INVERNESS, Calif. — At the height of California’s fierce wildfire season, the Sierra Nevada and North Coast forests are choked with tens of millions of dead and dying trees, from gnarly oaks to elegant pines that are turning leafy chapels into tinderboxes of highly combustible debris.
Ground crews wielding chain saws, axes and wood chippers are braving the intense summer heat in the Sierra’s lower elevations, where most of the pine trees have died. The devastation and danger are greatest in the central and southern Sierra Nevada, where the estimated number of dead trees since 2010 is a staggering 66 million.
Scientists say rarely is one culprit to blame for the escalation in the state’s tree deaths, and the resulting fire hazard. Rather, destruction on such a broad scale is nearly always the result of a complex convergence of threats to forest ecosystems.
Chief among them is a severe, sustained drought in the Sierra Nevada that is stressing trees and disabling their natural defenses. Climate change is raising temperatures, making for warmer winters. No longer kept in check by winter’s freeze, bark beetle populations are growing. Separately, a nonnative, potent plant pathogen is thriving in the moist areas of the North Coast, introduced to California soil by global trade. Opportunistic fungi are standing by, ready to finish the kill.
Factor in human shortcomings - poor or absent forest management, a failure to clear out ignitable dead wood, the darker temptation of arson, unchecked carelessness - and you have a lethal recipe.
“It’s never just one thing that brings down trees,” said David Rizzo, the chairman of plant pathology at the University of California, Davis. “It’s always a combination. The first may weaken trees; the next stresses trees over time. Then comes a third, shutting down the trees’ immune and defense systems. Finally, the last may come along to disrupt nutrient systems. When all this happens at once, or in rapid succession, trees are no longer able to save themselves.”
Two of California’s prized forest regions are in failing health because such conditions have stacked the odds against them. In the Sierra Nevada, the losses of pines and other conifers are concentrated and widening.
Along the North Coast, a picturesque blink of a town called Inverness and the surrounding Marin County woodlands are “ground zero,” Dr. Rizzo said, for the mysterious plant pathogen that began infesting coast oaks probably as far back as the mid-1980s.
Dr. Rizzo, who closely studies Phytophthora ramorum, also known as sudden oak death, returns often to this cozy glen on a peninsula overlooking the clear blue water. Graduate students, research associates and others accompany him to this patch of forest serving as an outdoor classroom, laboratory and demonstration plot.
It took years of research — detective work, really — before experts like Dr. Rizzo and Matteo Garbelotto, a professor of environmental science policy and management at the University of California, Berkeley, discerned that the funguslike pathogen had infested coast oaks years before their showy demise.
Dr. Rizzo estimated that five million to 10 million coastal trees had died because of sudden oak death.