Additive and non-additive responses of seedlings to simulated herbivory and drought data

Drought is a global threat, increasing in severity and frequency
throughout tropical ecosystems. Although plants often face drought in
conjunction with biotic stressors, such as herbivory or disease,
experimental studies infrequently test the simultaneous effects of drought
and biotic stress. Because multiple simultaneous stressors may have
non-additive and complex effects on plant performance, it is difficult to
predict plant responses to multiple threats from research examining one
stress at a time. Using an experimental approach in the greenhouse, we
investigated potential non-additivity in seedling growth and survival to
simulated drought and herbivory across a phylogenetically diverse pool of
ten Hawaiian plant species. Overall, seedlings showed limited tolerance,
defined as similar growth and survival in stressed compared to control
(non-stressed) plants, to simulated herbivory and drought, with the
combined effects of both stressors to be generally additive and negative
across species. Significant variation in stress tolerance was detected
among species, and species variation was explained, at least in part, by
functional traits such that species with larger root/shoot ratios and
smaller seeds, tended to demonstrate greater herbivory and drought
tolerance. Future research incorporating additional trait analysis and
different stressors could shed light on mechanisms underlying seedling
stress tolerance and clarify whether additivity, as detected in this
study, extends across other combinations of stressors. Such work will
provide needed insights into the regeneration of seedlings in tropical
forests under threats of herbivory and climate change.