As climate change continues to alter California’s landscape in amazing and often irreversible ways, researchers have targeted another victim of change: the forests of southern Sierra Nevada.
Between 2011 and 2020, wildfires, drought and bark beetle infestations contributed to the loss of nearly a third of all coniferous forests in the lower half of the range, according to a recent study published in the journal Ecological Applications. Eighty-five percent of the southern Sierra’s dense mature forests either lost density or became non-forest vegetation.
The losses could have serious consequences for California wildlife, including protected species like spotted owls and Pacific fishermen that rely on mature tree canopies for their habitat. The researchers said the findings are not only another clue to the state’s changing climate regime, but also offer new insights that could help drive forest management and conservation strategies.
“Thirty percent of the coniferous forests in southern Sierra Nevada are no longer considered forests,” said Zachary Steel, a research scientist at the United States Forest Service’s Rocky Mountain Research Station and lead author of the study. “They are either sparsely forested landscapes or, more commonly, transitioning to a shrubland-like system in the short or long term.”
The Sierra covers about a quarter of California’s land area, with the southern portion of the range extending from Lake Tahoe to Tehachapi. Hundreds of plants and animals call the region home, and the forest helps sequester carbon and store water for the state’s residents.
Steel, who conducted the study as a postdoctoral fellow at UC Berkeley, said the numbers are alarming.
“Most worrying is the pace at which this is happening,” he said. “There have always been fires in these landscapes, there has always been drought in these landscapes…but the decline is proceeding so rapidly that the succession or regrowth of these forests will not be able to keep up.”
The problem is multifaceted, he said. For starters, human-caused climate change is contributing to warmer, drier conditions that turn once-green forests to brittle tinder and lengthen the window for wildfires to burn each year.
In addition, thirsty trees are weaker and more vulnerable to deadly attacks from bark beetles, which burrow into them and gnaw at their inner cores. The study found that the combination of drought and beetle attacks caused even greater declines than areas where drought and wildfire overlapped.
But forest management is also part of the story, Steel said, as a century of suppressing wildfires and banning native fire practices has resulted in unnatural vegetation formation in the landscape. While forest fires used to regularly simmer along the forest floors, today’s mega fires burn with great violence and singe some trees to the top.
“One of the key takeaways is that if we still want old-growth forests in the landscape, we need to manage the rest to be more resilient,” Steel said. These include mandated burns, mechanical thinning and allowing naturally occurring fires to roam the landscape when safe to do so, he said.
Scott Stephens, a professor of fire science at UC Berkeley, said the study makes a good case for taking action on climate change.
“If it were actually the smaller or even medium-sized trees that were dying, I think that would be a different story in a way. But it’s the big ones, and the big ones already have deficits in many areas,” he said. “The peak of these last 10 years has actually been much more severe for the largest trees in the landscape, which we actually need the most.”
But while the loss of so much woodland may be staggering to humans, it’s downright devastating to some creatures that call the Sierra home. California spotted owls, which typically occupy nest sites with at least 70% canopy, have fewer opportunities in younger, sparser forests, the study said.
Decades ago, the state began establishing sanctuaries for spotted owls and other species to conserve them. But that strategy may have backfired as explosive vegetation accumulated in the protected areas as well. The study found that protected spots of activity for spotted owls saw a 49% decrease in canopy cover, compared to a 42% decrease in unprotected areas.
“It seemed to us in our naivety … that if we just put something in a box and left it alone, it would stay the same and we could protect it and we would be effective at that — and we have been for a long time,” Gavin said Jones, a research scientist at Rocky Mountain Research Station and co-author of the study.
“But this result that we found here, I think, contradicts some of these conventional paradigms and raises the question of whether we need to rethink them,” he said. “The places we’re trying to protect are harder to protect now because we’ve protected them.”
Jones said protection for these areas still serves important purposes and should not be eliminated, but that the approach could be refined by allowing some mandatory burns or other management methods.
Preventing further loss of such habitats, the study notes, “is likely to require a shift from a static, conservative habitat conservation paradigm to one that strives for sustained disturbance dynamics.”
The losses could have further cascading effects in California, the researchers said, as forests play an important role in the state’s water supply and also help with carbon sequestration. Jones said people still have some control over the outcome — at least when it comes to forest and habitat management.
“The bottom line is that we still have time, but what we’ve done so far isn’t working very well,” he said.
The state is ramping up efforts, including more than $2 billion in allocated funding for wildland firefighting and forest resilience projects over the past two years, much of it for forest thinning, mandated burns and other fire risk reduction projects. President Biden’s Inflation Reduction Act also calls for a $5 billion investment in forest management and reducing wildfire risk.
But Steel emphasized that climate change is a “threat multiplier” that leads to large – and often irreversible – disruptions over time.
“With changing fire patterns and climate change, there will be winners and losers,” he said. “We’re highlighting some of the ‘losers’ in this article and trying to figure out how to quantify that so we can work to ensure they don’t disappear from this landscape.”
https://www.latimes.com/environment/story/2022-11-01/a-third-of-southern-sierra-forest-lost-to-drought-wildfire A third of southern Sierra forest lost to drought, wildfire