Stressful environmental conditions and herbivores have major impacts on plant survivorship, physiology, growth, and reproduction. Variation in plant responses is a result of the timing, nature, and severity of each abiotic and biotic stress. Most studies to date have evaluated plant responses to herbivory and water variation in either leaves or roots, ignoring that these biotic and abiotic stressors can occur simultaneously. Furthermore, these stressors can be enhanced by anthropogenic climate change.
Here we experimentally simulated a key form of environmental stress – variation in water availability – and manipulated above and belowground herbivory in seedlings of the tropical shrub Solanum lycocarpum (Solanaceae) to test how their isolated and simultaneous effects would change plant survival and growth, biomass allocation, and the concentration of defensive terpenoids in roots and leaves. We addressed the following questions: (1) How changes in water availability and herbivory affect plant survival? (2) How changes in water availability and herbivory affect plant growth and defenses? (3) Are Solanum lycocarpum seedling responses to changes in water availability and herbivory specific to a plant-part or similar between above and belowground?
We found that 1) drought can severely affect plant survivorship and the effect is exacerbated by belowground herbivory, 2) herbivory has a greater effect on plant growth than variation in water, and the responses caused by AG and BG herbivores were distinct, making the impacts of belowground herbivores unique and context-dependent and 3) while water and herbivory can significantly alter plant defensive chemistry, the changes are not consistent in direction or magnitude.
These results emphasize that aboveground and belowground responses to biotic and abiotic stressors can differ strongly. Since some of these abiotic and biotic stressors are expected to vary more with climate change, understanding how some mechanisms in isolation or in combination affect the plant is essential for predicting how plant-herbivore interactions may affect community dynamics in the future.