How carbohydrate reserves change in conifers during drought and bark beetle attacks are poorly understood. We investigated changes in carbohydrate reserves and carbon-dependent terpene defenses in ponderosa pine trees 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. Our results showed that trenching did not influence carbohydrates whereas both biotic challenges reduced amounts of starch and sugars of trees. However, only the trenched-beetle attacked trees depleted carbohydrates and died within the first year of bark beetle attacks. While live trees contained higher carbohydrates than dying trees, amounts of constitutive and induced terpenes produced did not vary between live and beetle-attacked dying trees, respectively. Based on these results we propose that reallocation of carbohydrates to terpenes during the early stages of beetle attacks is limited in drought-stricken trees, and that the combination of biotic and abiotic stress leads to tree death. The process tree death is subsequently aggravated by beetle girdling of phloem, occlusion of vascular tissue by bark beetle-vectored fungi, and potential exploitation of host carbohydrates by beetle symbionts as nutrients.

Plants employ a diverse set of defense mechanisms to mediate interactions with insects and fungi. These relationships can leave lasting impacts on host plant genome structure such as rapid expansion of gene families through tandem duplication. These genomic signatures provide important clues about the complexities of plant/biotic stress interactions and evolution. We used a pseudo-backcross hybrid family to identify Quantitative Trait Loci (QTL) controlling associations between Populus trees and several common Populus diseases and insects. Using whole genome sequences from each parent, we identified candidate genes that may mediate these interactions. Candidates were partially validated using mass spectrometry to identify corresponding QTL for defensive compounds. We detected significant QTL for two interacting fungal pathogens and three insects. The QTL intervals contained candidate genes potentially involved in physical and chemical mechanisms of host-plant resistance and susceptibility. In particular, we identified overlapping QTLs for a phenolic glycoside and Phyllocolpa sawfly abundance. There was also significant enrichment of recent tandem duplications in the genomic intervals of the native parent, but not the exotic parent. Tandem gene duplication may be an important mechanism for rapid response to biotic stressors, enabling trees with long juvenile periods to reach maturity despite many coevolving biotic stressors.