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Summer of Fire: Drought Transforms Southern Sierra

There are an estimated 66 million dead trees in the southern Sierra Nevada. Jeffrey Moore of the Forest Service explains what it means for the region in the latest story in our series, Summer of Fire, on the state of California’s forests.

Written by Matt Weiser Published on Read time Approx. 7 minutes
Reddish-brown trees show the high rate of tree mortality in the Sierra National Forest.U.S. Forest Service

It was revealed in June that California’s southern Sierra Nevada is now stocked with an estimated 66 million dead trees, all killed directly or indirectly by the state’s ongoing drought.

The number is staggering and difficult to comprehend. Even more difficult is understanding what it means. In short, the southern Sierra – the highest section of California’s majestic mountain range – is undergoing ecological changes on a scale never seen before in human history.

The estimate was obtained by the U.S. Forest Service using surveys conducted during airplane flights over the range. The information will be used to assess the health of California’s federal lands, to plan firefighting strategy and land management policy.

But just as importantly, the survey of dead trees is intended as a wake-up call to the public and policymakers that the status quo is over. What we see in the mountains when we go there for recreation is changing dramatically, and how we manage that landscape is going to have to change.

More surveys are planned this summer and fall. To better understand the results and what they mean, Water Deeply recently spoke with Jeffrey Moore, aerial survey program manager with the Forest Service. Moore, a U.S. Army veteran who specialized in helicopter repair, has a degree in natural resources management and has been doing aerial surveys for the Forest Service for nearly 20 years.

Water Deeply: How serious is this. Is it unprecedented?

Jeffrey Moore: I hate to use the word unprecedented. Unprecedented means it’s never happened before. As far as we know, it hasn’t happened in Anglo written history. As long as we’ve been around, we haven’t heard of anything of this magnitude.

There are a lot of different things that may or may not happen. The future is going to tell how serious it is, in some ways. We don’t know how well a lot of these areas are going to come back to coniferous forest types. It may be a conversion of some type. To me, that is very serious.

Water Deeply: What kind of conversion do you mean?

Moore: Most likely, it would convert back to oak woodlands or shrub-steppe. These things do change and move up and down the slopes over the eons. At some point, it was even below water. It depends on how far back you want to go.

Water Deeply: How would such conversions affect the landscape, and it’s uses?

Moore: In the long term, you’re going to have less biomass being sequestered, less oxygen getting put back into the atmosphere, and less carbon being pulled out of the air. Also, if it converts back to a shrub-steppe type, that radically changes the fire regime. Basically, if it turns back into chaparral, the fire regime would convert into that as well, meaning extremely intense fire occurring regularly.

Water Deeply: What does this mean for climate change?

Moore: It definitely tips the balance, probably, of California forests becoming a net producer of CO2. I don’t think it will be the carbon sink that it has been. I don’t think anybody is arguing with that. They won’t produce as much biomass, they won’t sequester as much carbon.

Also, since coniferous forests use more water and transpire more water back to the atmosphere you’re going to change that, as well. There just won’t be that water going back into atmosphere. It could, to some extent, affect the weather and the overall temperature, as well.

Water Deeply: What contributes to all this tree mortality?

Moore: It’s drought-driven. The drought has set the stage for bark beetles to take advantage of the situation. And now we have a massive population of bark beetles out there, which will continue to kill trees even if we have normal to near-normal conditions the next few years. There’s always this lag time of a drought, with available (tree) material to take advantage of. You have a drought, and then a period of probably a few years later, the bark beetles are the main player at that point.

Water Deeply: So, will the number of dead trees continue to grow?

Moore: For this year, I’m expecting it to be much larger than last year. And I hate to say how much. And I would expect to have very high levels of mortality, at least in the next year, if not more.

Water Deeply: How are the trees surveyed?

Moore: The way we operate is to have two observers on the plane, one looking out either side of plane. We will each have a computer touch tablet in our lap, that is geo-referenced to where we are in relation to the Earth, and with some sort of satellite or aerial photographic imagery as a background on these tablets that scrolls along as we’re flying. We then draw polygons where we see mortality on the ground, and then attribute it as closely as we can to what’s really going on.

