Berkeley, California — A recent study has found that California’s treasured trees are facing significant threats from a rise in fungal pathogens, which are increasingly linked to declines and mortality in various species. Researchers from the University of California, Berkeley, highlighted the interconnection between climate change, urbanization, and these latent pathogens in a report that aims to reshape understanding of the factors reshaping the state’s landscapes.
Matteo Garbelotto, a specialist in forest pathology at UC Berkeley and lead author of the study, remarked on the importance of collaborative research efforts. He noted that partnerships among county advisors, landowners, and academia have provided a comprehensive framework for identifying environmental threats across large geographic areas.
Since the early 2000s, reports have emerged from landowners who observed a troubling increase in unhealthy trees throughout California. Specifically, species such as grey pines and blue oaks in the Sierra Nevada foothills have been notably impacted, alongside instances of dieback in coastal manzanitas and California bay laurels within the Bay Area.
While many attribute these tree deaths primarily to climate change, the study published in the journal Plant Pathology suggests that this is not the sole influence. Researchers discovered that previously dormant pathogens are causing severe damage to trees, particularly as drought stress becomes more prevalent.
From 2019 to 2024, scientists surveyed multiple California regions, collecting samples from trees that displayed symptoms of infection. The results revealed that ten distinct woody plant species were affected by nine types of fungal pathogens. Garbelotto emphasized that these latent pathogens activate during times of environmental stress, leading to decline and death in trees that might otherwise remain unaffected.
“The research confirms that latent pathogens are key drivers of tree decline, challenging the notion that climate change is the only factor,” Garbelotto explained. “Our findings provide concrete evidence that native, dormant pathogens can be as harmful in the context of climate change as invasive species.”
The alarming trend of tree diebacks raises concerns for landowners and conservationists alike. Garbelotto expressed hope that this study will clarify the underlying causes of tree mortality, enabling land managers to allocate resources more effectively by focusing on critical issues.
Igor Lacan, an urban forestry advisor for the Bay Area and co-author of the study, stressed the importance of water for maintaining the health of mature urban trees. “Drought-stressed trees are far more vulnerable to these latent infections,” he stated, underscoring the need for proactive strategies to safeguard high-value trees during extended dry spells.
As climate patterns shift, many trees are found outside their optimal growth zones. “Invasive species, urban encroachment, and climate change, along with latent pathogens, will all impact the natural distribution of plants not just in California but globally,” Garbelotto warned.
He advised that response strategies must change to accommodate the evolving climate, recommending that landscape planners prioritize native species suited to both current and anticipated future conditions. “Planting exotic species is increasingly problematic, as finding favorable climatic matches becomes more difficult,” he cautioned, pointing to the struggles of species like blue gums as evidence of this reality.
Garbelotto further noted that some native species, such as bay laurels and tanoaks, are thriving in unsuitable environments due to fire suppression practices, exemplifying the disruption of natural ecosystems. The ramifications of these shifts mean that the natural ranges of certain species may need to be reassessed moving forward.
The research findings will serve as a vital resource for understanding the complex interplay of factors threatening California’s forests, funded by contributions from the East Bay Regional Parks Foundation, the San Francisco Public Utilities Commission, and the U.S. Forest Service. The full study can be accessed online for those seeking an in-depth look at these alarming developments in the state’s ecology.