Combined drought and bark beetle attacks deplete non-structural
carbohydrates and promote death of mature pine trees
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.