Broader consequences of variable snow depth on plant-insect interactions with Artemisia tridentata

Background/Question/Methods

Plant-herbivore relationships may be the most common type of biotic interaction on earth. Selection pressure from these interactions has shaped, via co-evolution, the communities as they exist today. Even subtle changes in abiotic conditions can alter these interactions and thus change both the plant and herbivore communities, particularly in the case of insect herbivores.  Drastic changes to some abiotic conditions are predicted to occur as a result of our rapidly changing climate. It is important to understand how such changes will directly impact plant and insects, but also to study how these interactions might change. The ecosystem of the eastern Sierra Nevada is shaped by limited precipitation, which comes predominantly as snow; this climate variable is predicted to change in the future, although the direction of change is unknown.  Great Basin sagebrush, Artemisia tridentata, a dominant shrub in this ecosystem, is host to many species of gall-forming insects and other arthropods. Artemisia-dwelling insects represent a large portion of the insect biodiversity in this system.  The suitability of A. tridentata as a host for these insects will likely change under future climate scenarios. The purpose of our study was to observe changes in A. tridentata, as well as the insect community, across experimental snow depth treatments.

Results/Conclusions

To determine the effects of increased or decreased snow on plants and insects, we surveyed the galling and herbivory patterns on A. tridentata in areas with experimentally increased or decreased snow. On these same plants, we quantified plant characteristics including size, leaf area, photosynthesis, water potential, and leaf C:N. We found surprisingly high diversity of gall morphotypes (> than 20), as well as several species of aphid herbivores including three species tended by ants. Gall abundance increased with decreased snow.  With any manipulation of snow depth, leaf damage from aphid herbivores increased, and leaf litter volume also increased. Additionally, the aphid-ant mutualism appears to break down with any change to snow regime. Plant characteristics associated with rapid growth are greater on plots with increased snow depth (larger size, increased water potential and photosynthesis, greater seed production, higher C:N), and lowest on plots with the least snow pack. These patterns suggest that both plants and insects are sensitive changes in the snow regime. Changes associated with an altered plant-insect interaction may have far-reaching impacts which shape both the plant and insect community.