DIFFERENTIAL USE OF LARGE SUMMER RAINFALL EVENTS BY SHRUBS AND GRASSES - A MANIPULATIVE EXPERIMENT IN THE PATAGONIAN STEPPE

In the Patagonian steppe, years with above-average precipitation (wet years) are characterized by the occurrence of large rainfall events. T he objective of this paper was to analyze the ability of shrubs and gr asses to use these large events. Shrubs absorb water from the lower la yers, grasses from the upper layers, intercepting water that would oth erwise reach the layers exploited by shrubs. We hypothesized that both life-forms could use the large rainfalls and that the response of shr ubs could be more affected by the presence of grasses than vice versa. We performed a field experiment using a factorial combination of wate r addition and life-form removal, and repeated it during the warm seas on of three successive years. The response variables were leaf growth, and soil and plant water potential. Grasses always responded to exper imental large rainfall events, and their response was greater in dry t han in wet years. Shrubs only used large rainfalls in the driest year, when the soil water potential in the deep layers was low. The presenc e or absence of one life-form did not modify the response of the other . The magnitude of the increase in soil water potential was much highe r in dry than in humid years, suggesting an explanation for the differ ences among years in the magnitude of the response of shrubs and grass es. We propose that the generally reported poor response of deep-roote d shrubs to summer rainfalls could be because (1) the water is insuffi cient to reach deep soil layers, (2) the plants are in a dormant pheno logical status, and/or (3) deep soil layers have a high water potentia l. The two last situations may result in high deep-drainage losses, on e of the most likely explanations for the elsewhere-reported low respo nse of aboveground net primary production to precipitation during wet years.