Origin mechanisms of forest resilience

I use computer simulations and large-scale field experiments to identify the key climate and fire drivers drivers of subalpine tree regeneration. 

    The resilience of forests may erode in the 21st century causing them to transition to alternate states as temperature warms and natural disturbances increase in frequency and severity. There is tremendous interest in determining how and why regional forests may change because of the consequences for carbon storage, climate regulation, biodiversity, and provision of ecosystem services. However, changes in regional forests will likely emerge from aggregate effects of drivers acting on local processes, such as reproduction, seedling establishment, tree growth, and mortality. Thus, research identifying and characterizing fine-scale mechanisms underpinning regional forest responses to changing climate and disturbance is essential. 

    Transition to an alternate state requires an origin mechanism   ̶  an ecosystem process, that when acted on by external drivers (e.g., changing climate or fire regimes), is capable of producing fundamental change in the system. Tree-seedling establishment following stand replacing fires could be an important origin mechanism in conifer forests of western North America because the process is key to postfire resilience of subalpine and boreal forests, shaping stand structure and composition for decades. Robust postfire tree regeneration requires sufficient seed supply and delivery. Fire activity is projected to increase globally and seedling densities may be reduced if burned patch sizes exceed effective dispersal distances or if multiple fires reoccur before trees reach reproductive maturity. When seed is available, changing climate can also shape regeneration outcomes because tree seedlings are very sensitive to environmental conditions. Warming could reduce establishment if severe droughts follow fires.

Selected publications related to these ideas

Hansen, W.D. and M.G. Turner. In Press. Origins of abrupt change? Postfire subalpine conifer regeneration declines nonlinearly with warming and drying. Ecological Monographs.

Hansen, W.D., K.H. Braziunas, W. Rammer, R. Seidl, and M.G. Turner. 2018. It takes a few to tango: Changing climate and fire regimes can cause regeneration failure of two subalpine conifers. Ecology 99:966-977.

Hansen, W.D., W.H. Romme,  A. Ba, and M.G. Turner. 2016. Shifting ecological filters mediate postfire expansion of seedling aspen (Populus tremuloides) in Yellowstone. Forest Ecology and Management 362:218-230.