Golluscio R A, Sala O E, Lauenroth W K
Departamento de Ecología, Facultad de Agronomía, Universidad de Buenos Aires. Av., San Martín 4453 (1417) Buenos Aires, Argentina e-mail:
Department of Rangeland Ecosystem Science, Colorado State University, Fort Collins, CO80523, USA, , , , , , US.
Oecologia. 1998 Jun;115(1-2):17-25. doi: 10.1007/s004420050486.
In the Patagonian steppe, years with above-average precipitation (wet years) are characterized by the occurrence of large rainfall events. The objective of this paper was to analyze the ability of shrubs and grasses to use these large events. Shrubs absorb water from the lower layers, grasses from the upper layers, intercepting water that would otherwise reach the layers exploited by shrubs. We hypothesized that both life-forms could use the large rainfalls and that the response of shrubs could be more affected by the presence of grasses than vice versa. We performed a field experiment using a factorial combination of water addition and life-form removal, and repeated it during the warm season of three successive years. The response variables were leaf growth, and soil and plant water potential. Grasses always responded to experimental large rainfall events, and their response was greater in dry than in wet years. Shrubs only used large rainfalls in the driest year, when the soil water potential in the deep layers was low. The presence 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 higher in dry than in humid years, suggesting an explanation for the differences among years in the magnitude of the response of shrubs and grasses. We propose that the generally reported poor response of deep-rooted shrubs to summer rainfalls could be because (1) the water is insufficient to reach deep soil layers, (2) the plants are in a dormant phenological status, and/or (3) deep soil layers have a high water potential. The two last situations may result in high deep-drainage losses, one of the most likely explanations for the elsewhere-reported low response of aboveground net primary production to precipitation during wet years.
在巴塔哥尼亚草原，降水高于平均水平的年份（湿润年份）的特点是会出现大量降雨事件。本文的目的是分析灌木和草本植物利用这些大量降雨事件的能力。灌木从下层吸收水分，草本植物从上层吸收水分，截留了原本会到达灌木利用层的水分。我们假设这两种生命形式都可以利用大量降雨，并且灌木的反应受草本植物存在的影响可能比反之的情况更大。我们进行了一项田间实验，采用加水和去除生命形式的析因组合，并在连续三年的温暖季节重复进行。响应变量是叶片生长、土壤和植物水势。草本植物总是对实验性的大量降雨事件有反应，并且它们在干旱年份的反应比湿润年份更大。灌木仅在最干旱的年份利用大量降雨，那时深层土壤水势较低。一种生命形式的存在与否并未改变另一种生命形式的反应。干旱年份土壤水势增加的幅度比湿润年份高得多，这为灌木和草本植物反应幅度的年份差异提供了解释。我们认为，普遍报道的深根灌木对夏季降雨反应不佳可能是因为：（1）水分不足以到达深层土壤层；（2）植物处于休眠物候状态；和/或（3）深层土壤层具有较高的水势。后两种情况可能导致大量深层排水损失，这是其他地方报道的湿润年份地上净初级生产力对降水反应较低的最可能解释之一。
