Ecologists spark hope of protecting western American forests from the worst of wildfires

The western United States is getting hotter and drier, affecting forests. Ecologists have found that the climate is making it less likely that trees can regenerate after wildfires. However, a new study in the Proceedings of the National Academy of Sciences by Kimberley Davis and colleagues shows that changes in forest management could mitigate some of the effects of climate change.

Fires aren’t necessarily bad news for a forest. They can clear undergrowth and allow new seedlings to sprout. Older and larger trees can also survive fires, but things are changing. Hotter and drier conditions in recent decades have intensified how fires burn, resulting in more trees dying. This can result in fewer seeds available for forests to regenerate after wildfires. Even when seeds are available, a warming climate increasingly limits seedlings’ chances to establish and grow.

“Climate change increasingly limits tree establishment after wildfires because seedlings can be killed by hot temperatures and dry conditions,” says the study’s lead author, Kim Davis, in a press release.

The research examined how the severity of a fire – the number of trees it kills – in combination with the climate conditions after the fire affected the chances of tree seedlings regenerating to establish a new forest. It is the most extensive study to date, assessing the regeneration of eight major tree conifer species after 334 wildfires across the West, using information from over 10,000 field plots collected by more than 50 research teams.

Researchers found that warmer, drier conditions over the past four decades have led to a decline in tree regeneration after wildfires. This trend is expected to accelerate in the future. For example, from 1981-2000, 95 per cent of the areas studied had climate conditions suitable for tree regeneration after wildfires, but this is projected to decrease to only three-quarters of the West by 2050 under future climate scenarios.

The most vulnerable forests were in drier regions in the Southwest and California. In comparison, forests in the wetter and cooler regions of the northern Rocky Mountains and Pacific Northwest are still expected to support conifer regeneration in the near-term future.

“The impacts of climate change and wildfires vary across the West, and the large scope of this study allowed us to highlight where these changes are most concentrated and happening first,” says Philip Higuera, a co-author and professor of fire ecology at the University of Montana.

A short term solution is to change forest management tactics to make fire survival a priority. The team conclude that this is critical for almost half of the study region. In these areas regeneration after wildfires is projected to be likely only if future fires burn at lower severities because fewer trees that produce seeds needed for forest recovery are killed.

“We know from prior research that forest thinning and controlled burns in overgrown dry forests effectively reduces fire severity and subsequent tree death,” says study co-author Marcos Robles, lead scientist for The Nature Conservancy in Arizona. “Land managers can’t do much about drought and climate change in the short term, but they can reduce the area in which forests are vulnerable to severe wildfires by accelerating ecological based forest management.”

In the longer term, the ecologists predict that increasingly warm and dry climate conditions will overwhelm any effects due to fire severity or seed availability. In their article Davis and colleagues write:

We project that a substantial portion of the forests in our study region will experience declines in postfire conifer regeneration, which would have major implications for ecosystem structure and function. These results highlight the need to better understand what type of ecosystems will replace these forests when regeneration fails—likely to vary greatly by region—and the implications for carbon sequestration, hydrology, wildlife habitat, and other key ecosystem services on which society depends. Despite the pronounced impact of climate change, the stark contrast in the projections of conifer recruitment probability from the low- and high-severity scenarios emphasize how management actions taken to reduce fire severity can significantly shape postfire vegetation trajectories. Identifying whether, when, and where management intervention is appropriate to resist or direct trajectories of change in these forests will become more critical as wildfire affects more of the landscape each year. Importantly, by elucidating the interactive effects of climate and fire severity, we show that windows of opportunity for management intervention may decline as climate increasingly limits conifer recruitment in the near-term future.

Davis et al. 2023

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Davis, K.T., Robles, M.D., Kemp, K.B., Higuera, P.E., Chapman, T., Metlen, K.L., Peeler, J.L., Rodman, K.C., Woolley, T., Addington, R.N., Buma, B.J., Cansler, C.A., Case, M.J., Collins, B.M., Coop, J.D., Dobrowski, S.Z., Gill, N.S., Haffey, C., Harris, L.B., Harvey, B.J., Haugo, R.D., Hurteau, M.D., Kulakowski, D., Littlefield, C.E., McCauley, L.A., Povak, N., Shive, K.L., Smith, E., Stevens, J.T., Stevens-Rumann, C.S., Taylor, A.H., Tepley, A.J., Young, D.J.N., Andrus, R.A., Battaglia, M.A., Berkey, J.K., Busby, S.U., Carlson, A.R., Chambers, M.E., Dodson, E.K., Donato, D.C., Downing, W.M., Fornwalt, P.J., Halofsky, J.S., Hoffman, A., Holz, A., Iniguez, J.M., Krawchuk, M.A., Kreider, M.R., Larson, A.J., Meigs, G.W., Roccaforte, J.P., Rother, M.T., Safford, H., Schaedel, M., Sibold, J.S., Singleton, M.P., Turner, M.G., Urza, A.K., Clark-Wolf, K.D., Yocom, L., Fontaine, J.B. and Campbell, J.L. (2023) “Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States,” Proceedings of the National Academy of Sciences of the United States of America, 120(11). Available at: https://doi.org/10.1073/pnas.2208120120.