What we do is we kind of grid-fly an area and then we’re done with it. It will be 3-4 miles (5–6.5km) spacing, kind of like mowing your lawn: You fly down one way and come back over an adjacent track.

Water Deeply: How do you judge mortality?

Moore: I’ve been a little upset with some of the ways this has been worded in the media lately. When they say 26 million new dead trees since last October, that’s pretty crazy. What we’re seeing is trees that have changed color. When a tree first dies, it stays green for some time. Usually over several months to a full season later, it has changed over from green to a red or yellow color. That’s when we record it.

Typically, within the following season after that, that foliage will have dried up, blown off and be removed from the tree to some extent. Then it just looks gray. Then at that point, we no longer care about it from a survey point of view, and we don’t record that any longer. We try not to remap trees that way. What we generate, over time, is a trend, a timeline of tree mortality on the landscape.

Water Deeply: How do you approximate the count of trees?

Jeffrey Moore, shown at far left, with fellow survey crew members and the aircraft they use to count dead trees in the Southern Sierra Nevada. (U.S. Forest Service)

Jeffrey Moore, shown at far left, with fellow survey crew members and the aircraft they use to count dead trees in the Southern Sierra Nevada. (U.S. Forest Service)

Moore: When it’s a large area, we typically revert to a trees-per-acre calculation. Every time I fly near a football field (which is about an acre in size), I look at that and try to calibrate my eye. I just try to put down a series of those where I’m looking, and estimate how many trees will be in that football field.

So it’s a very rough estimate. But whether it’s 20 million or 30 million, the main thing is, we’re getting into some sort of a ballpark figure. We try to round our numbers. And maybe we should even round more. But people don’t like to hear about the possible spread.

Water Deeply: Is it possible the numbers are actually greater than what you are estimating?

Moore: Very possible. And in verification of prior studies, we do actually underestimate. Small trees can be hidden underneath bigger trees that we can’t see. We have that top-down, oblique angle that we’re using, and sometimes we just can’t see some areas very well.

Water Deeply: What’s it like to sit in that plane and fly over miles and miles of dead trees?

Moore: It’s a little gut-wrenching. But I think, like any doctor or nurse or social worker, you kinda get hardened to it after a while and try not to internalize it too much. And I think anybody in the natural resources arena does the same sort of thing, whether they look at it from the air or not.

Water Deeply: This degree of tree mortality has fundamentally changed the landscape in some areas. And the fire situation is very severe. How do we deal with fire risk on this scale?

Moore: I don’t think we can. This is just my personal opinion. I think it’s going to be a very severe fire year, and there’s going to be some fires that just can’t be helped. We may have to let them go. It’s happened before. I think in 2008 we had a number of fires that were burning out of control. If we get a big lightning event, I think that very well could happen again.

Water Deeply: Your surveys examine the southern Sierra. What about the northern Sierra?

Moore: It’s not anywhere near the (mortality) extent of the south. It’s much more sporadic on the landscape. There are small intense pockets of pine mortality and scattered white fir mortality on the landscape.

Water Deeply: Why is not as bad in the north?

Moore: The drought hasn’t been as severe or as prolonged, primarily. We have had more snowpack, more moisture availability and the trees have been healthier as a result.

Water Deeply: What other concerns does this great number of dead trees pose?

Moore: We have to be worried about hazard trees around people’s houses and power lines. That’s a huge concern, and we’ll have to do things to ameliorate that hazard.

Wildlife that are dependent on coniferous forests may not be able to live in some areas. Others that are fine with a mixture of oak woodlands and shrubs will do better. We did see a lot of oak mortality in some places, too, but I expect they will come back. Oaks have a lot of big seeds, and there tends to be a lot of small oaks just looking for opportunities to grow in these areas. But it will be a long time before they mature and start reproducing.

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