Tree mortality and fire
Tree mortality and fire: implications for the North Coast of CA
*file240829*Tree mortality has become a prominent issue in California in recent years, due to the extended drought, wildfires of unprecedented size and severity, and outbreaks of insects and disease, as well as some targeted activities associated with forest management and restoration. CAL FIRE estimates that the drought alone has killed 29 million trees across the state between 2013-2015—enough to inspire the Governor to issue an emergency proclamation in October 2015.
Fire hazard is one of the primary concerns associated with extensive tree mortality in the West. Understandably, people fear that dead trees will contribute to dangerous fuel loads and increase the risk of catastrophic wildfire. However, the relationship between tree mortality and fire is complex, and it is important to understand how the timeline of tree mortality and decay—as well as the location and arrangement of dead trees—can affect fire risk around homes and communities and in our forests and wildlands.
What do we know?
In a 2012 paper, Hicke et al. synthesized findings from 39 peer-reviewed studies on the effects of bark beetle-caused tree mortality on wildfire, and they developed a conceptual framework to describe expected changes in fuels and fire behavior.
- They found that time since mortality was the key factor influencing these changes, and that changes in fuels and fire behavior were variable—some elements of were enhanced by tree mortality, while others either stayed the same or were diminished.
- For example, canopy bulk density is greatly reduced in the years following mortality, as needles and fine branches dry out and fall to the ground. This can increase litter and fine fuels on the forest floor, thus contributing to surface fire potential, while simultaneously reducing the potential for active crown fire. Without canopy fuels, individual trees may torch, but it is unlikely that crown fire will be able to initiate and spread.
- In the decades following mortality, surface fuels continue to recruit to the forest floor as snags fall over, and ladder fuels increase as the understory redevelops. The exact timeframe for these patterns varies by forest type, but it’s important to understand that the nature of dead tree-related fire risk changes over time.
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Figures 1 & 2 from Hicke et al. 2012 showing the timeline of fuels and fire behavior following tree mortality. Click image to enlarge.
North Coast examples:
Research on sudden oak death provides a Coast Range example of these same patterns. Valachovic et al. (2011) characterized the fuels impacts of sudden oak death (SOD) in Douglas-fir/tanoak forests in northwestern California, and they found time since infestation to be the defining element of the fuels profile in diseased stands. In SOD-impacted forests, crown fire potential is highest in the period directly following tree mortality, when the leaves are still in the canopy but have significantly lower moisture contents than the leaves of living trees (Kuljian and Varner 2011). After that initial period, fire behavior is driven more by surface fuel accumulations from falling branches and snags—and live fuels from understory growth—than by canopy fuels. The Valachovic paper also looked at stands that had been treated with herbicides, and found that fuels in herbicided stands followed a similar trajectory as SOD-infested stands, though more condensed; herbicide treatment is a one-time disturbance that leads to a single surge of fuels, in contrast with SOD, which contributes a steady flow of new fuels over time as individual trees become infected and die. This is an important distinction, because at any given time, a SOD-infested stand could have trees in the initial phase of mortality (associated with increased crown fire potential), whereas all trees in an herbicide-killed stand will move beyond that phase at the same time. These findings demonstrate the importance of understanding the mechanisms and timelines of tree mortality and fuels, and designing hazard mitigation efforts accordingly.
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The location and arrangement of dead trees can also have important implications for fire hazard in affected stands.
- At the statewide scale, CAL FIRE has designated “fire hazard severity zones,” which describe the relative fire hazard levels of state responsibility areas in different parts of the state. On the North Coast of California, fire risk is lower than in other, drier regions, such as the Klamath Mountains, the Cascade Range, and the Sierra Nevada. A number of factors contribute to these hazard ratings, including the likelihood of ignition sources like lightning.
- At a more regional scale, tree mortality is of heightened concern around homes and communities, where it can threaten structures and other resources, complicate fire response, and compromise firefighter safety. This is the reason that recent tree mortality mitigation efforts, like those mandated in Governor Brown’s 2015 executive order, have focused explicitly on hazard tree removal near power lines, roads and evacuation routes, community infrastructure, and homes and other structures in the wildland-urban interface (WUI).
- At the stand scale, fire risk will depend on the proportion and arrangement of dead trees. A few scattered dead trees, such as snags that provide wildlife habitat or are a component of oak restoration-focused projects (see "girdling" below), are unlikely to have a major influence on potential fire behavior. This is in contrast with whole stands of dead trees, such as drought- and bark beetle-killed trees in the southern Sierra, which have the potential to alter fuels and fire behavior. Again, the extent to which mitigation efforts are needed to address changing fuels and fire behavior will likely be dictated by the proximity of those stands to WUI areas and other critical resources at risk, and the likelihood of ignition.
References:
Hicke, J.A., Johnson, M.C., Hayes, J.L. and Preisler, H.K., 2012. Effects of bark beetle-caused tree mortality on wildfire. Forest Ecology and Management, 271, pp.81-90.
Valachovic, Y.S., Lee, C.A., Scanlon, H., Varner, J.M., Glebocki, R., Graham, B.D. and Rizzo, D.M., 2011. Sudden oak death-caused changes to surface fuel loading and potential fire behavior in Douglas-fir-tanoak forests. Forest Ecology and Management, 261(11), pp.1973-1986.
Kuljian, H. and Varner, J.M., 2010. The effects of sudden oak death on foliar moisture content and crown fire potential in tanoak. Forest Ecology and Management, 259(10), pp.2103-2110.
For more information on specific causes of tree mortality: