Native bark beetles are important drivers of nutrient cycling and habitat modification in conifer forests. Bark beetles can destroy significant portions of a tree’s phloem while excavating egg-laying galleries, while plant-pathogenic fungi introduced by the beetles effectively inhibit water transport in the xylem. While it has been shown that blue-stain fungi contribute to the decline in stem water potential and transpiration in lodgepole pine in under a month, a summer attack on Douglas fir in 2018 found only slight variations in transpiration several months later. This suggests there may be potential differences between tree species and their respective bark beetles and fungal associates. This study tested the hypothesis that Douglas fir mounts a stronger defense response to blue-stain fungi infection than does lodgepole pine by producing more resin ducts and higher terpenoid concentrations. Thirty Douglas firs and twenty lodgepole pines were inoculated with representative fungi to simulate a mass attack by bark beetles. Tissue samples, including the current year’s growth ring, were collected at four time points following inoculation, alongside pre-dawn water potential and soil moisture measurements. Samples were analyzed via light microscopy and gas chromatography to determine traumatic resin duct formation and terpenoid concentration.
Results/Conclusions
Preliminary analysis of the tissue samples did not suggest large differences in traumatic resin duct production among treatment groups. Data from pre-dawn water potential measurements also did not differ significantly (p = 0.60) among treatment groups, correlating strongly (R2 = .99) with soil moisture. These results do not suggest a strong, rapid pathogenic blockage of the xylem by the introduced fungi, rejecting our hypothesis. This may indicate a stronger than expected defense response from both tree species, possibly due to differences in environmental conditions between a 2013 lodgepole pine study and the current growing season. Another factor that may be contributing to these observations is the difference in observed fungal communities between the sampled beetles and the expected populations. Occurrence of blue-stain fungi was lower in the beetle population that attacked our Douglas fir trees in the 2018 growing season when compared to beetles collected in other areas and in previous seasons.