Berkeley, California — A recent study reveals a significant threat to the state’s rich forestry: a surge in fungal infections is contributing to the decline and death of various tree species across California. Researchers from the University of California’s Agriculture and Natural Resources division conducted an extensive investigation into the links between tree health, latent pathogens, and environmental stressors, emphasizing the need for urgent attention to California’s changing landscapes.
The study’s lead author, Matteo Garbelotto, a forest pathology specialist at UC Berkeley, noted a shift in understanding the factors driving tree health as climate change intensifies. The findings illustrate a collaboration among university researchers, county advisors, and landowners, which offers an effective approach to detect new threats across extensive areas.
As early as the 2000s, reports from landowners and managers indicated a worrying trend in tree health. Observations of rapid die-offs were documented, particularly among gray pines and blue oaks in the Sierra Nevada foothills, alongside various manzanita species on the Northern California coast, and California bay laurels and tanoaks in the San Francisco Bay Area.
While many attribute these declines to climate change, the research indicates that latent pathogens—microorganisms that can lie dormant within tree systems—are playing a significant role in exacerbating this issue. The study, published in the journal Plant Pathology, highlights the crucial influence of these pathogens, revealing that they often activate under conditions of drought stress.
From 2019 to 2024, scientists collected samples from ten different woody plant species across six regions of California, identifying nine types of fungal pathogens responsible for the observed tree declines. This research challenges the assumption that climate variations alone are to blame, demonstrating that the impact of these latent pathogens is equally critical.
“Latent pathogens are indeed driving tree decline, and it’s important to recognize that climate change isn’t the only factor,” Garbelotto explained. He added that skepticism still exists among some experts regarding the role of these microorganisms, and the study provides the necessary evidence to support their significant impact.
As tree dieback catches the attention of conservationists and landowners alike, Garbelotto’s hope is that understanding the underlying causes will guide effective management strategies. “Directing efforts toward the real threats can prevent resource wastage and better protect our forests,” he said.
Igor Lacan, a co-author and urban forestry advisor for the Bay Area, emphasized the importance of maintaining adequate water supply for urban trees, especially during droughts. “Trees under stress from lack of water become substantially more vulnerable to latent pathogens,” he noted.
As rising temperatures push many tree species beyond their ideal growing conditions, the intertwined effects of invasive pathogens, urban development, and environmental changes complicate the challenges facing California’s forests. Garbelotto likened the situation to a “chronicle of a death foretold,” as numerous tree species find themselves on the brink due to habitat and climate shifts.
Looking ahead, Garbelotto urges landscapers and land managers to reconsider planting strategies. He recommends selecting native species better suited for the future climate, rather than relying on exotic plants that may not thrive as conditions continue to change.
The ongoing transformation of California’s natural landscape, influenced by both climate change and latent pathogens, indicates a pressing need for adaptive management practices. “This is a pivotal moment,” Garbelotto said. “We must reshape our approach and take into account the evolving interactions within our ecosystems.”
The study represents a collaboration supported by the East Bay Regional Parks Foundation, the San Francisco Public Utilities Commission, and the U.S. Forest Service, illustrating the importance of collective efforts in tackling environmental challenges. The full research article can be accessed through academic publications for those interested in a deeper exploration of the findings.