Anthropogenic activities such as cattle grazing and forest clearing have led to the establishment of early successional grass layers in some native forests, which may inhibit or entirely prevent native tree regeneration. We hypothesize that increased grass coverage reduces or eliminates the establishment of native tree seedlings by limiting water availability to seeds and seedlings. This study aims to evaluate the impact of grass cover on tree seedling survival under varying levels of soil water stress. We conducted a field experiment using 36 experimental exclosures in two northwest Patagonian valleys, representing a regional gradient in altitude, rainfall, and tree canopy openness. Additionally, a greenhouse experiment was performed with 36 pots, manipulating four levels of grass cover and three levels of water stress. Results from both experiments showed similarities: in the field, the likelihood of finding a live tree seedling was approximately 2.78 times higher in areas without grass compared with grass‐covered sites. In the greenhouse, the presence of grass reduced the final number of established seedlings by an average of 43% across all irrigation levels, indicating significant water competition. These findings suggest that management practices promoting grass invasion could severely hinder tree regeneration in forests not adapted to large herbivore intensive grazing. Such situation may be exacerbated in regions suffering water limitation in the growing season or where the climate change would intensify water stress.

中文翻译：

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禾本科植物对沿水分胁迫梯度的本地树木幼苗建立的影响：森林和温室试验的结果


放牧和森林砍伐等人为活动导致一些原生林中建立了早期演替草层，这可能会抑制或完全阻止原生树木的再生。我们假设，草覆盖率的增加通过限制种子和幼苗的水分供应来减少或消除本地树苗的建立。本研究旨在评估不同土壤水分胁迫水平下草覆盖对树木幼苗存活的影响。我们在巴塔哥尼亚西北部的两个山谷中使用 36 个实验封闭进行了一项田间实验，代表了海拔、降雨量和树冠开放度的区域梯度。此外，还对 36 个花盆进行了温室实验，处理了 4 个级别的草覆盖率和 3 个级别的水分胁迫。两个实验的结果显示出相似之处：在田间，与草覆盖的地点相比，在没有草的地区找到活树苗的可能性大约高出 2.78 倍。在温室中，草的存在使所有灌溉水平的最终已建立的幼苗数量平均减少了 43%，这表明存在严重的水竞争。这些发现表明，促进草入侵的管理做法可能会严重阻碍不适应大型食草动物密集放牧的森林中的树木再生。在生长季节缺水的地区，或者气候变化会加剧水资源压力的地区，这种情况可能会加剧。