Synergistic, additive and antagonistic impacts of drought and herbivory on Pinus sylvestris: leaf, tissue and whole-plant responses and recovery

Authors: Bansal, Sheel; HALLSBY, GORAN - Swedish University Of Agricultural Sciences; LOFVENIUS, MIKAELL - Swedish University Of Agricultural Sciences; NILSSON, MARIE-CHARLOTTE - Swedish University Of Agricultural Sciences

Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2013
Publication Date: 3/21/2013
Citation: Bansal, S., Hallsby, G., Lofvenius, M.O., Nilsson, M. 2013. Synergistic, additive and antagonistic impacts of drought and herbivory on Pinus sylvestris: leaf, tissue and whole-plant responses and recovery. Tree Physiology. 33:451-463.

Interpretive Summary: Drought and herbivory are two common stressors in forests that are expected to co-occur more frequently in the future. We hypothesized that when both stressors co-occur, their impact on plant traits would be synergistic (i.e., more than a simple additive effect from each stressor in isolation). We found that the two stressors generally had antagonistic effects (contrary to our expectation), thus demonstrating the need for empirical studies to predict the impacts from multiple stressors on plant performance.

Technical Abstract: Synergies among stressors could result in catastrophic dysfunction of plant and ecosystem processes from an otherwise recoverable situation. To date, studies that have specifically tested for synergies among multiple stressors have almost exclusively focused on the presence or absences of specific stressors, yet the impacts from multiple stressors may be strongly influenced by the relative intensities of each stressor. Moreover, most studies on multiple stressors have occurred in marine or freshwater systems, with relatively few studies in forests. Given that the frequencies and intensities of climatic or anthropogenic stressors are increasing in terrestrial ecosystems, it is critical to improve our understanding of how multiple stressors impact ecologically and economically important species. Here we applied two common stressors in forests, drought and herbivory, at three levels of intensity (control, moderate and severe) in a full factorial design on young Pinus sylvestris seedlings (one of the most widespread and propagated conifers in the world). We assess additive and non-additive (synergistic or antagonistic) effects on a suite of morphological and physiological traits at the leaf-, tissue- and whole plant-level. We additionally investigated if recovery from herbivory was dependent on relief from drought. The two stressors had synergistic impacts on specific leaf area and water use efficiency, additive effects on height and root:shoot ratios, but antagonistic effects on photosynthesis, conductance and most notably, on root, shoot and whole-plant biomass. Nevertheless, the magnitude and direction of the combined impacts were often dependent on the relative intensities of each stressor, leading to many additive or synergistic responses for specific stressor combinations. Also, seedlings did not achieve complete recovery from severe herbivory when drought stress was sustained, thus showing how recovery from one stressor was dependent on relief from a second stressor. Our study illustrates some synergistic effects from co-occurring herbivory and drought stressors, although the majority of impacts were additive or antagonistic, thus indicating that plant performance and ecosystem function may be relatively ‘safe’ from synergies among these stressors.