Effects of changes in rainfall amount and pattern on root dynamics in an arid shrubland

Ecosystems will likely be affected by changing rainfall patterns in dry environments and roots will play a major role in plant responses. However, little is known on root dynamics in response to changing rainfall in nature. In a rainout shelter experiment conducted in a semiarid shrub-steppe in south-eastern Spain, we manipulated both the amount (30% reduction all year round) and seasonal rainfall distribution according to climate change scenarios, with increases (30%) or subtle reductions (10%) in winter rainfall, and reductions in the remaining seasons (15% in spring and autumn or 40% in spring and autumn and 30% in summer). Minirhizotron tubes were used to monitor fine root dynamics beneath the canopy of the dominant shrub in a year that was above the average rainfall. We found higher root length density and enhanced root length production in the 30% reduction in annual rainfall compared to ambient (control) or any of the altered seasonality treatments. There were no differences among the three other treatments. By contrast, root length loss and root length turnover were not consistently influenced by rainfall treatments. These results evidence that moderate rainfall reductions all year round would have the largest impact on root dynamics in this arid shrubland. Increased or subtle rainfall reductions in rainy winters may have lower impact, due to moisture replenishment at the beginning of the growing season. These findings are stimulating for further research in larger time scales and in years with contrasting precipitation.