California's 2020 fires were unprecedented — and not just because they covered more than 4 million acres.

The Creek Fire, which burned east of Fresno in the western Sierra Nevada, flamed with such frenzy that it produced a cloud resembling an atomic bomb blast, with smoke reaching the stratosphere. That fire and others, like the huge, lightning-sparked North Complex fires in the Sierras north of Sacramento, didn't burn in the usual patchy fashion of wildfires, leaving lightly singed spots mixed with more intensely burned islands. They torched much of the acreage within their boundaries, killing even large trees that would have withstood smaller blazes.

The resulting charred landscapes, a consequence of decades of fire suppression policies and a warming climate, may represent a funeral for some forests, which struggle to regenerate on their own after such severe conflagrations. This new regime of ferocious flames threatens to completely change familiar forest ecosystems, tipping towering pine stands into lands dominated by squat scrub species. Forest ecologists warn that this may harm biodiversity, lower the capacity of forests to store carbon, and even threaten water supplies.

That's why researchers are working to refine how to restore areas that have been heavily burned. It's why many land managers are rethinking traditional restoration approaches with a view to future-proofing. As regions grow warmer and drier with the changing climate, trees that once thrived at a given site might not be suited for that site in future decades. So ecologists and foresters are turning to clues from the scars of earlier destructive fires to figure out what restoration approaches would work best, and exploring new methods of sourcing seeds. The results may hold broad lessons on how to rebuild forests across the western U.S. so they can withstand a hotter and drier future.

The Creek Fire in the mountains of California was so large and intense that it formed a massive smoke cloud known as a pyrocumulonimbus. Smoke clouds from other, smaller fires are also visible. | (NOAA, GOES-17 SATELLITE/ Courtesy Knowable Magazine)

Friendly fire

Many ecosystems in California and the West are adapted to, and sometimes even require, frequent fires. This includes the conifers of the Sierras, the shrubby chaparral of coastal and inland climes, and the oak savannahs of foothill zones. Historically, both lightning and Native Americans sparked regular blazes. For Indigenous peoples, fire aided in the cultivation and management of the land.

Those forest fires mostly burned through dry brush and lower vegetation, helping a diversity of plants to establish, returning nutrients to the soil, and maintaining mountain meadows by preventing pines from encroaching. But today's fires tend to include large patches of fierce crown fires, in which flames engulf the entire forest canopy.

These intense fires kill trees across broad areas, and the scorched expanses are too large for nearby surviving trees to reseed. As with apples, the cone does not fall far from the pine: Among foresters, the general rule is that seeds can move a maximum distance that's twice the height of the mother tree. "The seeds of the conifer trees are too heavy to disperse out into that area," says Matthew Hurteau, a forest ecologist at the University of New Mexico. "And then the other thing is, when you burn off all the tree cover, it gets a hell of a lot hotter and drier in that environment." That means the seeds that do sprout may have trouble surviving.

Decades of fire suppression have filled forests with densely packed trees and woody debris that fuel larger, more destructive fires, like this crown fire burning in Yellowstone National Park in 2013. In the aftermath of such fires, natural regeneration may not be enough to allow the forest to regrow. | (MIKE LEWELLING, NATIONAL PARK SERVICE / FLICKR/Courtesy Knowable Magazine)

Without intervention, a tangle of fast-growing shrubs — Ceanothus species and manzanita among them — crawl into the cleared space, forming a dense cover that young trees have trouble competing with. This accelerates another process driven by climate change: the expansion of shrub ecosystems as the cool climate favorable for forests gradually moves upslope.

When high-severity fires kill big sections of forest, they also harm biodiversity and habitat for numerous species. Large, intensely burned areas and the shrub cover that follows lack the varied habitat niches the forest would normally provide, and therefore have reduced biodiversity, says Malcolm North, an ecologist with the University of California, Davis. Sensitive animal species like the spotted owl and weasel-like Pacific fisher need fragments of dense forest.

Broad, post-fire shrub patches are dominated by just a handful of plant species, adds Wolfy Rougle, forest health watershed coordinator of the Butte County resource conservation district on the western slope of the Sierra Nevada. And while deer and elk may munch on the young sprouts and grasses that grow after a fire, once the dense thicket forms, there's little for them to eat.

When tall pines give way to scrubby chaparral, important ecosystem processes are also lost or diminished. In these times of rising atmospheric carbon dioxide, we need all the carbon storage we can get, but shrublands don't do that nearly as well. "Shrubs hold maybe 5 to 8 percent of the carbon that a forest might — it's a dramatic drop," North says.

All this means that foresters may need to replant saplings after severe burns such as those California experienced this year — and that need has been growing as megafires become more common. The acreage that must be replanted annually is now several times more than it was in the 1970s, North estimates.

Traditionally, foresters replanted forests on a grid pattern, leading to what some call "pines in lines" (left). But some experts now argue for a more flexible planting pattern of clumped trees and open spaces. This pattern mimics the structure of naturally sprouted forests that experience regular, lower-intensity fires, such as this area in Yosemite National Park (right). | (MALCOLM NORTH (LEFT); MARC MEYER (RIGHT)/ Courtesy Knowable Magazine)

But replanting isn't as simple as it sounds: Trees in California are often highly adapted to their particular elevation, aspect, and soil type. Some people might roll their eyes at yet another claim that the Golden State is special, but in this case it's true. In other parts of the country, deglaciation occurred relatively recently: The Great Lakes ecosystem emerged from being covered in glaciers only 12,000 years ago, for example. The region now known as California, however, was never fully glaciated, so its flora has had hundreds of thousands, if not millions, of years to evolve.

That, in addition to the variety of geologic landforms within the state, helped give rise to extraordinary biodiversity, says Bruce Baldwin, an evolutionary biologist at the University of California, Berkeley, who authored a review of the state's exceptional plant diversity in the Annual Review of Ecology, Evolution, and Systematics.

Take the prickly-coned Pacific ponderosa pine, which dominates many lower mountain slopes from southern British Columbia to San Diego. On the west slope of the Sierras in northern California, the pines enjoy an annual rainfall of 69 inches, but they can also thrive on the drier eastern slopes. All the trees are of one species, but each population can be so specialized to its particular niche that a seed planted in a spot with different rainfall, soils, or temperature will often do poorly.

The clearest evidence for such specialization comes from experiments called provenance tests that were done starting in the 1930s, in which researchers planted tree seeds at various elevations. The general pattern that came out was that plants grew best within about 500 feet up or down from their source. "Anything more than 500 feet was really moving them more than what was optimal," says North.

For that reason, the U.S. Forest Service has a rule of thumb that trees should not be replanted outside their original 500-foot elevation band. The rule is codified in the California tree seed zone map, first published in 1946. It's had a couple of revisions since then, but it's still the document that silviculturists refer to when sourcing their seeds.

Sourcing seeds for the future climate

In their efforts to revegetate with the most local seeds possible, reforestation workers found the 500-foot rule satisfactory — until the impacts of climate change began to reveal themselves. As temperatures warmed, trees' historic habitats sometimes no longer matched their preferred climate. That's making experts think twice about what seeds to plant, says Emily Moran, an ecologist at the University of California, Merced. "There's been a lot more interest in trying to find what are actually the seed sources we want to be planting — to make sure that the areas we're restoring are going to be healthy in the future," she says.

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