By Cynthia H. Craft

New York Times News Service

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,” Rizzo said, for the mysterious plant pathogen that began infesting coast oaks probably as far back as the mid-1980s.

Rizzo estimated that 5 million to 10 million coastal trees had died because of sudden oak death.

No one knows exactly where Phytophthora ramorum first showed up in California, but Rizzo said the pathogen was probably a stowaway aboard a shipment of imported nursery plants.

The pathogen spreads by rain splash. The forest’s Typhoid Mary is the bay laurel tree, a faithful host for the disease but one that never succumbs to its perils.

When rain falls on bay laurel leaves, contaminated droplets scatter, reaching neighboring trees. If those neighbors are coast oaks or tanoaks, the pathogen penetrates bark with ease and establishes residence. At the end of outstretched filaments too small for the naked eye to see, the pest launches lethal spores that sap the tree of its nutrients. Native fungi follow, as do pests. Within a couple of years, the tree is dead.

On the other side of California, the tree die-off in the Sierra Nevada continues to worry firefighters and public officials. Under stress in the fifth year of severe drought, the ponderosa pines, the pinyons and the sugar pines lack the moisture needed to manufacture the sticky resin that prevents bark beetles from burrowing into their trunks.

With nothing to stop the voracious pests — no bigger than a grain of rice — they bore into the pines, where they produce larvae. In turn, the larvae feed off the trees’ nutrients, and the tall, proud pines die in place, standing upright like matchsticks waiting for a light.