Forest fires across California have become significantly more destructive over the past four decades, with new research showing that high-severity blazes now dominate many forest landscapes.
Scientists from UCLA found that while the total area affected by forest fires has increased tenfold since 1985, the land burned by the most damaging fires has surged by a factor of 30.
Published in PNAS, the study examined wildfire patterns between 1985 and 2024, revealing a fundamental shift from low and moderate-intensity fires, which can benefit ecosystems, to severe fires that destroy large areas of forest and leave limited opportunity for natural recovery.
Researchers attribute the trend to a combination of climate-driven aridity and increasingly dense forest vegetation.
The findings suggest California’s forests are approaching a tipping point, with extensive tree loss raising the likelihood that some landscapes could permanently transition from forest to grassland or shrubland.
The researchers argue that adapting forest management practices could help reduce the risk of future high-severity forest fires.
Severe forest fires now outnumber beneficial burns
Historically, many California forests experienced relatively frequent, low-intensity fires that removed undergrowth while leaving mature trees largely intact. These natural cycles helped maintain healthy ecosystems and reduce the build-up of combustible vegetation.
However, the UCLA analysis found that this balance has shifted dramatically. High-severity forest fires, which kill vast numbers of trees and fundamentally alter forest ecosystems, overtook low-severity fires in 2012 and have remained the dominant fire type every year since.
The trend mirrors the state’s recent wildfire history. According to the study, eight of California’s ten largest recorded fires have occurred within the past decade, while 2020 and 2021 rank as the two largest years on record for forest area burned across the western United States.
Climate change and dense vegetation driving fire severity
The researchers identified two key factors behind the growing intensity of forest fires.
The first is increasing environmental dryness. Rising temperatures have created a warmer atmosphere capable of holding more moisture, increasing what scientists call the vapour pressure deficit.
As the atmosphere draws more moisture from vegetation and soils, forests become significantly more susceptible to extreme burning conditions.
The study found a strong relationship between these increasingly dry conditions and the expansion of high-severity fires over the past 40 years.
The second factor is fuel density. Forests containing dense vegetation, thick undergrowth, and accumulated dead plant material experienced the fastest growth in severe fire activity.
Researchers suggest decades of aggressive fire suppression have unintentionally contributed to this build-up of combustible fuel.
While preventing smaller fires may reduce short-term risk, it can allow forests to accumulate more vegetation, creating conditions for much larger and more destructive wildfires when fires do occur.
Long-term risks for California’s forests
The ecological consequences extend well beyond the immediate destruction caused by the fires themselves.
High-severity burns often eliminate large areas of mature trees while damaging soils, making natural forest regeneration far more difficult. Without nearby seed sources, forests may take decades to recover, if they recover at all.
Instead, many affected areas could gradually transition to grasslands or shrublands, fundamentally changing California’s landscapes. Such shifts would also influence how future forest fires behave, altering the relationships among vegetation, drought, extreme heat, and wind.
These changes could create long-lasting feedback loops that reshape fire behaviour across the state for decades.
Economic and environmental impacts continue to grow
The loss of forests also threatens a range of environmental and economic benefits.
Healthy forests play an important role in improving air quality, regulating regional climate, reducing flood risk by managing water flow, and supporting industries, including timber production and tourism.
By contrast, high-severity fires release substantially more smoke and pollutants into the atmosphere while increasing the likelihood of soil erosion, flooding, and long-term damage to watersheds.
Researchers warn that these cascading effects could place additional pressure on California’s already strained water resources, particularly as prolonged droughts become more common.
Forest management could reduce future risk
While climate change remains a major driver of worsening wildfire conditions, the study concludes that improved forest management can still make a meaningful difference.
Reducing fuel loads through manual vegetation clearing and expanding the use of prescribed burns could help restore more natural fire regimes and limit the spread of high-severity blazes.
The researchers argue that combining these management strategies with broader efforts to address climate change will be essential if California hopes to preserve its forests and reduce the escalating impact of future forest fires.