A test of the stress-gradient hypothesis including abiotic stress and consumer pressure during an extreme drought year

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.

During an extreme drought in the arid central valley of California, USA, we artificially manipulated a soil moisture gradient and erected animal exclosures to examine the interactions between dominant shrubs and the subordinate annual community.

There was a high frequency of positive interactions between shrubs and the annual community at all levels of soil moisture and consumer pressure. Shrub facilitation and water addition displayed similar effect sizes on plant communities, however, the shrub facilitation effect was significantly stronger in watered plots.

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.