Wildfire and droughts in Sierra Nevada forests

With much of the state’s water supply originating in the mountains as precipitation on the forested landscape, the health of the forests are paramount to California’s water supply. In this presentation, Wildfire and droughts in the Sierra Nevada forests, Susie Kocher, Natural Resources Advisor for the University of California Cooperative Extension Service discusses drought and wildfire in the Sierra Nevada, and the importance of fire to the health of the forest. Ms. Kocher is a registered professional forester covering El Dorado, Amador, Calaveras and Tuolumne counties, where she works with forest land owners to help them understand how to better manage their forests. In her presentation, she discusses the fire history of the Sierra Nevada, the types of forests found there, the consequences of fire suppression and possible remedies.

Ms. Kocher began with a short fire history of Sierra Nevada forests. “We know from the scientific record that Sierra forests are adapted to frequent, low intensity fire,” she said, displaying a slide with a picture of a Ponderosa pine with a fire scar, noting that the fire burned up to the side of this tree and went by but the tree survived. “We have a lot of evidence, including ethnographic interviews with Native American tribes. Pre-EuroAmerican settlement. Native tribes did a lot of burning in California – they did it to keep the forests open, promote acorns which was their primary food source, and meadow foods – plants, tubers, that sort of thing. They did to suppress insects in the forests and develop pliable shoots in shrubbery for basketry materials. There are still plenty of Native Americans who can tell us about their fire and burning histories so we know that was one of the sources that the forests were actively managed for. We also had plenty of lightning fires, so there were natural occurrences as well.”

Tree rings are another source of data. “The study of tree rings is called dendro for trees, and chronology for sequencing or time,” she said, displaying a picture of a tree ring sequence. “These black lines are all individual fires that burned next to this tree while it was still alive. You can actually go through and measure one ring per year and figure out how often a fire came by this particular site. You can do it on a single tree or on several trees, and that is called a fire history, and you can understand what the natural plus Native American burning fire regime was in a particular area.”

A recent estimate showed that California was burning all the time in the past, she said. “Between 4 and 12 million acres in California burned every year, so that’s between 5 and 12 percent of the landscape annually through Native and natural sources,” said Ms. Kocher. “So before fire suppression started, the skies were probably smoky in the summer and fall all the time. C.H. Merriam was the chief of the U.S. Biological Survey and in 1898 he commented that, ‘Of the hundreds of people who visit the Pacific slope every year to see the mountains, few see more than the immediate foreground, and a haze of smoke, which even the strongest glasses is unable to penetrate.’”

“So the idea that fire is controlled and rare as an occurrence is a really modern idea that we probably need to abandon, because we’ve gotten to the point where we can’t really control all those wildfires anymore like we used to,” Ms. Kocher pointed out.

Ms. Kocher then turned to the different forest types, which in California vary according to elevation and latitude. “Our individual native trees are adapted to a wide range of moisture, elevation, snow loads, and light. They are found in natural groupings throughout the Sierra and the rest of the state, and each one of these groupings has its own characteristic fire return interval in the past that we can tell from tree ring records.”

Ms. Kocher noted that the light green areas on the map are the mixed conifer forests, the dark green are true fir, and on the east side of the Sierra, there are lodgepole pines. “We have clumps of different types of forests depending on the elevation and latitude. The time that each of these areas evolved to burn is called the fire return interval, which is the time between two successive fire events at a given site. In the Ponderosa pine belt, my records show every five to twelve years, a low intensity fire would come through on most acres. Mixed conifer going up, 8 to 20, Red fir higher up 15 to 50, sub alpine between 25 and 60 years.”

She then presented a map of the fire return interval in the Sierra Nevada, explaining that before fire suppression, the brown areas on the map burned on average every eleven years, the yellow areas about 40 years, the green areas about 60 years, and the darker greens every 150 years, on average. “Obviously not like clockwork, it’s an average number,” she said. “The reason that these numbers go up with the elevation is that the higher up you go, the moister it is, and the slower it is for the vegetation to grow back naturally after a fire. In this brown area, there’s enough productivity in the forests that the needles and branches are dropping, and it can carry a fire every 11 years, but when you get up higher in those snowy areas, it takes a lot longer for that area to build up the fuels necessary. It’s also moist longer during the year, so it’s a less frequent fire regime.”

