Physiological responses of oaks to extreme water deficit, fungal pathogens, and insect herbivores through historic drought conditions in California
California is on the verge of entering its fourth year of what has been the most severe drought in over 160 years of state meteorological records and an estimated 1200 years of recent dendroclimatological reconstructions. This drought has provided a unique opportunity to investigate the physiological responses of different plant species to exceptionally severe water deficit, and to better understand the implications of such responses on the process of carbon starvation, hydraulic failure, and drought-induced mortality. Since negative interactions with natural enemies such as fungal pathogens and insect herbivores can exacerbate these processes, their physiological costs on plant water and carbon relations are critical to identify and quantify. In the summer of 2014, I surveyed several coexisting populations of blue oak and valley oak (Quercus douglasii & Q. lobata, Fagaceae) that spanned a gradient of drought severity from Sonoma County to San Luis Obispo County, CA. To address the question of whether the prevalence and overall impact of foliar disease and herbivory is correlated with increased water deficit, I took the following measurements across 87 trees: pre-dawn and midday xylem pressure potential, stomatal conductance, leaf area, specific leaf area, percent foliar disease, and percent foliar herbivory.
After coupling an analysis of n=3470 measurements of leaf size, shape, disease (necrotic and chlorotic tissue area), and herbivory (tissue area lost to insect herbivores) with n=348 water potential measurements and n=174 measurements of stomatal conductance, I found strong positive correlations between natural enemy pressure and indicators of drought stress. This was most pronounced in measurements of foliar disease, such that almost all blue oak and valley oak individuals across my three sites exhibited increased percent disease with decreasing pre-dawn and midday water potential through the summer. Midday stomatal conductance also tended to increase with increasing disease burden, suggesting that foliar disease may reduce the ability of plants to effectively regulate water loss. Taken together, my results suggest that foliar pathogens and insect herbivores have the potential to interact with and exacerbate the negative effects of severe water limitation on plant performance and water-carbon relations. Ongoing and future work includes experimental greenhouse and common garden approaches to further disentangle the relative physiological and metabolic impacts foliar pathogens and insect herbivores have on different plant species, which will allow for novel inferences into the importance of biotic interactions in mediating plant survival and mortality through rapidly changing environmental conditions.