Opinion | Will These Ancient Trees Survive a Drying West?

About two hours north of Los Angeles, on the steep slopes of Mount Pinos, stands an ancient grove of big-cone Douglas fir trees. The telltale signs of decades-long droughts from centuries past lie deep within the trunks of its oldest trees. But inscribed just beneath the bark are traces of the worst drought these trees have ever withstood.

Across California, primeval forests are under threat. Pests are decimating the state’s bristlecone pines, the oldest trees on Earth. Two wildfires in September killed thousands of mature sequoias (the world’s largest trees by volume), a fate similar to large areas of coastal redwood forests (the world’s tallest) the year before. Last week, the Washburn Fire scorched the flank of the Mariposa Grove, a group of over 500 giant sequoias whose protection was a driving force for the creation of Yosemite National Park.

The rings inside the Douglas firs in the Mount Pinos grove record a continuous climate history stretching back 500 years, nearly five times farther than rain-gauge records. But even though these trees are well adapted to this rugged landscape, they cannot survive without sufficient rainfall and moisture. For some of the trees, their unbroken story may be coming to an end.

As a dendrochronologist, I examine tree rings to study climate. I hunt for old trees and use a simple hand tool — which does not harm the tree — to bore deep into trunks looking for evidence of ancient drought and deluge. For 20 years, first with my teachers and later with my students, I’ve visited old-growth forests across the American Southwest, watching dry season after dry season pile up.

These withering years are killing trees. We have watched their habitats shrink, as warmer temperatures pull moisture out of the ground at lower elevations and drive woodlands to higher, cooler slopes. I’ve taken samples from healthy younger trees and returned years later to find them dead or dying. The matriarchs, the largest, oldest and most deeply established trees in many groves, seem to be hanging on for now. But no scientist is sure how much more they can endure.

A recent study published in Nature Climate Change analyzed the rings of thousands of living trees and architectural wooden beams from around the Southwest to reconstruct a 12-centuries-long timeline of climate extremes. The authors concluded that there was probably not a drought as severe as today’s in the past 1,200 years.

Our planet produced decades-long megadroughts well before humans had a discernible impact on the climate. Tree rings and weather records make it possible to see how and why today’s megadrought is so different from those that came before: Rising temperatures over the past two decades, clearly attributable to the burning of fossil fuels, have greatly increased the severity of the Southwestern drought.

We have spiked the climate system, releasing heat-trapping carbon dioxide into the atmosphere faster than at any time in at least the past 50 million years. Human-fueled megadroughts like this one will be more frequent and severe in the future. In the Southwest, there will be more years with declining snowpack, more years with water shortages and a more dire mismatch between where water is available and where it is needed.

A century of successful wildfire suppression has ensured that when the forests of our dry lands finally catch flame, they will burn much hotter than before. Rare, high-intensity wildfires kill trees that the low-intensity fires of the past did not. Tree rings we are able collect this season might not be around to sample in the next.

That danger was evident last summer, when our research group pulled off the highway near Mount Pinos to extract core samples from living big-cone Douglas fir trees. Years ago, we wouldn’t have hesitated to bring chainsaws to collect entire discs of tree trunks from fallen trees. But not last year. We would leave the fallen trees where they were. We could not chance an errant spark that could ignite a wildfire.

The forest was a tinderbox.

The risk is even more acute at lower elevations, where temperatures are higher. That is the case for the vast oak woodlands that range across the dry foothills of California.

Blue oaks, named for the color their leaves take on deep in the summer season, are revered among dendrochronologists. They can live for more than 550 years, survive on as little as 10 inches of average annual rainfall and are among the most drought-adapted of any tree species in the state. Kelly Redmond, a leading climate scientist and observer of climate change across the American West, referred to blue oaks as natural rain gauges. They grow at low elevation and early in the season, in tight synchronicity with the rainwater of winter and spring. And they lack the growth quirks of some other tree species that can muddy the signals of heat and precipitation. The conventional wisdom for years has been that oaks never miss a ring.

But when we arrived at a site low in the foothills, we found dozens of dying trees with sickly branches barren of leaves. Looking at core samples back at the lab, we found missing rings where 2014 and 2021 should have been.

Blue oaks are drought resistant, but only to a point. As we worked our way up the slope, we found oaks with healthier canopies, surviving despite the intense stress. At higher elevations, the summer California heat is slightly tempered, and the rain falls just marginally more, enough to keep these trees alive.

Climate change will push the blue oak’s survivable range upslope, like a tidal wave engulfing an island, forcing inhabitants to gather at the highest ground as the dryness rises.

Across the Southwest, forests are now living on a knife’s edge. If wildfires find groves like these amid this intense drought, they may be deadly — to the Douglas firs and blue oaks that we study, to the plants and animals that live among them and to people who have built their homes near these weakened trees. Will a new forest be able to grow out of the ashes?

Elder trees should still have many seasons left to grow, saplings to nurse, ecosystems to support and stories to tell. They deserve better.

So do the future generations of humanity. Our fates are inextricably intertwined.