Drought, Tree Mortality, and Wildfire in Forests Adapted to Frequent Fire
Massive tree mortality has occurred rapidly in frequent-fire-adapted forests of the Sierra Nevada, California. This mortality is a product of acute drought compounded by the long-established removal of a key ecosystem process: frequent, low- to moderate-intensity fire. The recent tree mortality has many implications for the future of these forests and the ecological goods and services they provide to society. Future wildfire hazard following this mortality can be generally characterized by decreased crown fire potential and increased surface fire intensity in the short to intermediate term. The scale of present tree mortality is so large that greater potential for “mass fire” exists in the coming decades, driven by the amount and continuity of dry, combustible, large woody material that could produce large, severe fires. For long-term adaptation to climate change, we highlight the importance of moving beyond triage of dead and dying trees to making “green” (live) forests more resilient.
According to recent estimates, more than 100 million trees have died in California primarily in the southern and central Sierra Nevada (USDA–FS 2016a) prompting the Governor to declare a state of emergency. Why there, and why now? The answer lies in the management of fire-dependent ecosystems exacerbated by the recent episode of acute drought. Most western US ecosystems are fire dependent, meaning that for millennia, the flora and fauna depended on periodic fire to maintain ecosystem integrity. The distribution and structure of these ecosystems were sustained by fire up until Euro-American settlement in the late nineteenth century, particularly in the low- to mid-elevation yellow pine and mixed-conifer forests where fires recurred at intervals of a few years to several decades (Safford and Stevens 2017). Frequent-fire (FF) in these forests selected for, and protected the majority of large, old trees by limiting biomass accumulation and thinning competitors in the understory.
Historical FF forests were highly variable with a mixture of tree densities and size classes, but as a whole, were much more open than they are today (Taylor and Skinner 2003, Knapp et al. 2013, Collins et al. 2015, Stephens et al. 2015, Levine et al. 2017). As a result of aggressive fire suppression promulgated by land managers in the early 1900s, FF forests soon experienced prolific tree regeneration (Show and Kotok 1924, Covington et al. 1997). This was welcomed for timber-production purposes, because foresters believed that fire had kept historical FF forests at only a fraction of their stocking capacity (Show and Kotok 1924). However, for over a century, fires have been excluded under almost all practical circumstances, with the limited hectares burning in wildfires mostly relegated to extreme weather when suppression efforts are largely ineffective (North et al. 2015).
Paradoxically, aggressive and largely successful fire suppression has left FF forests increasingly vulnerable to the negative effects of fire and other tree mortality agents (Young et al. 2017). Removal of frequent, generally nonlethal fires effectively stores fuel for those dry and windy conditions when fires exhibit extreme behaviors. The result is often extensive tree mortality, occurring in large contiguous patches (Lydersen et al. 2014, Jones et al. 2016). In many wildfires burning in FF forests, tree mortality patches are an order of magnitude or two larger than those that occurred historically (Mallek et al. 2013, Stevens et al. 2017). In areas that have not yet burned at uncharacteristic severity, fire suppression-caused forest densification has increased competition among trees for water and other resources, destabilizing many FF forests by making them prone to mortality from other agents such as bark beetles (Dendroctonus, Ips, Scolytus spp.; Kolb et al. 2016). The recent Sierra Nevada tree mortality associated with these agents is unprecedented and far more extensive than fire-caused mortality in individual wildfires (Asner et al. 2016). In dense FF forests, tree vigor is reduced as a result of competitive stress, and the potential for native bark beetles to mass attack is greater because of the closer proximity to host trees and other factors (Fettig et al. 2007). These combined effects increase susceptibility to bark-beetle-caused tree mortality, but the trigger that leads to actual widespread mortality is often a multiyear drought (Young et al. 2017).
The recent massive tree mortality has many implications for the future of FF forests and the ecological goods and services they provide to society (recreation, wildlife habitat, water storage, timber, aesthetics, carbon storage, etc.). It could be surmised that because FF forests have seen such dramatic increases in tree density relative to historical conditions, the bark-beetle-caused tree mortality could be helping to produce more resilient forest conditions (we define resilience as the ability of a forest to maintain characteristic structural components, such as large trees, and broad functionality following disturbance and/or chronic stressors). The actual outcome, however, will likely be forests that are very different from their historically resilient condition. For one, many of the trees killed by bark beetles are the largest trees (van Mantgem et al. 2009) and not the trees that would be preferentially killed by low- to moderate-severity wildfires or targeted for removal in restoration projects (i.e., small- to moderate-sized trees). Second, bark-beetle-killed trees are often not removed, as is commonly the case in restoration projects involving mechanical thinning or in forests subject to centuries of frequent fires. Tree biomass therefore remains on site, just shifted from the live “pool” with high moisture content to the dead “pool” with low moisture content (figure 1). This shift has the potential to significantly alter fire behavior and forest succession in FF-adapted forests.
