Increases in drought frequency in combination with overgrazing may result in degradation of (semi‐) arid ecosystems. Facilitative interactions between plants are a key mechanism in preventing degradation, but it is poorly understood how they respond to increased stress by combined drought and herbivory. In this study, we used an ecohydrological model, to simulate the plant growth of two plant species interacting with each other under different rainfall and herbivory pressure scenarios. The functional traits of the two modeled plants were based on a prior field experiment in southeastern Spain, in which an unpalatable “nurse” species protected a palatable protégé’ species from herbivory. Moreover, the nurse species was more drought‐resistant; that is, it had a lower wilting point, whereas the protégé species had a higher optimal growth rate. Firstly, we investigated the coexistence of the two plant species growing under a single limiting resource, focusing on the effect of intra‐seasonal rainfall variability. We found that longer periods without rainfall within the wet season resulted in stable coexistence, whereas nearly constant rainfall led to competitive exclusion of the protégé by the nurse species. Secondly, we investigated how plant interactions varied along our studied gradients. Using the neighbor effect intensity and importance indices, we found that competitive effects increased with more constant rainfall. Moreover, higher herbivory rates resulted in increased facilitative effects of the nurse on the protégé species, but facilitative effects could only prevail over competitive effects under currently observed or higher intra‐seasonal rainfall variability. This study highlights the relevance of intra‐seasonal rainfall variability in explaining coexistence of species in dryland ecosystems and shows that increasing intra‐seasonal rainfall variability or herbivory pressure can result in more facilitative effects from a nurse species. This information is crucial to obtain a better insight into the long‐term coexistence of species, and the resulting stability of dryland ecosystems in response to future climate change.

中文翻译：

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季节内降雨的变异性和食草性影响两种旱地植物物种的相互作用结果

干旱频率增加与过度放牧相结合，可能导致（半）干旱生态系统退化。植物之间的促进性相互作用是防止降解的关键机制，但人们对它们如何应对干旱和食草综合胁迫的反应知之甚少。在这项研究中，我们使用了一个生态水文模型，来模拟两种植物在不同降雨和食草压力情况下相互影响的植物生长。两种模拟植物的功能性状是基于西班牙东南部的一项先前的田间试验，在该试验中，难吃的“护士”物种保护了可口的protégé'物种免受食草动物的侵害。此外，护士种类更耐干旱；也就是说，它的萎point点较低，而门生物种的最佳生长率则更高。首先，我们研究了在单一限制资源下生长的两种植物的共存，重点是季节内降雨变化的影响。我们发现，雨季内没有降雨的较长时间导致稳定的共存，而几乎恒定的降雨导致护士物种竞争性地排斥门生。其次，我们研究了植物相互作用如何沿着我们研究的梯度变化。使用邻居效应强度和重要性指数，我们发现竞争效应随着降雨的增加而增加。此外，较高的食草率导致护士对门生物种的促进作用增强，但在当前观察到的或季节内降雨变化较大的情况下，促进作用仅能胜过竞争作用。这项研究强调了季节内降雨变异性在解释旱地生态系统中物种共存方面的相关性，并表明，季节内降雨变异性或食草压力的增加会导致护士物种产生更多的促进作用。这些信息对于更好地了解物种的长期共存以及干旱地区生态系统对未来气候变化的响应所带来的稳定性至关重要。