Drivers of forest change in the Greater Yellowstone Ecosystem

A nuanced examination of high elevation forests from the byproduct of three consecutive years of dendro-ecological workshops in the Northern Rockies.

The Beartooth mountains in the distance

In the summer of 2017, I traveled a few hours southeast of Bozeman, MT, to a remote field station just north of the small, eclectic town of Cody, WY. My mode of transport was an old Prius, and my driver was an ex-Mormon. He had an imposing beard and long hair and, as it turned out, was kind of a hippie. My new friend and I were headed to the forests of the Beartooth mountains for the next week to learn the science and methods of ecological dendrochronology, or dendroecology.


Tree cores drying and held down by masking tape

Dendrochronology is the science of dating and examining the growth of trees based on their growth patterns which can be measured in their annual tree rings. Dendroecology emphasizes the ecological implications of using such a powerful tool, such as examining how the growth of different species of trees has changed through time. In 2017, my new hippie friend and I were part of the first group of three consecutive field campaigns over the next three years to collect data to understand how these montane forests are changing while also learning how to extract tree cores, sand them, and count their rings using a microscope.



Example of dating a tree using software

My group, in particular, sought to explore the role of drought and tree mortality in a high-elevation environment. And just recently, five years later, the data from the three consecutive field seasons have been compiled and published in the Journal of Vegetation Science thanks to the persistence and hard work of the lead author, Erika Blomdahl.


From the three years of field campaigns, we found the upslope movement of the treeline and a shift from grassy meadows at the tops of mountains to forest stands, starting around the 1950s. The two species that had made their residence in the former meadows were lodgepole pine and whitebark pine. Notably, we found that subalpine fir lower in elevation replaced lodgepole, a common indication of forest succession in the absence of wildfire in these ecosystems. However, that lodgepole had more seedlings higher upslope than mature trees, indicating that the species is continuing its upward march.


Study region and transect of data collection by year. My cohort was the 2017 year

Perhaps most interesting was that whitebark pine was distributed across all elevations. This is an uncommon finding. Whitebark pine is the charismatic tree of the Northern Rockies but is disappearing in many places due to climate change. Indeed, many other studies indicate that we may lose whitebark pine as the climate warms and the trees eventually run out of upslope-runway. However, we note that it may also be competitive at lower elevations, which bodes well for its future longevity. In fact, we find that whitebark pine’s main threat may be mountain pine beetle, evidenced by whitebark pine’s disproportionate mortality during a large outbreak between 2008-2012 that was synchronous with drought during that period.


Overall, our findings hint that these aging forests may harbor an increased susceptibility to beetle-caused mortality in the future. But our paper also provides evidence of hope for whitebark pine forests, counter to the common narrative describing these forests. Trees in the study area responded individually to global change stressors, which acted on these forests in complex ways and led to both ecotone shifts and stability. As such, this study highlights the interplay between succession, disturbances, and climate-related growth in driving forest compositional change in these high-elevation forests.


I should also mention that the event is called the North American Dendroecological Fieldweek (NADEF), and is still ongoing every year (but in different forests across the US). NADEF is led by some of the best in the field of dendrochronology (including those who actually wrote the book on the topic!) I would HIGHLY recommend it to anyone interested in using dendrochronological tools to better understand the environment through the lens of some of Earth’s best teachers: the trees. You may meet your very own ex-Mormon hippie friend and get a paper out of the experience in the process!

Group photo of my 2017 cohort at the top of our transect

Authors: Erika M. Blomdahl, James H. Speer, Margot Kaye, Nicole E. Zampieri, Maegen Rochner, Bryce Currey, Denise Alving, Gabriel D. Cahalan, Ben Hagedorn, Hang Li, Rose Oelkers, Lissa Pelletier, Ichchha Thapa, Kevin Willson, Brian D. Woodward, R. Justin DeRose Online Link: https://onlinelibrary.wiley.com/doi/10.1111/jvs.13141 Access to the PDF: Here