On a gentle slope along the western flank of the Sierra Nevada, where a towering canopy of ponderosa pines stood for centuries, wildfires have converted roughly 1,700 acres into barren, brown shrubland.
Scientists say that forest is gone for good. Spiky shrubs and poison oak have replaced the majestic trees, leaving behind only charred logs and dead branches.
This transformation is unfolding across the Western United States, from Oregon to New Mexico, as larger, more intense wildfires sweep through forests that have not evolved to withstand such high-severity blazes or a warming climate. Recent research estimates that up to 40 percent of Western conifer forests could transition to shrubland by 2100.
“This is a permanent shift from one ecosystem type to a fundamentally different one,” said Winslow Hansen, a forest ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y.
The consequences for both people and wildlife are expected to be profound.
Researchers warn that as tree cover disappears, winter and spring snowpack will melt earlier, impacting wildlife and communities already grappling with water scarcity. Some Western states derive as much as 75 percent of their water supply from snowmelt.
“When you remove the forest, that snow becomes far more susceptible to early melting,” said Benjamin Hatchett, an interdisciplinary scientist at Colorado State University. He added that a warmer, more arid atmosphere dries out soils and draws moisture from vegetation, increasing flammability. “That thirstier atmosphere is a major concern,” he said.
Forests also sequester planet-warming carbon dioxide. Large, older trees store significantly more carbon than younger trees or shrubs; the loss of conifer forests today portends a hotter future.
To better understand these cascading effects, Dr. Hansen and Johan Eckdahl, a postdoctoral researcher at the University of California, Berkeley, surveyed the Sierra Nevada this month. On a 100-foot hillside plot burned in the 2022 Oak Fire, they cataloged every flowering plant and shrub, collected soil samples, and measured ground moisture and temperature with specialized probes.