The net balance of positive (facilitation) and negative (competition) interactions between plant neighbors affects population dynamics, and therefore community composition. It is unclear, however, how joint effects from biotic (e.g. herbivory) and abiotic (e.g. drought) stresses alter that balance. Here, we evaluated whether water stress modified the sign and strength of interactions between focal palatable plants and unpalatable neighbors along a grazing intensity gradient. We hypothesized that the presence of unpalatable shrubs benefits palatable neighbors in grazed areas, regardless of water supply. Conversely, water availability alters the balance of plant-plant interactions under low or no herbivory. Field studies were conducted in the semi-arid Caldén (Prosopis caldenia) forest of central Argentina. First, we explored the performance of several herbs under cattle herbivory when living inside and outside the canopy of Baccharis ulicina, a common unpalatable bush in the region. Second, we transplanted individuals of the dominant grass Piptochaetium napostaense inside and outside Baccharis canopies at increasing distances from watering sources and in cattle exclosures, and for two years we established a factorial water addition treatment to separate effects from biotic and abiotic stress on the palatable grass performance. We recorded consumption, size, and fecundity of grass individuals inside and outside Baccharis.
Results/Conclusions
We observed that Baccharis facilitated several palatable species. Individuals of Piptochaetium were less consumed, taller and produced up to five times more flowers when growing inside than outside Baccharis neighborhoods, suggesting that facilitation occurred through protection against herbivores. In the experiment, individuals of Piptochaetium were always less consumed when growing beneath Baccharis canopies. Under heavy grazing pressure, Piptochaetium transplants were taller and produced more flowers inside than outside Baccharis. Differences in fecundity between plants inside vs. outside Baccharis disappeared as grazing pressure decreased away from the water source and inside exclosures. In addition, we found no significant effect of water supply on grass fecundity at any level of the grazing gradient. These results suggest that the net balance of interactions changed from positive under heavy grazing to neutral at low and no herbivory, and that water availability did not alter that balance. We conclude that in this system the balance of plant-plant neighborhood interactions is largely driven by herbivory pressure.