Grazing modifies ecosystem function through direct effects on plants, but also through indirect effects mediated by floristic changes induced by grazing. Although both types of effects occur in the long term, only the direct effects are evident in the short term. We evaluated the short-term direct effects of sheep (Ovis aries) grazing on a Patagonian steppe during one growing season. We measured plant aerial cover in permanent transects located at increasing distances from a watering point in three paddocks with different stocking rates through the growing season. We also measured frequency of defoliation for vegetative and reproductive phases of different plant species located along these transects. Sheep grazing directly (a) reduced aerial cover and/or increased frequency of defoliation of certain preferred grasses and perennial forbs, (b) did not increase the aerial cover of any life form, but only the proportion of bare soil, (c) did not change the litter aerial cover, and (d) defoliated the flowers of even the least preferred shrub. Result a) was coincident with previous plant aerial cover long-term studies; but results (b) and (c) were contrary to long-term studies, probably because they resulted from indirect rather than direct grazing effects. Result (d) was not detected by long-term studies, probably because flower defoliation through grazing is undetectable when measuring shrub plant aerial cover. Our study showed that grazing has short-term direct effects mainly on the most preferred species. This could be useful for rangeland management and conservation of Patagonian steppes because short-term effects may be more easily reversible than long-term ones, and may provide early warning of rangeland condition deterioration.

放牧通过对植物的直接影响，也通过放牧引起的植物区系变化介导的间接影响，来改变生态系统的功能。尽管这两种类型的影响都是长期发生的，但只有短期的直接影响是明显的。我们评估了绵羊（Ovis aries）的短期直接影响。）在一个生长季节里在巴塔哥尼亚草原上放牧。我们测量了三个生长区在距放养点距离越来越远的三个永久性样地中的植物空中覆盖，在整个生长季节中，它们的放养率不同。我们还测量了沿这些样带分布的不同植物物种的营养和生殖期的脱叶频率。直接放牧绵羊（a）减少某些首选草和多年生草的气垫和/或增加落叶的频率，（b）没有增加任何生命形式的气垫，但仅增加了裸土的比例，（c） （d）使最不喜欢的灌木的花朵脱落。结果a）与以前的植物空中覆盖物长期研究一致；但是结果（b）和（c）与长期研究相反，可能是因为它们是间接的而不是直接的放牧作用。长期研究未发现结果（d），这可能是因为在测量灌木植物的空中覆盖时，无法检测到由于放牧而导致的花朵脱落。我们的研究表明，放牧主要对最喜欢的物种具有短期直接影响。这可能对牧场管理和巴塔哥尼亚草原的保护很有用，因为短期影响可能比长期影响更容易逆转，并且可以提供牧场状况恶化的预警。可能是因为在测量灌木植物的空中覆盖时，无法检测到由于放牧而导致的花朵脱落。我们的研究表明，放牧主要对最喜欢的物种具有短期直接影响。这可能对牧场管理和巴塔哥尼亚草原的保护很有用，因为短期影响可能比长期影响更容易逆转，并且可以提供牧场状况恶化的预警。可能是因为在测量灌木植物的空中覆盖时，无法检测到由于放牧而导致的花朵脱落。我们的研究表明，放牧主要对最喜欢的物种具有短期直接影响。这可能对牧场管理和巴塔哥尼亚草原的保护很有用，因为短期影响可能比长期影响更容易逆转，并且可以提供牧场状况恶化的预警。