Forests cover ~30% of the Earth’s land surface and provide many important ecosystem services. Global change has impacted forests through changes in disturbance regimes; in the western US, change has resulted in extensive and severe bark beetle outbreaks. These outbreaks have the potential to impact forest function through the selection of certain phenotypes. In this study, we asked: (1) How does the growth of beetle-killed versus non-beetle-killed lodgepole pine (Pinus contorta) differ?, (2) How do growth differences change across different elevations and aspects?, and (3) How do observed differences in growth between beetle-killed and non-beetle-killed trees relate to climate variables? We collected 482 tree cores from 12 plots across three elevations on 800 m transects in the Beaverhead-Deerlodge National Forest, Montana according to aspect (N/S). At each plot, cores were collected from both beetle-killed and non-beetle-killed trees. All cores were processed following standard dendrochronological techniques and growth between 1950 and 2005 was estimated as basal area increment (BAI). Differences between mortality categories, and across elevations and aspects, were analyzed using analysis of variance both overall and during drought periods. Relationships between climate variables and BAI were analyzed using general linear mixed effects models.
Results/Conclusions
Preliminary results suggest higher growth in beetle-killed trees for the duration of the BAI time series. Growth differences between beetle-killed and non-beetle-killed trees were substantially larger at low elevation sites. Mid and high elevation sites showed smaller differences between mortality categories and no significant difference between the two elevations. Growth differences did not vary by aspect. Mixed effects models created for each aspect-elevation combination demonstrated that growth was impacted by late summer temperatures at all plots. High climatic water deficit (CWD) and low annual precipitation negatively impacted growth at low elevations, and higher fall minimum temperatures positively impacted growth at mid and high elevations. Models of growth differences at each plot demonstrated that late summer temperatures and CWD were important for determining the difference between beetle-killed and non-beetle-killed growth.
These results suggest that faster growing trees may be more impacted by drought conditions and therefore more susceptible to bark beetle attack. Overall, these results suggest phenotypic selection for slower-growing trees in the Beaverhead-Deerlodge National Forest in Montana due to a severe bark beetle outbreak. Low elevations showed greater differences in BAI and greater impacts of drought-related conditions on BAI, suggesting that phenotypic selection may be stronger in more drought-stressed environments.