Researchers from Stanford University in California have drawn attention to a troubling trend they describe as zombie forests spreading through the Sierra Nevada. The findings appear in a recent issue of a major scientific periodical, which focuses on how warming climates are altering mountain ecosystems and the resilience of conifer species in arid environments.
Experts explain that rising temperatures and longer dry seasons have pushed climate conditions beyond what many high elevation conifers can endure. In the Sierra Nevada, as heat waves intensify and winter snowfall becomes more sporadic, up to roughly one in five conifer trees may lose the ability to survive under extreme heat and drought, according to the study’s modeling across multiple climate scenarios. The result is a precarious forest where traditional evergreen canopies struggle to maintain their fundamental physiological processes, and soils increasingly fail to support vulnerable root networks.
The situation is driven by water stress that affects the shallow roots common to several dominant species, including ponderosa pines, a variety of pinyon types, and the Douglas fir. Water scarcity undermines nutrient uptake and gas exchange, leading to slower growth, heightened susceptibility to pests, and greater vulnerability to timberline shifts. As trees drop vigor, a transition unfolds where drought-tolerant shrubs and chaparral begin to dominate hillslopes, altering fire regimes, soil stability, and the local microclimate in ways that reinforce the new vegetation patterns.
An ecologist who co-authored the report notes that while the term zombie forest may sound sensational, the observed decline aligns with measurable physiological stress in trees. The researchers stress that the label is a shorthand for a broader danger: woody ecosystems losing their capacity to function as they did historically, even while some life remains visible. The study examined several global warming trajectories and found that a rapid rise in greenhouse gases and accelerated climate change could render nearly the entire Sierra Nevada unsuitable for conventional conifer forests by the century’s end, reshaping the region’s biodiversity and forest structure.
Although global efforts to curb fossil fuel emissions are rising, the study warns that zombie forests could still expand in area over this century. This expansion would occur even as some emissions reductions are implemented, because ecological responses lag behind atmospheric changes. The researchers emphasize that timing matters: the sooner climate stabilization acts, the higher the likelihood that forest composition will retain more conifers and related evergreen species, preserving typical carbon storage and habitat roles for native wildlife.
If severe droughts, intense fires, or widespread pest outbreaks finally eradicate large swaths of these forests, the landscape is likely to be overtaken by a mix of shrubs, hardwoods, and opportunistic nonnative species. Such a shift would reduce regional carbon sequestration capacity and could diminish the stability of soils and watersheds. It would also disrupt habitat networks that countless native plants and animals rely on, potentially enabling invasive species to gain a foothold and displacing long-established plant communities and food webs.
Earlier warnings about climate-driven forest transformations have already signaled broad changes across other continents and biomes. In Europe, scientists have flagged long-term alterations to many forest ecosystems as temperatures rise and precipitation patterns shift. The Sierra Nevada findings add a clear local example of how climate stress translates into tangible changes in tree health, forest cover, and ecosystem services that communities depend on for water, timber, recreation, and biodiversity. The research team continues to explore mitigation and adaptation strategies that could help preserve more of the forest’s original character and function for future generations, even under challenging climate scenarios. Attribution for these conclusions is based on the presented data and modeled projections from peer-reviewed sources.