Large and severe wildfires, like those recently experienced across much of the West, are expected to become more common as the climate changes. In addition to the obvious damage and disruption these wildfires wreak on human communities, there is a lesser-known side effect of growing concern to managers and researchers — that some forests may not be able to recover from more intense wildfires under hotter and drier conditions brought on by climate change.
We know that more frequent and severe wildfires can prevent tree seedlings from establishing between fires, and that drought conditions after fire also make it hard for seedlings to root and grow. However, the extent to which patterns of large-scale, recurring drought affect forest recovery after fire have remained largely unexplored, until now.
A new NW CASC-funded study, led by former NW CASC Postdoctoral Fellow Dr. Caitlin Littlefield, with NW CASC Researcher Dr. Solomon Dobrowski and co-authors at the University of Montana, found that recurring drought patterns that span the continent have both short- and long-term impacts on the regrowth of ponderosa pine in the Interior West. They also point to a critical period in post-fire recovery, showing that conditions in the year or so following fire can influence the condition of the forest decades later.
This issue of forest recovery after fire is just one piece of a larger climate change phenomenon — ecosystem transformation — which describes ecosystems shifting to new states once they can no longer cope with changing disturbances and climatic conditions caused by climate change. The NW CASC and national CASC network are focusing on this emerging climate issue to better understand what research is needed to help manage our lands in the face of ecosystem transformation.
The researchers uncovered a climate dipole, a recurring fluctuation in temperature and moisture that spans regions, and showed that this dipole affected forest regrowth. This dipole is like a “drought seesaw,” in this case, describing how drought conditions alternate between the Northern Rockies and the Southwest every few years. When the seesaw is active, conditions in the Southwest may be relatively moist while the Northern Rockies are dry — or vice versa.
They found that ponderosa pine regeneration in each region tracks the seesaw, with greater tree regrowth in the relatively moist region and greater seedling mortality in the relatively dry region. In other words: if, and how, a forest recovers from wildfire is largely determined by whether the conditions are relatively moist and cool in the few years following the fire. This is because, in the period after fire, ponderosa pine seedlings need moist soil to germinate and put out roots quickly before they’re outcompeted by other species. Seedlings are also highly susceptible to drought.
The drought seesaw switches on average every three to four years, yet its effects can be long-lasting. Since even a single very-dry year shortly after fire was shown to reduce the longer-term presence of young trees compared to postfire conditions with more moisture, the influence of this drought dipole can affect the cumulative likelihoods of ponderosa pine presence multiple decades after fire.
The results of this study highlight a critical window of opportunity for forest recovery within the first few years after fire. Since this study also confirms overall increases in aridity across the western US, climate-smart management strategies that take advantage of this window of opportunity will become even more important as climate changes.
Littlefield describes, “In some ways, the drought pattern we’re looking at is like a seesaw on an elevator. The Northern Rockies and the Southwest alternate in terms of where there’s drought and where there’s not — that’s the seesaw. But overall, aridity is increasing and making it increasingly hard for tree seedlings to regenerate in both regions — that’s the elevator. It’s important that we get a handle on these patterns so that we can anticipate the impacts of climate change. We’re losing millions of acres of forest to wildfire each year and that trend isn’t likely to stop. But I’m glad that policy-makers are increasingly recognizing the dire need for dry forest restoration and post-fire replanting, and I hope this sort of research can help us develop the best strategies for promoting forest recovery.”
This research was funded by the Northwest Climate Adaptation Science Center, the Aldo Leopold Wilderness Research Institute, NASA and the Joint Fire Science Program.