Intra‐seasonal rainfall variability and herbivory affect the interaction outcome of two dryland plant species

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 ﬁ eld experiment in southeastern Spain, in which an unpalatable “ nurse ” species protected a palatable prot ´ eg ´ e species from herbivory. Moreover, the nurse species was more drought-resistant; that is, it had a lower wilting point, whereas the prot ´ eg ´ e 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 prot ´ eg ´ e 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 prot ´ eg ´ e 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. scenarios (from low to high intermittency, IM = 1, 8, or 11 d) and three herbivory scenarios (from low to high herbivory intensity, h = 0, 0.1, 0.3 per yr). When calculating the indices, biomass values below 1g/m 2 were considered as equal to zero to avoid numerical arti-facts. A value of 1 for NInt C or NImp C represents maximum facilitative effects; a value of − 1 represents maximum competitive effects. An increase in intermittency resulted in a decrease in competitive effects. A combination of high herbivory and currently observed or high intermittency resulted in facilitative effects. a herbivory gradient (with steps of 0.1 per yr) for three intermittency scenarios (IM = 1, 8, or 11 d) and three annual rainfall scenarios (100, 300, or 500 mm/yr). A value of 1 represents maximum facilitative effects; a value of − 1 represents maximum competitive effects. When calculating the indices, biomass values below 1 g/m 2 were considered as equal to zero to avoid numerical artifacts. An increase in herbivory resulted in higher nurse effect intensity or importance.