Once again California is ablaze. A combination of hot, dry weather and a relatively rare lightning storm sparked hundreds of fires in the northern half of the state in recent weeks. Several of them exploded into major conflagrations—the kinds of intense, destructive fires that some research suggests will become more likely as temperatures rise and create more conducive conditions for a longer portion of the year.
The current fires, which include the second- and third-largest in the state’s history, have collectively burned more than one million acres. They have blanketed the region in smoke that makes the air unhealthy to breath, destroyed hundreds of homes and other buildings and—to widespread dismay—burned through Big Basin Redwoods State Park in the Santa Cruz Mountains south of San Francisco. Images of scorched bark and fires burning inside holes in the massive trees—which can grow to hundreds of feet and live for hundreds of years—spread on social media.
Kristen Shive, a fire ecologist and researcher at Save the Redwoods League, has worked in Big Basin and studied the effects of blazes on landscapes across California. Scientific American spoke with her about how the redwoods are likely to fare and how that outlook could potentially change in the future because of the impacts of global warming.
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[An edited transcript of the interview follows.]
What makes redwoods well adapted to fire? And how do we know the answer?
We understand their adaptations to fire from what we’ve been able to observe directly. They have this incredibly thick bark that insulates them from the heat of the fire. When they do burn pretty severely—where, say, the whole crown is consumed—in most other conifers, they’d be goners. But redwoods have the ability to sprout from their trunks, and they come back. They’ll look a little bit beat up for a while, but they do survive. The research that’s been done on other wildfires has shown that most of them tend to make it, even if they have that more severe damage.
But we also know that they are fire-adapted because we do have tree-ring records that have documented frequent fire, more or less throughout their range. Scientists can use tree rings to date trees, and then some of those rings will have fire scars. And then scientists can quantify how often this forest may have burned in the past. One thing that’s interesting about coastal redwoods is that if you look at the scale of the entire range, what you see is: it’s highly variable as to how frequent fires were in the past. And the reason is: even now there’s not a lot of lightning on the coast. Most of the past fires, we think, were lit by Native Americans tending the land. Where those folks were living and managing the landscape influences how often things were burning. They primarily burned grasslands and oak woodlands, and we think that then fire would periodically end up going through a redwood stand. Most of what they were trying to manage were plants—for food and for basketry. But, also, fires were probably [used] to clear out the underbrush to facilitate hunting and presumably to reduce fuels around their own communities—much as we do now with prescribed burning.
[Learn more about how prescribed burning works.]
But some areas—for example, the farther south you are, the warmer it is in the range—and fires were more frequent in the record there. We do have one fire history study that was done in the Santa Cruz Mountains, and on average, fires happened every 40 years. But if you look at individual sites, sometimes they were much more frequent than that even. So these places have evolved with a lot of fire. And then we excluded fire for more than a century in most of these places [by aggressively putting out wildfires], so there has been fuel accumulation since then, and that can change how fires are burning. But, like I said, even if there were some more fuels on the surface, and it burned a little bit hotter than it would have, because they have such significant fire adaptations, the trees are going to really be fine—most of them.
How might the trees in Big Basin have fared?
There’s what we call old growth and second growth. And the second growth are in areas that have been logged and are recovering, so they’re young stands of redwoods. But in the old growth, you have these big, kind of open stands. In those areas, a lot of the trees have been around for several hundred years, maybe up to a couple thousand years old, and so they sustained fire damage from all these fires that used to burn through. In some cases, there are places where fire might burn out a good hole in the middle of the tree—there’s one called the Chimney Tree—and they can cumulatively get a little bit of injury from each fire. Eventually, that could cause enough structural damage that they fall over or their crown snaps off or something like that. And I suspect where we get mortalities, especially in the old growth, is mostly going to be there.
The [question of the survival of the] second growth, it feels like, is a little bit more open. We’ve heard reports that it primarily burned at pretty low severity, but we don’t know that yet. We haven’t been out there. But it’s a little bit more of a concern because when you cut coast redwoods, they sprout from their base. Then they are similar-sized, and there are a lot of them—because where one big, ancient redwood maybe fell over, there might be 10 sprouts now coming up. So those areas also hadn’t burned in a long time, but they also had higher densities of trees, which could be more contiguous fuel for the fire. But even in that case, it sort of depends on how old they are. In areas that were logged more than 100 years ago, I would expect that they are starting to have bark thick enough to have pretty good defenses from fire. Because that thick bark that insulates them from fire can be up to a foot thick on the old growth, it takes some time to develop that, [so younger trees may not have as much of a defense]. But even in these extreme cases, where it burns so hot that the tree isn’t even sprouting back along its trunk, you’d still probably get sprouting from the base. It’s sort of like the individual would still persist.
How is climate change potentially affecting redwoods, both broadly speaking and more specifically in terms of how it might influence fire behavior?
We have a project called the Redwoods and Climate Change Initiative. The lead researcher is Steve Sillett at Humboldt State [University], and he has been tracking growth patterns in coast redwoods all across the range. What he found is: at the core of the range—up in Humboldt County, where the old growth is right on the coast—they have been thriving, even as a climate has warmed in recent decades. Even through the recent drought, I think, they barely noticed it. But having said that, on the edges of the range (the eastern edge or the southern edge, some of which is burning now), the trees did show signs of stress during the drought. That makes sense from what we know about population distribution—that’s sort of the edge of where they’re in their ideal habitat. Those are the places where there could be some level of environmental stress that makes it more difficult for them to recover from wildfire.
[And then], in the very long run, even though the trees live to be superold, you have to think about “How does the population perpetuate itself?” And that’s through new seedlings. For most trees, whatever the range of climate conditions is that they can survive once they’re established, it is usually broader than the conditions they need to regenerate. Because those little seedlings are much pickier. So in the really long run, are we going to stop seeing seedling regeneration at those parts of the range [that are marginal]? That’s a really long timescale, though, because mostly they regenerate through sprouting. When they sprout from their roots, they have a lot of energy stored below, so that’s also going to make them a little bit more resilient than trees that can only regenerate by distributing a seed and then coming up from the soil. So I’m a little bit less worried about them than, say, their giant sequoia cousins or some other tree species.
But the other thing is: yes, as we get more hotter and drier days, climate change will influence wildfire. The fires that are burning on the coast right now, they’re not burning super uncharacteristically, in the sense that we’ve had hot days in the past. But what I think is of concern is that we do know fire seasons are getting longer and that we’re having more heat waves. If we keep having more of those, we’re going to be more fire-prone in these areas. I feel like this really triggers a management question: All these [tree] communities are fire-adapted. We live in a fire-prone place, so we have to decide how we’re going to live with fire. One way could be to reintroduce more fire safely and do prescribed burning and get these stands into a resilient condition so that when we start having more and more conditions that are ripe for fire, these forests are ready for it. They’re true survivors, so most likely they’ll probably make it anyway. But I think a good example might be with the old growth: if we would rather see them persist—not have to lose their entire crowns and sprout back to life—if we think that by keeping fuel loads lower that we can have a more low-severity fire and have fire more regularly, I just think that’s going to set us up better for when they do burn in the future.