Abiotic and biotic stressors can have large impacts on plant traits and phytochemistry, yet surprisingly little is known about the interactive effects of these stressors. Understanding how climate change will interact with other stresses like herbivory is important to both species conservation and management. Water is a key selective agent in natural plant populations and, in the arid Western United States, its availability is predicted to decrease as the climate changes. Herbivores are also important selective agents, which impact plant chemistry and traits. Because plants induce trait changes to deal with individual stressors, the extent and nature of plant responses are likely to be impacted by multiple stressors occurring simultaneously. In this study, we investigated the interactive effects of drought and herbivory on plant water-use efficiency, aboveground growth, and defensive traits (latex exudation, trichomes, leaf-mass per area, foliar water content, C/N ratio, and secondary metabolites) of narrowleaf milkweed (Asclepias fascicularis). We also characterized differences among populations in their induced responses to drought and herbivory by planting seeds collected from along a gradient of climatic water deficit into a common garden. Plants were exposed to a two-level, full-factorial experiment with herbivory and drought treatments.
Results/Conclusions
Herbivory marginally increased plant biomass, whereas drought-stress substantially marginally decreased plant biomass. Drought-stress increased both the richness and the diversity of UV-absorbent secondary metabolites in the absence of herbivore damage. Surprisingly, this response disappeared when plants experienced both stressors. The drought responses of water-use efficiency and C/N ratios were also muted by simultaneous herbivory, though not as strongly as secondary metabolites. Foliar water content was reduced only in plants experiencing both stressors. Latex exudation, trichomes, and leaf-mass per area were not impacted by either stressor. In general, A. fascicularis grown from seeds collected in drier areas showed stronger induced responses to stress than plants from wetter areas. Interestingly, the resulting phenotypes varied among populations from along a water deficit gradient. This suggests that environmental stress gradients may shape trait responses within a species. Our results further suggest that there are interactive effects between drought-stress and herbivory that limit the presumably adaptive induced responses of A. fascicularis. Herbivory could thus intensify the threats posed by climate change to the survival of maladapted plant populations, such that understanding the effects of local adaptation will be critical to developing effective conservation plans.