Conifers are equipped with an array of complex defense mechanisms to combat insects and pathogens, but forests are now facing new challenges associated with anthropogenic disturbances including increases in the occurrence of drought and altered pest population dynamics. Secondary plant metabolites (SPMs) play a critical role in plant resistance to both abiotic stress and biotic agents, mediating a variety of interactions between trees and pests. Despite their important role in tree resistance, we lack a comprehensive understanding of how SPMs in mature conifers change as drought progresses, thresholds at which investment in SPMs becomes limited, and how altered chemical defenses may translate into insect host choice and invasion success.
Monoterpenes are the dominant SPMs in conifers and are part of an intimate relationship with one of the most destructive conifer pests--bark beetles. The impact of monoterpenes on bark beetle behavior is complex and is impacted in part by environmental effects on tree metabolism, including heat and drought stress. Here, we quantify the dynamics of pinyon pine monoterpene chemistry in the needles, phloem, and volatile emissions as a function of predicted and prolonged drought stress implemented at the SUrvival MOrtality (SUMO) experimental site at the Los Alamos National Laboratory.
In both woody and needle tissues, total monoterpene concentrations in ambient trees were not significantly different from those observed in trees exposed to heat stress, but trees experiencing drought showed higher total concentration while heat + drought trees were observed to have the highest levels (2-fold increase over ambient). These treatment effects were sustained over a two-year period despite seasonal variation in tree water status; however, total concentration in the xylem and phloem were closely coupled to tree water potential and treatment effects took longer to manifest relative to the needles. Individual compounds responded differently to the treatments, potentially suggesting cyclase-level enzyme regulation, with α-pinene dominating the total monoterpene concentration dynamics. Mass resin flow measurements show heat + drought trees having higher resin flow in 24 h relative to ambient (P=0.03), consistent with both the needle and phloem monoterpene data. Furthermore, we applied community diversity approaches to assess how volatile “phenotypes” differ between treatments. Diversity profile plots show the heat + drought profile to be the most dissimilar relative to the emissions from the other treatments. These results have important implications for pinyon-bark beetle interactions during drought and provide a missing link between drought-induced physiology and insect behavior.