How a Changing Climate is Affecting Forests in the Western United States

Katarina Samurović

Updated:

Scientists have long warned that climate change will change the face of the Earth – not only due to disastrous events such as global sea-level rise but also through shifting and transforming the Earth’s biomes eco-regions.

What most people don’t realize is that the process is already underway.

The forests in the Western United States consist mostly of various native conifer trees spread across the vast and rugged land. From the Sierras to The Rockies to Alaska, the area holds some of the most iconic landscapes of North America.

Also, Western forests are notoriously fire-prone, especially in their southern range. Still, no matter how bad the fire, the forests were eventually able to regenerate. 

In the process of post-fire succession, at first, the earlier (pre-forest) vegetation type would return to the site. That primary recolonization would then allow for a series of succession stages, allowing the new tree saplings to sprout, grow, and eventually mature in the process. Upon the return of the original tree cover, the succession would come a full circle, and the new-old plant community was considered “stable.”

However, what happens when temperatures start to rise above the ecological sweet spot of typical Western conifer species – all the pines, firs, and spruces?

It turns out that the forests we know them might not be making a comeback – at least not a uniform one.

The failure of post-fire forest regeneration

A study from 2019 led by Kimberley Davis, a plant ecologist at the University of Montana, already declared there are now many sites where the new, global warming-enhanced conditions fail to support the development of young pine trees. 

While adult trees can still handle the harsher conditions due to their extensive root systems, thick protective bark, and the mutual shade, young trees do not have such luxuries – especially when they find in themselves sprouting on the fire-cleared terrain. 

A pine forest with a dirt road running through it in Oregon.
A pine forest in Oregon. Photo: USGS, public domain.

In these areas, the “new normal” summer temperatures can directly weaken and kill the seedlings before they develop their protective bark. Continuous dry spells also rid them of the needed water – and they have to rely on precipitation rather than the underground sources. Lastly, with the “parent” tree generation now gone because of the fires, there is nothing to shade them in the harsh summer sun, further amplifying the local heat and the drought.

The consequences are straightforward. Once the post-fire seedling generation is gone – or fails to sprout – there is no new production of seeds and no sufficient amount to replenish the forest. Even if there were, the seedlings would likely fail to thrive in the new conditions.

Even if forests do return, the recovery is not likely to be uniform. Kyle Rodman, a research scientist at Northern Arizona University studying forest recovery, has led the study published under the title “A changing climate is snuffing out post-fire recovery in montane forests” in 2020.

His team found that, in general, the south-facing slopes and the lowland forest fringes of the Rocky Mountains are least likely to support new young pine and fir trees because those are parts of forests with the highest temperatures. Instead, the forest returns to the shaded northern sides of the mountain while the treeline moves to higher, cooler elevations.

Then, the shift occurs.

A view across a forest with dead trees from a prolonged drought in California.
View of dead trees from the Colony Mill Trail in Sequoia National Park, taken in 2015 during California’s extreme drought. Photo: Nate Stephenson, USGS Western Ecological Research Center, public domain.

Instead of the native species we’re used to, those better adapted to deal with hotter and drier conditions take over.

Once again, it should be emphasized that it is not the fire that directly kills off the western forests. The changing climate in these areas is already inhospitable to the new generation of conifer seedlings. By destroying mature trees that were still able to cope, the fires simply make new climatic conditions more apparent – and the novel plant communities more in tune with them.

These pioneer plant communities differ across the West. Still, there are a couple of general rules: 1) the communities of hotter, southern, lower elevations are moving to previously cooler, northern, higher elevations 2) shrubland is replacing forests.

In the Southwestjuniper and drought-tolerant oaks are replacing the pines. In northern California, dense thickets of manzanitas and the nitrogen-fixing California lilies (Ceanothus sp.) tend to occur. In the northern Rocky Mountains, the original forest covers are replaced by a mix of grasses, and again, the ceanothus shrubs.

The climate-fueled spread of invasive species

Unfortunately, the well-adapted plants often include invasive ones. For example, the Old World-native drooping brome (Bromus tectorum) is one of the invasive grass species that now thrive in the American West. In the United States, it is also known as cheatgrass because it becomes dry earlier than native grasses, thereby denying foraging herbivores – both wild and domesticated – nutritious fresh food. 

A view of hills in a burn area with burnt tree trunks in the foreground and grassy hills in the background.
Cheatgrass turning red in the fall in the Squirrel Creek burn area, Medicine Bow National Forest, Wyoming. Photo by Amanda West, USGS, public domain.

A study says that invasive annual grasses have become dominant in one-fifth of the Great Basin rangeland – on more than 77,000 square kilometers – replacing the native sagebrush (Artemisia spp.)-dominated communities. State-wise, that includes Oregon, Nevada, Idaho, Utah, and California. Higher-elevation habitats are also becoming susceptible to non-native species, which was previously not the case.

The Role of Snow Cover

As hinted, it is not just the excess of heat, but also the lack of precipitation that is bringing about the biome change. However, that doesn’t include only the rain. Snow pack plays a vital role in forest regeneration – and it is decreasing quite rapidly. 

Anne Nolin, a hydrologist and a geography professor at the University of Nevada, has led a study that looked at the environmental conditions and their relation to post-fire forest regeneration in the Pacific Northwest. 

When they looked at the data from across the 260,000 sq. mile Columbia River Basin, they found that rain precipitation has remained the most critical factor for successful forest regrowth – which could hardly come as a surprise. 

