Aspen fire ecology and climate change: Disturbance dynamics in an uncertain future

Aspen is the only deciduous tree species with substantial extent across much of the western United States, and decline of aspen woodlands due to climate change would likely alter ecological processes and result in cascading losses of animal and plant species in the region. Recent aspen mortality events in the western U.S. might be harbingers of future aspen disturbance dynamics under climate change, and some climate-based vegetation models suggest substantial loss of aspen over the next several decades. However, the role of future disturbance, particularly fire, complicates our ability to predict aspen distribution and productivity under climate change. Aspen is typically seral to conifer tree species, re-sprouts prolifically after fire and, thus, is often considered to be a fire-dependent species. However, some aspen populations may be stable in the absence of fire, and a recent review suggests at least five aspen fire regime types are possible. Thus, although hotter and drought-prone future climates may increase aspen mortality in some locations (e.g., at lower elevations), these same climate conditions are also predicted to increase wildfire activity across much of the western United States, and this could be beneficial to aspen in other locations (e.g., at higher elevations where conifers now dominate). Here, I present an overview of aspen-fire dynamics in the western U.S., including describing different aspen fire regimes, and discussing how future climate-fire interactions might affect aspen populations. I also present preliminary modeling results as a case study for potential changes in aspen distribution and abundance under alternative future climate scenarios and fire regimes in the northern Great Basin. I then discuss the relevance of this research to aspen restoration efforts, highlight key areas of scientific uncertainty, and suggest high-priority research areas

Effects of blowdown, salvage logging, and wildfire on regeneration and fuel characteristics in Minnesota’s forests

The patchiness resulting from a sequence of recent disturbances – blowdown, salvage logging, and wildfire – provided an excellent opportunity to assess the impacts of these disturbances, singly and in combination, on (1) wildfire severity (2) post-disturbance vegetation responses, (3) ecosystem carbon stocks, and (4) soil mercury (Hg) accumulation or loss in jack pine (P. banksiana) forests of northern Minnesota. Considering issue 1, our results suggest that salvage logging reduced the intensity (heat released) of the subsequent fire. However, its effect on severity (impact to the system) differed between the tree crowns and forest floor. Considering issue 2, our results suggest that disturbance combinations (blowdown and fire with and without salvage logging) resulted in similar woody plant communities, largely dominated by trembling aspen (Populus tremuloides). By comparison, areas experiencing solely fire were dominated by jack pine regeneration, and areas experiencing solely blowdown were dominated by regeneration from shade-tolerant conifer species. Considering issue 3, our results suggest that various disturbances cause dramatic shifts in the proportion of carbon in different pools, suggesting that potential increases in multiple disturbance events may represent a challenge for sustaining ecosystem carbon stocks. Considering issue 4, our results suggest that when disturbance combinations are considered in addition to singular disturbances, unexpected consequences in atmospheric Hg emission, soil Hg accumulation, and risk to aquatic biota may result. Taken together, these results lend themselves to improved forest management strategies, particularly regarding post-disturbance harvesting prescriptions

Appendix A. Keys for tree crown and forest floor fire-severity index classes.

Keys for tree crown and forest floor fire-severity index classes