An aggressive prescribed burning program is needed to manage the massive number of trees killed during the California drought. U.C. Berkeley fire scientist Scott Stephens says there’s limited time to tackle the problem.

The drought in California may be officially over, but that doesn’t mean all the problems are fixed.

One of the biggest lasting problems from the state’s five-year drought is a vast quantity of dead trees. In November the U.S. Forest Service, based on aerial surveys, estimated the drought killed more than 102 million trees on national forests in California, with the greatest concentration in the southern Sierra Nevada. New surveys set to begin in June may raise that number even higher.

The dead trees represent a massive fire risk that could harm nearby communities, habitat, water quality and air quality. Scott Stephens, a U.C. Berkeley professor and an expert on fire ecology, says the only realistic solution is to begin a massive program of controlled burning. This means using intentional practices to restore a natural fire regime to the forests.

California forests evolved to cope with fire. Regular lightning-caused fires created forests that were much more open than they are today. For 100 years our management of forests was misguided: We saw fire as a bad thing, and suppressed every fire. This resulted in forests that grew too thick with small trees, creating a fire risk today that is potentially catastrophic.

We have about five years, Stephens says, to start chipping away at this fire threat before it becomes unmanageable.

Water Deeply: How are forests changing as a result of these tree deaths?

Scott Stephens, U.C. Berkeley professor of fire science, says that prescribed burns are necessary to deal with California’s dead trees. (Photo Courtesy UC Berkeley)

Scott Stephens: The sad part is, mostly the larger trees are dying versus the smaller ones. The trees that get us most concerned are the ones that are 200, 300 years old. A lot of these are very old pines, like ponderosa and sugar pine, which aren’t regenerating as well because of our management practices. So, it’s taking out the linchpins of the forest ecosystem. Of course, the forest is going to continue, but it will be different.

With most of the dead standing trees, there is no plan for removal. That means in about 10 to 15 years, probably 80 percent of them will be on the ground. That’s when, from a fire perspective, at least in areas of high mortality where 25 to 50 percent of the trees have died, having that much biomass on the ground available to burn … that worries me more even than what happens in the next couple years. Since they’ll be dry, they’ll be available to burn in the next wildfire.

Water Deeply: What can we do about it?

Stephens: This is something the Forest Service has been thinking about. If you’re not going to remove these trees mechanically – because you don’t have enough sawmill capacity or roads – one thing you can do when the trees go into the “red phase” is begin new burning to remove some of that fine fuel that’s just accumulated. The red phase is when all the needles and the smallest branches have fallen on the ground and the tree itself is still standing there. So you begin to work by going in there and burning out the understory fuels.

And then as more and bigger material starts coming down from all those dead trees, in 10 years or 15 years, you do it again. You’re taking out the accumulated fuel in layers. Just don’t wait for the whole thing to come down and sit there until the next wildfire comes along and you have a disaster on your hands.

Water Deeply: Have we ever attempted prescribed burning on that scale before?

Stephens: No, we haven’t done it. It’s a challenge. We don’t have a lot of experience with it.

Down in the San Bernardino and San Jacinto mountains, they had a similar mortality event from 1999 to 2002. It looked a lot like what’s happening in the Southern Sierra today. It was a big deal. They did a lot of work down there, spent tens of millions and most of the material was taken down and chipped in place. Some logs were shipped by rail to Northern California to make into wood products.

But I’ve heard of nothing like that going on in the Southern Sierra. So, I think it’s likely the trees will go into natural succession and just fall on the ground. If that’s the case, you have to look at those issues of hazard trees and wildfire potential. A lot of these areas are unroaded and have steep slopes. So the only thing you can use to manage the risk is fire, in some capacity.

Water Deeply: What are some of the challenges involved in doing this much prescribed burning?

Stephens: There’s no doubt funding would be needed to do this, and the work is not going to generate money. I know there’s been some requests by the Forest Service in California for additional funding from USDA to do this type of work. But I don’t know of any outcomes of those requests.

There’s another thing that’s really challenging about doing this at the scale we’re talking about. This is a lot of standing dead material, which is very dangerous. A forester will tell you one of most dangerous things they can do is enter a forest with lots of standing dead trees. More firefighters are killed by falling trees than anything.

Water Deeply: You co-authored a study that found prescribed fire actually increases moisture in forests and helps trees survive drought. How does that work?

Stephens: That study focused on a place in Yosemite National Park, about a 45,000-acre [18,210-hectare] area called the Illilouette Creek Basin. Beginning in 1972, the National Park Service began to allow lighting fires to burn there. They allowed maybe 80 percent of the fires to burn without suppression, so fire’s been back in there for 45 years or so. And it really has changed it.

If you go in there today, there’s a lot of forest that is relatively thin, low-density – big trees with lots of space. The Hoover Fire burned in there about 2001, and it burned an area we visited a bunch of times. I remember just being there, walking in that system, and it was a lodgepole pine forest, a pretty place. And after the fire we were walking in 6 inches of water. A wetland got formed. And it was even there last year, after four years of drought. And I thought, this forest has got to be changing.

In the study, we found the amount of water leaving that basin is either unchanged or has increased over time, and the areas around that basin that never had fire going back 45 years, they’ve decreased in moisture. We also know from the paper that the resilience – the ability of that forest to survive drought and insects – is much, much greater. So the tree mortality is much, much lower.

Water Deeply: So, is this what forests might have looked like historically?

Stephens: I think so, yes. We’ve done some statistical analysis based on tree-ring records. The pattern of fire that’s come back to the Illilouette Basin since 1972, compared to the late 1800s – amazingly the regime is almost identical. They did go through 100 years of fire suppression – the Park service was just as good as the Forest Service at putting out fires. So the pattern we see today, the reoccurrence of lighting fire, really was able to sculpt that forest.

Water Deeply: Does that mean we actually have ourselves to blame for all these tree deaths? The drought killed so many trees because we let the forest become too dense?

Stephens: I think you’re right. No doubt the drought was pretty intense. But I think it’s fundamentally a result of the forest structure we have right now. We’re seeing mostly in California a manifestation of unsustainable forest conditions. Not everybody would agree with that. In a condition the forests were probably in 200 years ago – with lower density, larger trees – you get a drought like that and a fire on it and you’re still going to maintain the vast majority of the trees. And today we’re not.

Water Deeply: How do you get people accustomed to this idea that the forests we have today need to change?

Stephens: I do think a lot more people today are at a place where they understand there’s a problem. I think there has been some tide change in terms of people becoming more accustomed to a little bit of burning in the forest.

But if you just talk about the inevitability of what’s going to happen in the future – if we don’t do anything we’re going to have so much more severe fires, more smoke, potential damage to the urban interface and homes. To me there isn’t a no-fire option. You have to maintain the forests as best you can. But it’s not going to be simple. You start small with maybe 10,000 acres [4,050 hectares] of prescribed fire, and maybe in year three you’re burning 400,000 acres [161,875 hectares]. You ramp it up and show people what you can accomplish. I worry much more about the standard fare of just watching what happens if we do nothing. I just can’t support that.

Never miss an update. Sign up here for our Water Deeply newsletter to receive weekly updates, special reports and featured insights on one of the most critical issues of our time.