A view across a canyon with hoodoos, snow, and cedar trees.
Snow-covered cedar forest, Bryce Canyon, Utah. Photo: Alex Demas, USGS, public domain.

However, snow precipitation trails right behind as the second most crucial component. There was a positive correlation between the winter snowpack and the summer greenness in many localities. In fact, the study found that the snow cover was the decisive variable for revegetation in the Oregon and Washington Cascades.

The study concludes that when we consider the “trends of increasing wildfire activity, lower snowpacks, and earlier snow disappearance dates across the Pacific North West, forests will likely experience more frequent drought conditions that will impact post-wildfire vegetation regrowth.

And indeed, other studies are foreseeing a “low-to-no snow future” in the American West, with the possibility of no or very little snow becoming a reality in the next 30 years. The current trend is already alarming: we went from a total of 600 inches in 2017 to only 180 inches in 2020. Unfortunately, that doesn’t have disastrous implications only for the forests, but for water supply and agriculture as well.

Forest Management Implications

Although all the evidence can seem quite bleak, there are still things that forest managers can do to mitigate the impact of forest fires in times of climate crisis. 

This knowledge may be used to facilitate adaptive post-fire management policies and decisions to ensure long-term forest health,” Nolin explains, “For example, depending on the sub-region and species composition, reseeding efforts following low snow winters might employ more drought-tolerant species or, replanting could be delayed one to two years until snowmelt and soil moisture conditions are more favorable for seedling propagation.

A view of dead ponderosa trees on a hillside.
Drought and climate change are causing dieback in the forests of the western United States.  Photo: dead ponderosa pines in New Mexico’s Jemez Mountains, caused by a combination of drought stress and bark beetle attacks on vulnerable trees. Photo: USGS, public domain.

Still, Nolin also explained climate change implications that are likely out of our control.

Climate change projections and shifting wildfire regimes have increased concerns about post-fire regeneration (…) The snowpacks’ role in aiding revegetation will become increasingly important across the West. And where snowpacks have declined, there likely will be ecosystem transitions that look like a shift from forest to non-forest and from evergreen to deciduous vegetation.

The conclusion of stories like this one is always unsurprisingly the same – addressing climate change itself is the most logical course of action to preserve our native forests, water resources, and the iconic snowy mountaintops.

Resources

Studies:

Davis, D.T., et al. (2019) Wildfires and climate change push low-elevation forests across a critical climate threshold for tree regeneration.

Proceedings of the National Academy of Sciences Mar 2019, 116 (13) 6193-6198; DOI: 10.1073/pnas.1815107116  https://www.pnas.org/content/116/13/6193

Rodman, K.C. et al. (2020) A changing climate is snuffing out post-fire recovery in montane forests. Global Ecology and Biogeography. doi: 10.1111/geb.13174. https://www.fs.usda.gov/treesearch/pubs/61334

Siirila-Woodburn, E.R., Rhoades, A.M., Hatchett, B.J. et al. A low-to-no snow future and its impacts on water resources in the western United States. Nat Rev Earth Environ 2, 800–819 (2021). https://doi.org/10.1038/s43017-021-00219-y https://www.nature.com/articles/s43017-021-00219-y#citeas

Smith, J. T. et al. (2022). The elevational ascent and spread of exotic annual grass dominance in the Great Basin, USA. Diversity and Distributions, 28, 83– 96. https://doi.org/10.1111/ddi.13440

Wilson, A. C., Nolin, A. W., & Bladon, K. D. (2021). Assessing the role of snow cover for post-wildfire revegetation across the Pacific Northwest. Journal of Geophysical Research: Biogeosciences, 126, e2021JG006465. https://doi.org/10.1029/2021JG006465

Zwolinski, M.J. (1988) Fire Effects on Vegetation and Succession. Conference paper. Effects of Fire in Management of Southwestern Natural Resources conference. Tucson, AZ, November 14-17, 1988 https://www.fs.fed.us/rm/pubs_rm/rm_gtr191/rm_gtr191_018_024.pdf

Articles:

A California without snow? Lajja Mistry. USC Annenberg Media. 30 November 2021. https://www.uscannenbergmedia.com/2021/11/30/a-california-without-snow/

Invasive grasses are taking over the American West’s sea of sagebrush. Brianna Randall. Science News. 6 December 2021 https://www.sciencenews.org/article/invasive-grasses-spread-wildfire-plants

Study: Snow cover critical for revegetation following forest fires. This Is Reno. November 26, 2021 https://thisisreno.com/2021/11/study-snow-cover-critical-for-revegetation-following-forest-fires/

Western Forests. The National Wildlife Federation. https://www.nwf.org/Educational-Resources/Wildlife-Guide/Threats-to-Wildlife/Climate-Change/Habitats/Western-Forests

Western Forests & Mountains. U.S. Global Change Research Program https://www.globalchange.gov/browse/educators/wildlife-wildlands-toolkit/eco-regions/western-forests-mountains

Wildfires are erasing Western forests. Climate change is making it permanent. Nathanael Johnson. Grist. 29 November 2021 https://grist.org/climate/climate-change-forest-loss/

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About the author
Katarina Samurović
Katarina Samurović is an environmental analyst and a freelance science writer. She has a special interest in biodiversity, ecoclimatology, biogeography, trees, and insects.

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