The next thing to consider is the severity of the fire. “A fire that’s very frequent doesn’t allow a lot of fuels to build up, and so generally these are low severity surface fires,” she said. “There’s a little bit of fuel built up. The flame lengths are very low, anything with less than about 4 feet is considered low severity, and at that level, it’s not very likely to kill very many trees. It can be suppressed directly by people just using hand tools. And so this kind of fire takes some of the understory fuels and is considered to be what was the most common in the past before fire suppression.”

“There were probably pockets of higher severity,” she said. “This is a picture of a moderate severity fire – there are individual trees torching out, especially in the understory, and these ladder fuels which can conduct fire up into the crowns of the trees and kill all the rest are burning in small pockets, so you have flame lengths up to 10 feet tall and some trees are killed. It’s a little patchy.”

The highest severity of fires are crown fires, she said. “The fires are burning through the crowns of the trees and there are surface fuels to support it to continually stay up there. The behavior is unpredictable and most of the trees are killed outright or might die from the heat. They are very hard to suppress. These are the ones that are out of control that we worry about a lot more than moderate and low severity fires.”

“The fire return interval and the severity of wildfire are linked,” said Ms. Kocher. “This is in a natural fire regime. They all had pretty quick fire return interval, less than 35 years, and were low and mixed severity. Most of the trees survived just fine. Rocky mountain lodgepole – those forests are evolved to burn at high severity, they all burn every 35 to 200 years and they start over. It’s a stand replacing fire leading to new seedlings coming in afterwards. It’s really important to keep in mind where you are and in which ecosystem you are in to understand how the systems there evolved, and what kind of fire they evolved in.”

She noted that, for example, the Ponderosa pine has a nice thick bark and it naturally self prunes so there aren’t a lot of branches at the base, so a little fire can come by and it doesn’t get up into the canopy; it out competes others which are not so well adapted.

Ms. Kocher than gave a brief history of fire suppression. “Our forests evolved to very frequent fire regime, which, starting in about the late 1800s, which was then suppressed,” she said. “There were some really big fires. In 1910, a fire burned 5 million acres in Montana and the west, and it killed over 79 people. There were firefighters who were killed, there were cities that burned down, and it was considered a huge problem, so after that, the forest service stressed fire prevention and the need to control fires as quickly as possible.”

More fires in the 1930s, including the Tillamook burn of 1933, in which 3 million acres were burned, let to the establishment of a ‘no-burn’ policy, she said. “All fires had to be controlled as quickly as possible, preferably during the very first afternoon, and if you didn’t’ get it by the first day, you were going to get it by the next day at 10am, or the next day, and that was called the 10am policy.”

“It wasn’t until about the 1970s that managers, scientists, really started to appreciate the role of fire in the natural ecosystem,” she said. “There are many proponents of more use of prescribed fire, although we have many institutional barriers to move that forward, so our understanding has evolved, but that doesn’t necessarily mean that our policies have evolved to where they need to be to improve this situation.”

“There are a lot of consequences to fire suppression,” she said, displaying a picture series from the Bitterroot Forest in Montana after fire was excluded in 1909. “Starting here on the left, you can see 1895, by 1909 it’s starting to grow in, and here are the conditions today. Without a small, low severity fire to burn up the small understory, it’s gotten crowded and overstocked.”

She next presented a slide from the east branch of the north fork of the Feather River under snow in the 1890 noting that there’s been no logging at this location. “It was naturally very widely spaced. The white is the snow. On the right is the same location about 100 years later. All those areas have filled in with a lot of living trees and fuel, and that’s one reason why our fires are getting so much more severe over time.”

