The stress gradient hypothesis original purposed the frequency of plant interactions along countervailing gradients of abiotic stress and consumer pressure. However, research to date has studied these two stressors in isolation rather than together, thereby potentially neglecting the interaction of these factors on plant composition. In the arid central valley of California, we artificially manipulated a soil moisture gradient and erected animal exclosures to examine the interactions between dominant shrubs and the subordinate annual community. We conducted this experiment in an extreme drought year (2014) and a year of above-average rainfall (2016).
Results/Conclusions
Shrubs positive affected the abundance and biomass of the annual community at all levels of soil moisture and consumer pressure. In the drought year, shrub facilitation and water addition produced similar positive effect sizes on plant communities; however, the shrub facilitation effect was significantly greater in watered plots. During the year with higher rainfall, there was no observed water or exclosure effect, but shrubs still significantly increased biomass of the subordinate plants. Shrubs and positive interactions maintain productivity of annual plant communities at environmental extremes despite reductions in droughts stress or consumer pressure and these positive effects are even more pronounced with water addition. The relationship between consumer pressure and abiotic stress on plant interactions is non-linear particularly since shrubs can facilitate understorey plants through a series of different mechanisms.