Xylem traumatic resin ducts (TRDs) in Douglas-fir form in response to mechanical injury, fire, and root pathogens, but it is unknown if these form at the stem in response to bark-beetle-associated fungi. Meanwhile, TRDs are rarely documented in lodgepole pine. In the southern Rocky Mountains, TRD formation in the two species from sterile (Control) and fungal inoculation treatments (Aggressive, Weak (Douglas-fir only)) were compared; predicting the following: (1) both trees would produce TRDs in response to fungal treatments; (2) in Douglas-fir, Aggressive treatment would promote denser and larger TRDs than Weak or Control treatments; and (3) interspecifically, Douglas-fir would produce a higher density of TRDs than lodgepole pine in Aggressive treatments. Two months post-treatment, the position of TRDs indicated these were only induced on all Douglas-fir treatments. Aggressive and Weak treatments had similar responses, except a second TRD line formed in two Douglas-fir Aggressive treatments. Douglas-fir produced >7× more resin ducts that were twice the size of those in lodgepole pine. Douglas-fir’s stronger induced response indicates better resistance traits against bark beetle fungal associate colonization. Understanding the characteristics of TRD produced in reaction to specific damage in Douglas-fir can improve past disturbance reconstructions and explain interspecific tree response differences conducive to bark beetle resistance.

Bark beetles and their associated fungi kill trees readily, but we often ignore which organism is the leading cause of tree mortality. While phloem feeding beetles inhibit photosynthate transport, their associated fungi block the tracheids disrupting transpiration. Within the family Pinaceae, knowledge of tree physiological decline following bark beetle and associated fungi colonization is limited to the genus Pinus. Here we investigate the physiological response of Pseudotsuga (P. menziesii) to bark beetles or its fungi. We hypothesized that fungi block water transport in Douglas-fir causing faster mortality than by bark beetle activity alone. We successfully lured Douglas-fir beetle to attack a subset of trees in our experimental area using pheromones and compared Beetle-Killed trees with mechanically Girdled, and Control trees. During spring snowmelt, nine months after treatments were applied, Control, Girdled, and five trees that Survived beetle attack had higher transpiration rates and less negative pre-dawn water potential than five Beetle-Killed trees. Declines in transpiration and leaf water potential in our Beetle-Killed trees occurred much earlier than those in studies of beetle-attacked lodgepole pines, suggesting stronger defensive traits in Douglas-fir. Our data suggest that, as in pines, bark beetle-associated fungi are the leading cause of mortality in Douglas-fir beetle-attacked trees.

Fungal and mite associates may drive changes in bark beetle populations, and mechanisms constraining beetle irruptions may be hidden in endemic populations. We characterized common fungi of endemic-level Jeffrey pine beetle (JPB) in western USA and analyzed their dissemination by JPB (maxillae and fecal pellet) and fungivorous mites to identify if endogenous regulation drove the population. We hypothesized that: (1) as in near-endemic mountain pine beetle populations, JPB’s mutualistic fungus would either be less abundant in endemic than in non-endemic populations or that another fungus may be more prevalent; (2) JPB primarily transports its mutualistic fungus, while its fungivorous mites primarily transport another fungus, and (3) based on the prevalence of yeasts in bark beetle symbioses, that a mutualistic interaction with blue-stain fungi present in that system may exist. Grosmannia clavigera was the most frequent JPB symbiont; however, the new here reported antagonist, Ophiostoma minus, was second in frequency. As hypothesized, JPB mostly carried its mutualist fungus while another fungus (i.e., antagonistic) was mainly carried by mites, but no fungal transport was obligate. Furthermore, we found a novel mutualistic interaction between the yeast Kuraishia molischiana and G. clavigera which fostered a growth advantage at temperatures associated with beetle colonization.