She then presented a graph produced by Carl Skinner, a fire ecologist at the Pacific Southwest Research Station of the U.S. Forest Service, depicting the fires and the rate of tree establishment. “They did a tree study that could show when all the trees in this particular trees were established; they could also see the fires,” she said, noting that the green bars represent the fires and the yellow bars are new trees establishing. “So some trees would establish, there would be a fire, some trees would live, some would die, and overtime you have a fully stocked forest.”

After they started suppressing the fires, there is a huge pulse of trees establishing in 1900, 1910, 1925, and you get down into the 70s and pretty much all the growing space is occupied, she said. “These stands are getting older and having some natural mortality that leads to the fuels on the ground. This is how we’ve gotten into a big problem in the Sierra Nevada.”

There are ecological consequences to fire suppression and drought, she said. “The ecological consequences of fire suppression are basically increased stress due to water competition between trees that now have so many more neighbors than they used to; those trees are more vulnerable to insects and disease. There is a lot less different kinds of understory herbaceous plants in the understory because everything’s shaded out. Many shrubbed areas are probably now all in conifers where in the past, they may not have been able to grow conifers with frequent fire. There is displacement of deciduous vegetation in riparian areas. The meadows are being taken over by conifers – there is a reduction and loss of meadows for that, the reduction and loss of more open areas. The biggest one that concerns people from a safety point of view is the huge buildup in the forest fuels because dead things are not getting burned up.”

“One of the biggest consequences of the buildup in fuels has been the increase in high severity fires,” she said, displaying a picture of the Rim Fire, noting that in the background, you can see some areas that burned at very high severity. “Fires that burn now are much more likely to be high severity, meaning that all or most of the trees are killed.”

Weather is a factor in the severity of a fire event, she said, displaying a slide of the 2006 Hancock Fire, noting that the areas in red are high severity and areas in green are low severity. “There are patches of high severity where all the trees have been killed, but those are mostly on south-facing slopes, which means that it’s drier and hotter there. High severity is not a very high percentage here.”

“On the other hand, here in 2002 is a picture of the Biscuit Fire in Oregon, and you have much larger areas of red, which means large areas of high severity fire patches where all these trees were killed,” she said.

“So there’s a variety, but on average, we’ve documented that the percentage of area burning at high severity has increased dramatically, from 17% in 1984 to about 30% in 2006.” She noted that the statistic is from almost ten years ago, and it would be much higher if it included the Rim Fire. “The reason that this is a problem is that the larger the patch is, the less likelihood there will be a living tree which could lead to a seed source which could lead to a new tree. The patches themselves are getting larger.”

Ms. Kocher then presented a map of the Rim Fire, noting that the areas in red are the high severity fire where almost every tree is killed. “One hundred thousand acres almost, which is about 40% of the area. You can see there are huge areas without any living tree, and left to its own devices, seeds would have to blow in. That’s a very large area, so it could be a very long time before the forest is reestablished without some intervention like replanting, so this is why we are so concerned about it. In some areas, you’ll be seeing some type conversion to shrublands and forests not growing back.”

How does drought affect this trend? “Basically it just makes it more so,” said Ms. Kocher. “In a drought, you’ll have more fires because there’s reduced moisture in the fuels and in the live vegetation so they are more flammable, and there’s reduced relative humidity, so anything that starts has a higher chance of getting away.”

“Drought also affects the snowpack in the Sierra Nevada,” she continued. “We had very low snowpack this year, so the snow melting sooner increases the length of the fire season; it just gives you that many more months to have a big fire. It also reduces the water availability for fire fighting. There are some areas in some of the counties I work with where they are starting to plan for emergency supplies to be able to catch the fires if they get one because their water is so low.”

Drought also affects tree health, making them more vulnerable to insects and disease. “If a tree is attacked by a bark beetle, what it needs to do is spit out sap to try and repel and flush the beetles out,” she said. “If they don’t have enough moisture, then they don’t have the vigor or even the moisture itself in order to produce the sap and they are much more likely to die from any insect or disease outbreak, and from any wildfire because they don’t have a lot of resilience.”