A theoretical depiction of vegetation and fuel dynamics following severe pine mortality due to bark-beetle attack in a mixed-conifer forest. Initially (1–2 years following mortality), the primary change would be reduced moisture content of canopy fuels (a). In the intermediate time period (3–10 years), there would be an overall loss of canopy fuels as dead foliage and branches are deposited on the forest floor, and there would be a corresponding increase in dead and live surface fuels as tree seedlings and shrubs establish (b). The longer-term changes (11–20 years) would include continued low canopy fuels—although this could be offset by the growth of residual overstory trees taking advantage of the available growing space—and considerable increases in large surface fuel (c). Increased surface fuels would be in both the dead (primarily fallen snags) and live (regenerating trees and shrubs) pools (c).
In this article, we summarize research that may improve the understanding of the near- and longer-term effects of the massive tree mortality event in FF forests in California. It presents data and results from a recent wildfire illustrating how drought-induced tree mortality affected fire behavior and suggests management practices that might reduce future mortality and increase forest resilience and adaptation to climate change. The rapid and extensive tree mortality in the Sierra Nevada has surprised many observers and challenges management to proactively respond to what will likely become a more common occurrence under changing climate conditions (Fettig et al. 2013).
The impacts of tree mortality on wildfire
Tree mortality has long been known to play an important role in altering fuel dynamics within forests. The process of reducing live canopy fuels and subsequently increasing dead fuels alters the arrangement, composition, and quantity of fuel available for combustion. These fuel changes directly influence the spread and intensity of wildland fires, and indirectly influence micrometeorological conditions that can drive fire behavior and effects. Although a number of studies have directly evaluated alterations in forest fuel beds following bark-beetle-caused tree mortality (e.g., Hicke et al. 2012), empirical investigations of the effects of mortality on fire behavior remain limited. Instead, studies have mostly relied on the use of fire behavior modeling. Furthermore, most of the published literature has focused on investigations in more mesic forest types, such as lodgepole-pine (Pinus contorta), Douglas-fir (Pseudotsuga menziesii), and spruce-fir (Picea-Abies) forests (Agne et al. 2016), rather than in more seasonally xeric FF forest types, such as Ponderosa pine (Pinus ponderosa), Jeffrey pine (Pinus jeffreyi), and mixed conifer. Outcomes from the mesic forest types, which historically experienced infrequent (75–300 years), generally more intense fires are, in many cases, not directly applicable to the xeric forest types that historically experienced frequent (every 5–25 years), generally low- to moderate-intensity fire, and where the effects of fire exclusion on stand density are most pronounced.
Following the conceptual framework of Hicke and colleagues (2012), the impacts of bark-beetle-caused tree mortality on forest fuels are best understood by using broad temporal categories, or phases, to characterize changes to the fuels complex. During the initial phase, often termed the red phase, the conversion of live to dead canopy fuels reduces foliar moisture content and alters foliar chemistry. Both of these contribute to increased flammability within the tree by decreasing the heat requirements for ignition (Jolly et al. 2012, Page et al. 2012). The magnitude of this effect depends on the proportion and timing of tree mortality. If mortality is acute and extensive, increases in flammability would be expected. If mortality is more gradual, however, the increased flammability from recently killed trees can be somewhat mitigated by the loss of crown fuels from neighboring trees that died earlier, because most dead needles fall to the forest floor within 1 or 2 years following tree mortality.
In general, it is believed that the rate of fire spread and fire line intensity are increased during the red phase, at least under more severe burning conditions (figure 2); however, there is uncertainty surrounding this generality. A number of simulation studies have suggested that the decreased mean foliar moisture content of the canopy during this time period results in increased fire rates of spread and fireline intensities (Hicke et al. 2012, Hoffman et al. 2013, Linn et al. 2013). Other studies have revealed that there may be a decrease in these metrics during this time period especially in cases in which tree mortality was gradual and resulted in reduced canopy fuel loads (Simard et al. 2011). The discrepancy between studies may also in part be due to the use of simplified fire behavior models that do not account for the increased ignition characteristics of dead canopy fuels. Hoffman and colleagues (2015) used a physics-based model to compare the effects of a rapid and gradual tree-mortality event on simulated fire behavior and found that during the near term (1–3 years following mortality), the gradual and rapid scenarios increased fire rates of spread by 1.2- to 2.7-fold, suggesting that conversion of live needles to dead is an important characteristic driving near-term fire behavior. To our knowledge, the only study that has directly quantified the impacts of recent mortality (red phase) on fire spread is that by Perrakis and colleagues (2014), who in forests dominated by lodgepole pine found a rate of fire spread 2.7 times greater than that expected pre-outbreak.