Bark beetles of the genus Dendroctonus are native pests in many coniferous ecosystems where they are important drivers of nutrient cycling and habitat formation. Bark beetles destroy significant portions of a tree’s phloem building egg-laying galleries, while plant-pathogenic fungi introduced by them infiltrate and occlude the sapwood. Both of these actions can potentially kill the tree; however, it has been shown that blue-stain fungi contribute substantially to a rapid decline in stem water potential and transpiration in lodgepole pine (Pinus contorta). This study aims to determine whether this also occurs in Douglas-fir (Psuedotsuga menziezii) after attack by its primary bark beetle enemy, the Douglas-fir beetle (D. psuedotsugae). In the spring of 2018, we selected a cluster of 30 trees within a 60m diameter plot assigning three random treatments: control trees, girdled trees, and trees baited with an attractant for Douglas-fir beetle. Measurements of transpiration, using Granier sap flow sensors, and mid-canopy pre-dawn water potential were recorded before and after beetle attack through November 2018. Data will continue to be collected starting early March 2019, before trees become active again. We hypothesized that transpiration levels and water potentials in beetle-attacked trees would decline rapidly as fungi infiltrated the sapwood.
Results/Conclusions
Only a few attacked trees showed changes in transpiration by late October, when soil moisture levels were lowest. This was contrary to our previous findings where transpiration declined rapidly in all attacked lodgepole pine following a natural mountain pine beetle attack. However, after late October precipitation events, the control and girdle treatments showed a larger increase in transpiration to the newly available soil moisture than did Douglas-fir beetle attacked trees, suggesting that blockage of xylem by blue-stain fungi was affecting water transport. These results are preliminary, as data collection will continue following winter dormancy. We expect that the control and girdle treatments will recover to normal transpiration levels at the beginning of the growing season while beetle attacked trees will exhibit minimal transpiration rates and extremely negative leaf water potentials.