of a nitrogen-limited montado ecosystem.

To date the implications of greater intra-annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on soil and vegetation carbon and water fluxes in the understorey of a Mediterranean oak woodland in response to increasing precipitation variability, with an extension of the dry period between precipitation events from 3 to 6 weeks, without altering total annual precipitation inputs. With prolonged dry periods soil moisture did breach the stress thresholds for ecosystem processes, which led to short-term treatment differences in photosynthesis, but not in system carbon losses, with subsequent short-term decreases in net ecosystem exchange. Independent of treatment, irrigation events rapidly increased carbon and water fluxes. However, contradicting the predictions drawn from the 'bucket model', over the course of the growing season no all-over treatment differences were found in system assimilation and respiration, nor in evapotranspiration and ecosystem water use efficiency. This lack of responsiveness is attributed to the ecosystem's resilience to low soil moisture during the growing season of the herbaceous understorey, with temperature rather than soil moisture controlling key ecosystem processes. Moreover, severe nitrogen limitation of the studied ecosystem may explain the lack of moisture effects on net system carbon dynamics. Thus, although the bucket model predicts changes in soil water dynamics with increasing precipitation variability, ecosystem responses to more extreme precipitation regimes may be influenced by additional factors, such as inter-annual variability in nutrient availability.