Drought-Mediated Changes in Tree Physiological Processes Weaken Tree Defenses to Bark Beetle Attack

Interactions between water stress and induced defenses and their role in tree mortality due to bark beetlesare poorly understood. We performed a factorial experimenton 48 mature ponderosa pines (Pinus ponderosa) in northern Arizona over three years that manipulated a) tree water stress by cutting roots and removing snow; b) bark beetleattacks by using pheromone lures; and c) phloemexposure to biota vectored by bark beetlesby inoculating with dead beetles. Tree responses included resin flow from stem wounds, phloemcomposition of mono- and sesqui- terpenes, xylem water potential, leaf gas exchange, and survival. Phloemcontained 21 mono- and sesqui- terpenes, which were dominated by (+)-α-pinene, (−)- limonene, and δ-3- carene. Bark beetleattacks (mostly Dendroctonus brevicomis) and biota carried by beetles induced a general increase in concentration of phloemmono- and sesqui- terpenes, whereas water stress did not. Bark beetleattacks induced an increase in resin flow for unstressed trees but not water-stressed trees. Mortality was highest for beetle-attacked water-stressed trees. Death of beetle- attacked treeswas preceded by low resin flow, symptoms of water stress (low xylem water potential, leaf gas exchange), and an ephemeral increase in concentrations of mono- and sesqui- terpenescompared to surviving trees. These results show a) that ponderosa pine can undergo induction of both resin flow and phloem terpenesin response to bark beetleattack, and that the former is more constrained by water stress; b) experimental evidence that water stress predisposes ponderosa pines to mortality from bark beetles.