Background/Question/Methods Physiological changes in conifers during drought and insect attacks are poorly understood, particularly how non-structural carbohydrate (NSC) reserves and defense metabolites interact in trees confronted with multiple stressors that can individually cause mortality. We investigated changes in NSC reserves and carbon-dependent terpene defenses in ponderosa pine trees that were experimentally subjected to two levels of drought stress (via root trenching) and two types of biotic challenge treatments (pheromone-induced bark beetle attacks or inoculations with crushed beetles that include beetle-associated fungi) for two consecutive years. We hypothesized that the combination of drought and attacks by the mutualistic bark beetle-microbe more rapidly deplete NSC reserves in tree stems than either stress agent alone, thereby eliciting tree mortality. We addressed the following four questions: (1) Do drought stress, bark beetle attacks, and inoculations affect NSC reserves? (2) Does drought magnify the effect of bark beetle attacks on NSC reserves and terpene concentrations? (3) Do changes in NSC reserves alter terpene composition? (4) Can interactions between NSCs and terpenes in dying trees explain their mortality? This is the first field study quantifying seasonal changes in NSCs and defense metabolites through mortality of any conifer species during drought and bark beetle attacks.
Results/Conclusions Trenching increased water stress and reduced photosynthesis but did not influence NSCs whereas both biotic challenges reduced amounts of starch and sugars of trees. However, only the combined trenched-bark beetle-attacked trees depleted NSCs and died during the first year of attacks. Three possible mechanisms can explain these results. First, girdling tree phloem and xylem by beetles and symbiotic fungi likely disrupted carbon and water transport in the tree stem, promoting cellular dehydration. Second, once inside the tree, the fungal hyphal growth in the phloem and xylem became a major carbon sink as the growth of these fungi requires carbon, which are either directly acquired from tree carbohydrate pools or the carbon released from the degradation of terpenes. Finally, dying trees had higher concentrations of terpenes than live trees, suggesting dying trees remobilized more NSCs to terpene biosynthesis. This reallocation likely reduced the NSC reserves, potentially resulting in carbon starvation in the tree stem. Together these mechanisms likely depleted the NSC reserves in dehydrating cells of bark beetle attacked trees, resulting in tree mortality. These results support our hypothesis that bark beetle attacks can deplete tree NSCs by limiting carbohydrate production and transportation and promoting the production of carbon-based defense metabolites.