On a gentle slope on the western side of the Sierra Nevada Mountains, where a towering green canopy of ponderosa pines had stood for centuries, wildfires have transformed roughly 1,700 acres into brown, bare shrubland.
That forest is gone for good, scientists say. Spiky shrubs and poison oak have replaced the majestic trees. All that is left of the pines are burned logs and dead branches.
It’s part of a wider transformation that’s taking place across the Western United States, from Oregon to New Mexico, as bigger and hotter wildfires sweep through forests that have not evolved to survive such high-intensity blazes and a warmer climate. Recent studies estimate that as much as 40 percent of Western conifer forests will turn into shrubland by 2100 as a result.
“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, an independent research center in Millbrook, N.Y.
The consequences of the shift, for both people and wildlife, are expected to be profound.
Researchers say that as trees disappear, winter and spring snow will melt faster, affecting wildlife as well as communities that are already struggling with water shortages. Some Western states get as much as 75 percent of their water from snowmelt.
“When you remove the forest, that snow is more susceptible to melting earlier,” said Benjamin Hatchett, an interdisciplinary scientist at Colorado State University. And he noted that a warmer, more arid climate dries out soil and pulls moisture from plants, making them more susceptible to wildfire. “That thirstier atmosphere is a big concern,” he said.
Forests also absorb planet-warming carbon dioxide from the atmosphere. Bigger, older trees store more carbon than younger trees and shrubs. The loss of conifer forests today means a hotter tomorrow.
To better understand these cascading effects, Dr. Hansen and Johan Eckdahl, a postdoctoral researcher at the University of California, Berkeley, visited the Sierra Nevada this month. On a 100-foot hillside plot that burned in the 2022 Oak fire, they recorded each flowering plant and shrub, filled plastic bags with soil, and checked the ground moisture and temperature with a special probe.
