Dominik Kulakowski -- The consequences of changing disturbance regimes for aspen in the western U.S.
Forest ecosystems are being affected by both the indirect and direct results of climate change. Indirect drivers include increasing extent, magnitude and/or frequency of various forest disturbances such as wildfires and insect outbreaks. Direct drivers include droughts and altered temperature and precipitation regimes. Together these drivers are likely to affect the composition of Rocky Mountain forests, including the dominance and extent of quaking aspen (Populus tremuloides). Here I review recent work on these topics and propose possible future consequences for quaking aspen. Warm and dry conditions generally result in increased wildfires and bark beetle outbreaks, particularly in coniferous forests. Both of these disturbances have the potential to increase aspen dominance due to aspen’s ability to regenerate in and dominate post-disturbance environments. Furthermore, compounded disturbances (i.e. two or more disturbances occurring in short succession) also appear to favor regeneration of aspen over conifers and could further increase aspen dominance if compounded disturbances increase with projected shifts in climate regimes. However, aspen demography is contingent on favorable climatic conditions. If the same warm and dry conditions that bring about disturbance regimes potentially favorable to aspen dominance also characterize post-disturbance environments, theses climatic conditions may actually inhibit the ability of aspen to regenerate, grow, and survive. If aspen is able to increase dominance due to changing disturbance regimes and a changing climate, this will likely affect forest susceptibility to subsequent disturbances. As aspen stands are generally more mesic than adjacent conifer stands, the former are less likely to burn. Aspen stands are also less susceptible to bark beetle outbreaks that affect conifers and to wind disturbances. Thus any change in the amount of aspen in the landscape has the potential to feedback to the overall disturbance regime at broad scales. The consequences of changing disturbance regimes for quaking aspen in the western U.S. are likely to be complex and contingent on post-disturbance climatic conditions as well as on feedbacks among climate, disturbances, and forest species composition.