Many trees in the Rocky Mountains were alive long before I was born- before my grandparents were born. These trees bore witness to an unprecedented rise in CO2 concentrations, and have weathered the associated changes in climate. In the past decade, however, many trees that survived two centuries of climate change have been killed by a tiny insect: the mountain pine beetle.
The mountain pine beetle is a native insect roughly the size of a grain of rice. These tiny insects have co-evolved with native pines, and mountain pine beetle outbreaks tend to occur cyclically every few decades. The recent outbreak, however, is the largest ever observed, and part of the reason for this may be related to climate change.
Rising temperatures can favor the mountain pine beetle in two ways: by inhibiting the defensive mechanism of trees and by increasing overwinter survival of beetles. Trees defend themselves against beetle attacks by producing a sticky substance called pitch, with oozes out of the holes produced by beetles and can block their entry. When a tree is stressed due to drought, however, it cannot produce enough pitch to keep out all of the attacking beetles. Warmer winters also favor the mountain pine beetle because beetle larvae cannot survive particularly cold winters, which usually help to keep their population in check. Increases in beetle outbreaks can be considered an indirect consequence of climate change, and this disturbance is having a far greater impact on forests than the direct effects of climate change alone.
In addition to the large and immediate impact the beetle outbreaks have on forest structure and ecosystem dynamics, this disturbance may also drive a rapid period of species turnover that serves to facilitate climate–driven range shifts. Because trees are long-lived and can tolerate a wide range of climatic conditions, they often persist in areas that are no longer suitable for seedling establishment. Disturbances like fire or insect outbreaks that kill mature trees can usher in a period of rapid change by reducing competition, which allows new species to establish that are better adapted to the current climate.
My dissertation research investigated the potential for mountain pine beetle disturbance to accelerate shifts at the range margins of lodgepole pine (Pinus contorta), a species common throughout the intermountain west. With support from the land managers at Rocky Mountain National Park, I was able to re-sample a large number of vegetation transects that were established twenty years ago. Roughly half of these transects had been impacted by the mountain pine beetle outbreak. This allowed me to compare how species composition had changed over a 20-year warm period both with and without disturbance.
We found that lodgepole pine seedling densities were higher at the upper (cooler) range margin, which suggests a moderate climate change response independent of disturbance. At lodgepole pine’s lower range margin, changes were much more dramatic. Mountain pine beetle disturbance removed most of the mature lodgepole pine trees, and yet few new lodgepole seedlings were found. Instead, the disturbance was associated with a pulse of new Douglas-fir seedlings. This suggests that the current climate may no longer be suitable for lodgepole pine recruitment. The new Douglas-fir seedlings will eventually grow to replace the mature lodgepole pines that died in the beetle outbreak, perhaps weathering another century of rising temperatures.
More information on the research can be found in two related publications:
Renwick, K.M., Rocca, M.E., and Stohlgren, T.J. In Press. Biotic disturbance facilitates range shift at the trailing but not leading edge of lodgepole pine’s altitudinal distribution. Journal of Vegetation Science
Katie Renwick is a postdoctoral researcher with the North Central CSC.