In this third year of drought, there has already been an increase in fires, she said. “CalFire has responded to date to 1900 fires in their state and local responsibility areas, which is 300 more than last year at this time,” she said. “That doesn’t even count forest service starts. They went to peak staffing two weeks early and had to have extra appropriations in order to respond to wildfires, so we know there’s a financial effect as well.”

This is only expected to worsen because of climate change, she said. “There’s no way to know if one drought is caused by climate change, but we do know that as things are warming over time, that we expect this trend to continue,” she said. “If its warmer, you’re getting lower snowpack and therefore your season is longer and this is what you end up with. The yellow is 10% more fires, the red is 2.5 times more fires, by 2050 you can see some areas of red in the Sierra Nevada, and by 2100 you see some really dark areas where you expect 7.5 times more wildfires based on just the increase in temperatures and lack of snowpack. So that’s the bad news.”

So what do we do? “Basically the message on what to do is not that different from what we suggest people do to maintain a resilient forest at any other time, and that’s to make sure that you’re reducing moisture stress by having a properly thinned forest that doesn’t have a whole lot of extra trees,” she said. “The only way to get the moisture levels closer to what trees can survive on is to have them thinned to the appropriate stocking level so there’s not as much competition and what moisture is there goes into one tree instead of 20. That way, the individual trees is more vigorous. Thinning is used as a way to substitute for the historical fire regime by altering the forest structure.” She added that thinning also helps vulnerability to insects and disease.

She noted that there could be grants available to help landowners with the costs, or in some cases, the trees can be sold. “There have been a lot of great funding programs from the state to try and help landowners get their forests more resilient, so that’s the number one way of improving your chances to survive drought that we can recommend.”

We know that generally thinning works and helps trees survive wildfires, she said, presenting a slide of the results of a study which combined data from six fires in 2010, looking at treated and untreated areas, noting how many trees survived. “Of the areas that were treated or thinned, 80% of the trees survived the wildfire that came through,” said Ms. Kocher. “In the untreated areas, not thinned, but just hadn’t been burned in 100 years like a lot of the Sierra, only 30% of the trees survived, so a huge difference in tree survival. That’s what we mean by it worked – it doesn’t mean that we stopped it, it’s that we reduced the severity of the fire.”

The study also looked at the severity of the fires in both the treated and untreated areas. “This graph also shows from that same study the percentage of the area that burned at what severity,” she said. “The white is low severity, so 60% of a treated or thinned area burned at low severity; 30% at moderate meaning some trees died, and about 10% at high severity, meaning all the trees died, compared to an untreated area, high severity fires, 70% all the trees died. This shows that it works and it’s also getting us back to closer to how the forests were before we started suppressing fires so much … We know that thinning seriously promotes resilient forests.”

Prescribed fire is a great way to reduce the stocking level, but there are a lot of barriers, she said. Ms. Kocher noted that a lot of these things should have done two years ago or the year before, but are not emergency responses. “Trees are such long-lived organisms and forest management is a hundred year proposition, so we would encourage you to think about this as a warning, wake up call for the next drought, and to be planning ahead in all of your forest management operations.”

“Homes are actually the biggest fuel in the forest,” said Ms. Kocher, noting that embers can blow in the wind and hit a house, and if they find something flammable, it will catch on fire. “Almost all houses that burn in big fires burn in the first few hours because there aren’t enough firefighters. In this case, they were all out at the head of the fire, the fire started at 2 in the afternoon and all the houses burned down by 6 in the evening that very night. … In general, homes are vulnerable to wildfires and so spending time looking at the home and reducing its ability to catch fire is going to give you a lot of payoff in the long run and increase the risk of your house surviving a fire.”

Note: Most – but not all portions of this presentation were covered. Those portions not covered were portions pertaining specifically to landowners who manage forested landscapes, and some details on steps homeowners in forested landscapes should be taking to guard their homes against wildfires. If these are your concerns, please review the last 15 minutes of the webinar.

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This post is derived from a webinar from the series, Insights: Water & Drought Online Seminar Series, produced by the University of California Agriculture and Natural Resources.