By Kristen Pelz
This past summer, I attended a meeting about aspen ecology and management. I presented my thoughts about how aspen (Populus tremuloides) will respond to widespread lodgepole pine (Pinus contorta) mortality caused by mountain pine beetle (MPB) (Dendroctonus ponderosae). I was nervous and excited to present to a small group of accomplished scientists and high-level managers from around the country. I thought I had a complete theory about why aspen had not increased in the way we had expected following a 1980s MPB outbreak that I studied for my Master’s thesis. But, meeting and talking with many people really broadened my perspective and reminded me—once again—about the importance of avoiding tunnel vision in research.
So, let me tell you my story. First, a little background: Quaking aspen and lodgepole pine co-occur on millions of hectares throughout the Rocky Mountains. Both species are fast-growing, sun-loving tree species that regenerate prolifically following fires and other intense disturbances (like clear-cut logging). Mountain pine beetle has killed pine on nearly 25 million hectares in western North America (Raffa et al. 2008). Many have speculated that this huge disturbance will favor aspen because of increased light and resource availability in forests (Kaufmann et al. 2008, Diskin et al. 2011).
Part of my Master’s research set out to test this idea. Did aspen dramatically increase in the decades following past MPB? I re-visited areas where MPB killed the majority of trees in the 1980s and compared current conditions to the 1980s forest. I did not find the explosion of aspen that many had expected—though there were pockets of post-MPB aspen regeneration (Pelz and Smith 2012). It was definitely a surprise.
Why? Where were the aspen that we thought would have grown in the 30 years following the opening of the forest canopy? After doing extensive research about the physiology of aspen sprout initiation, I felt that there was no clear mechanism for MPB to induce aspen sprouting. Some sort of damage to aspen seemed necessary to get a large sprouting response (sensu Frey et al. 2003). Perhaps aspen regenerated only where aspen mortality or damage occurred? This could explain why there were patches of aspen regeneration following 1980s lodgepole pine mortality but why the response was not pervasive.
This was basically the argument I presented at the meeting I attended in June. But, at the end of my talk a prominent aspen researcher asked, “What about browsing?” Well, I responded, I didn’t think browsing was a problem in my study area. I had discussed how browsing by animals can kill aspen sprouts in my talk, but I had discounted the idea of this being a problem in my sites because I hadn’t observed browsing damage. I was embarrassed to have such a lame response, but I remained convinced that browsing wasn’t a problem in my study area for some time.
In part, I didn’t question my conviction at first because my questioner was from Utah. Utah has a notorious over-grazing problem, where, by autumn, everything edible will have been eaten by cattle or sheep. Browsing seems to be the number one problem for aspen in Utah—if managers took away the cattle and sheep, it would do the tree species a great service. My study area, in central Colorado, didn’t seem to have this problem. I felt that my questioner was projecting his experience in another region on to my study sites, which I felt I knew best.
However, as time passed, I realized I had been the one suffering from tunnel vision. Coincidentally, my fiancé’s father had helped his friends run cattle in my study area during the 1980s and 1990s. This was something I had known, but not considered fully, until after I attended June’s aspen ecology meeting. I had never seen cattle in my study areas, so I didn’t think about their effects. The more I thought about it, though, the more important these cattle seemed—they, along with elk and deer, could have caused substantial aspen damage in the 1980s and 1990s. Despite the lack of browsing evidence today, it seems likely that past browsing could have had a major effect on aspen densities in this area. My change in perspective, however humbling, has led me in exciting new directions. I am looking more carefully at the effects of browsing on aspen as part of my PhD.
Have you had “tunnel vision” experiences in your own research? If so, how did you realize, and come to terms with, what you were neglecting? How have you used this to move your work forward? I am sure this will happen to me many more times in the future – but I am also confident there will be someone to remind me (hopefully gently…) about what I am missing.Citations: Diskin, M., Rocca, M. E., Nelson, K. N., Aoki, C. F., Romme, W. H., 2011. Forest developmental trajectories in mountain pine beetle disturbed forests of Rocky Mountain National Park, Colorado. Can. J. For. Res. 41, 782–792. Frey, B., Lieffers, V., Landhausser, S., Comeau, P., Greenway, K., 2003. An analysis of sucker regeneration of trembling aspen. Can. J. For. Res. 33, 1169–1179. Kaufmann, M., et al., 2008. The status of our scientific understanding of lodgepole pine and mountain pine beetles – a focus on forest ecology and fire behavior. GFI technical report 2008-2. The Nature Conservancy, Arlington, VA. Pelz, K. A., Smith, F. W., 2012. Thirty year change in lodgepole and lodgepole/mixed conifer forest structure following 1980s mountain pine beetle outbreak in western Colorado, USA. For. Ecol. Manage. 280, 93–102.