The area burned by wildfire in the Sierra Nevada has increased by 274% over the last 40 years and the area impacted by stand-replacing fire has also increased. The forests in the Sierra Nevada are important for provision of clean water and are also part of the state’s climate action plan. As a result, figuring out how to reduce the chances of large, hot fires presents a large challenge.
We know that the current pace and scale of forest treatments to reduce the risk of large, hot fires is inadequate given the scale of the problem and the area burned by wildfire is projected to increase with on-going climate change. In a recent study led by Shuang Liang, we set out to determine how the pace of large-scale treatment implementation would alter carbon storage across the Sierra Nevada. We ran simulations under projected climate and wildfire and two management scenarios. Both management scenarios included applying thinning and prescribed burning treatments to low and mid-elevation forests. These are forests that have been most impacted by fire suppression. In the distributed scenario, we simulated an equal portion of the area treated at each time-step and with full treatment implementation by the end of this century. In the accelerated scenario, we simulated the same treatments over the same area, but schedule the treatments so they were complete by 2050. We included a control scenario that assumed no active management for comparison.
The area burned between all three scenarios was fairly consistent because we used the same fire size distributions in our simulations (black line in Figure 1). However, the proportion of burned area that was burned by stand-replacing fire (severity 4 and 5) decreased substantially. The faster pace of treatment under the accelerated scenario increased the proportion of area burned by surface fire (severity 1 and 2) and decreased the area burned by stand-replacing fire at a much faster rate than the distributed scenario.
Both the accelerated and distributed treatments ended up storing more carbon than the control by 2100 (Shown by the difference in Figure 2).
However, what was most striking was how these treatments influence the carbon balance of Sierra Nevada forests as a percentage of California’s 2020 emissions limit from the Governor’s Climate Action Plan. Initially, total carbon losses are higher in the treatment scenarios, with the accelerated treatment having the largest loss (Figure 3). However by 2030, carbon loss is similar amongst all three scenarios and by 2050 the accelerated scenario has lower emissions than the wildfire emissions under the control.
As we demonstrated in a previous study, changing climate and the increase in area burned has the potential to increase wildfire emissions by 19-101% by later this century. The results from this study demonstrate that restoring surface fires to the low and mid-elevation forests in the Sierra Nevada can reduce the magnitude of future emissions and maintain a larger amount of carbon stored in these forests.
2016 has been an interesting year for wildfire research. A couple of studies published this year have identified linkages between the increasing area burned by wildfire and increasing temperature. Leroy Westerling published a study where he looked at the increase in area burned by large wildfires over the period 1970-2012. He found that across the western US, area burned by large wildfires has increased by 556% over the 1983-1992 average (Figure 1 top). His analysis shows that increasing temperatures correlate with longer fire seasons. Average fire season length increased by 84 days between the first decade of his analysis (1973-1982) and the last decade (2003-2012)
John Abatzoglou and Park Williams published a study where they looked at the relationship between fuel aridity and area burned in the western US. Fuel aridity is a measure of how dry the material in the forest is and the drier it is the more flammable it is. Their results show a strong relationship between this measure of dryness and area burned (Figure 1 bottom). Increasing temperature is also playing a role here and they attribute approximately half of the forest area burned by wildfire to human-caused climate change over the period 1984-2015.
Fire suppression costs by year from the National Interagency Fire Center website show that from 1985 to 2015 we spent approximately $36.6 billion on fire suppression in 2015 dollars. While the year with the highest total fire suppression cost was 2015 ($2.13 billion), the year with the highest per acre suppression cost was 1998 ($455/acre, Figure 2). Area burned in any particular year explains about 51% of the variability in suppression costs. A number of factors account for the remainder of the variability in suppression cost, including proximity to developed areas. As an example, the 2016 Soberanes fire in southern California burned 132,127 acres and cost an estimated $260 million to suppress, that works out to $1967/acre.
When we look at how decadal averages in suppression cost have change over time, the picture is similar to Leroy Westerling’s results (Figure 3). It is important to note that the first average suppression cost only covers the period 1985-1992 and the last average suppression cost bar is only for the years 2013-2015. The majority of the suppression expenditures are by the US Forest Service and in a 2015 report they showed that fire suppression accounted for 52% of their budget. Given that area burned by wildfire in the western US is increasing as the temperature goes up and suppression expenditures are on the rise, this really begs the question – how sustainable is our current relationship with fire?
The 2011 Las Conchas fire burned 156,593 ac (63,370 ha) on the east flank of the Jemez Mountains in northern New Mexico. While certainly not the largest fire in recent years, this particular fire burned through three areas previously impacted by wildfire. This New York Times article provides a good overview of the issues facing this particular landscape and many western landscapes in general.
In the southwestern US, the area burned by wildfire has increased 1266% over the 1973-1982 average. A recent estimate suggests that climate change contributed an additional 10.3 million ac (4.2 million ha) of forest fire over the period from 1984-2015, meaning that in the absence of climate change we would have expected about 50% less burned area over that period. As a result, severely burned landscapes like the east flank of the Jemez Mountains are becoming more common.
Increasing fire size and larger patches of severe fire, where the majority of trees are killed, create a challenge for reforestation. The first challenge is distance to mature trees that provide the seed for tree establishment. This challenge can be overcome by planting seedlings. However, trees modify the climate conditions at ground level. Under the canopy of a forest, the air temperature is cooler and the relative humidity is higher, making it a moister environment. These factors matter because hot, dry conditions can be lethal to seedlings. In a burned patch where all of the overstory trees have been killed, these microclimatic conditions may be too harsh for seedlings to establish.
We are in the process of establishing an experiment funded by the Joint Fire Science Program to figure out how the post-fire environment influences the ability of planted seedlings to survive and grow. The post-Las Conchas fire landscape has a mix of shrub and grass cover. Our hypothesis is that shrub cover could create more favorable growing conditions for tree seedlings. To test this hypothesis, we constructed exclosures in shrub and non-shrub patches and are in the process of planting seedlings.
We are instrumenting these sites with sensors that measure temperature and relative humidity at the height of the seedlings.
We’ll link the temperature and humidity data with data collected at weather stations that we are deploying around the experiment. This will allow us to model how microclimate (ground level temperature and relative humidity) vary across the larger area and predict how these factors influence tree seedling survival and growth. Stay tuned for updates on this project.