The effects of arthropod galling on the physiological function of arctic willows

Background/Question/Methods

The increase in abundance of arctic deciduous plants such as dwarf birch (Betula nana) and willows (Salix sp.) is coincident with warming trends and will likely contribute to changes in regional carbon dynamics. However the influence of herbivores, particularly specialist herbivores such as gall-inducing arthropods, on plant-level carbon uptake in arctic deciduous species remains poorly studied. Hence, significant natural-enemy impacts on fundamental processes such as (leaf-level) carbon gain and metabolite allocation have been overlooked. We examined the impacts of galling, a widespread phenomenon among arctic willows, by two eriophyoid mites on a suite of ecophysiological traits in leaves of two willows (Salix pulchra Cham. and Salix glauca L.) in the Alaskan arctic tundra. Maximum photosynthetic capacity (A_max), stomatal water conductance (g_s), water use efficiency (WUE) and photosystem II efficiency (F_V/F_M) were measured in the field on galled leaves, neighbouring ungalled leaves and gall-free leaves from gall-free (control) plants. In addition, leaf pigment concentrations were assessed from leaf spectral reflectance measurements and foliar metabolites (glucose, fructose, sucrose, starch) along with C:N content were determined across the three treatments from leaves collected in the field.

Results/Conclusions

Galled leaves showed significant declines in A_max, F_V/F_M, g_s and WUE in S. pulchra leaves and in A_max and F_V/F_M of S. glauca leaves.  A_max in galled leaves decreased by 40% and 35% compared to control leaves from gall-free plants and 55% and 38% compared to neighbouring gall-free leaves of S. pulchra and S. glauca respectively, suggesting compensation by neighbouring leaves on stems with gall-infected leaves. Galled leaves in both species showed similar decreases in g_s and WUE compared to neighbouring gall-free leaves.  Depression in PS II efficiency in response to galling suggests that the impact of galling on the photosynthetic light harvesting apparatus might in part account for the suppression of stomatal activity seen in galled leaves. Mite gall-infested tissue had significantly higher concentrations of glucose and fructose compared to gall-free leaves, suggesting a preference for these over other soluble metabolites (sucrose and starch). Increased anthocyanin and decreased chlorophyll concentrations were seen in galls of both species, and likely contributed, at least in part, to the observed decreases in photosynthesis. Based on our findings, we suggest that galling mites - ubiquitous but poorly examined in the tundra - have significant impacts on photosynthetic processes and are likely to affect whole-plant functioning in arctic willows.