Simulating post‐wildfire forest trajectories under alternative climate and management scenarios

Abstract

Post-fire predictions of forest recovery under future climate change and management actions are necessary for forest managers to make decisions about treatments. We applied the Climate-Forest Vegetation Simulator (Climate-FVS), a new version of a widely used forest management model, to compare alternative climate and management scenarios in a severely burned multispecies forest of Arizona, USA. The incorporation of seven combinations of General Circulation Models (GCM) and emissions scenarios altered long-term (100 years) predictions of future forest condition compared to a No Climate Change (NCC) scenario, which forecast a gradual increase to high levels of forest density and carbon stock. In contrast, emissions scenarios that included continued high greenhouse gas releases led to near-complete deforestation by 2111. GCM-emissions scenario combinations that were less severe reduced forest structure and carbon stock relative to NCC. Fuel reduction treatments that had been applied prior to the severe wildfire did have persistent effects, especially under NCC, but were overwhelmed by increasingly severe climate change. We tested six management strategies aimed at sustaining future forests: prescribed burning at 5, 10, or 20-year intervals, thinning 40% or 60% of stand basal area, and no treatment. Severe climate change led to deforestation under all management regimes, but important differences emerged under the moderate scenarios: treatments that included regular prescribed burning fostered low density, wildfire-resistant forests composed of the naturally dominant species, ponderosa pine. Non-fire treatments under moderate climate change were forecast to become dense and susceptible to severe wildfire, with a shift to dominance by sprouting species. Current U.S. forest management requires modeling of future scenarios but does not mandate consideration of climate change effects. However, this study showed substantial differences in model outputs depending on climate and management actions. Managers should incorporate climate change into the process of analyzing the environmental effects of alternative actions.

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

“…Increasing background mortality may be a result of greater competition from CO 2 -enhanced tree growth Phillips 2008, Doughty et al 2015 Forest thinning can reduce drought risk D 'Amato et al 2013, Giuggiola et al 2013, Tarancón et al 2014 MT 10 Table 2. Risk factors leading to greater vulnerability to tree mortality during hotter drought.…”

“…Tested models poorly replicate observed die-offs Steinkamp and Hickler 2015 PJ 9 Thinned forests can still die during hotter drought Tarancón et al 2014, Elkin et al 2015 MT 10 The scale of forest management is limited and projected warming rates and magnitudes of hotter drought extremes could overwhelm the effects of management actions Bonan 2008, Kurz et al 2008, Phillips et al 2009, Reichstein et al 2013, IPCC 2014, Lindner et al 2014 Table 3. Ten contrasting perspectives on the relative vulnerability of forests to tree mortality from hotter drought, summarizing the broad spectrum of evidence associated with perspectives of both lesser and greater vulnerability presented in the main text and Tables 1 and 2, which also provide supporting references.…”

“…Warmer conditions and greater drought stress can amplify additional tree-killing disturbance processes and their interactions, including increased insect pest outbreaks (Raffa et al 2008, Weed et al 2013) and more frequent and severe fires (Pechony and Shindell 2010, Flannigan et al 2013, 2014, Brando et al 2014, IPCC 2014. As projected hotter droughts intensify in coming decades, post-mortality forest recovery could be limited, delayed for long time periods, or impossible because hotter droughts in association with more frequent and severe disturbances will create less favorable conditions for regeneration of many currently dominant species (Suarez and Kitzberger 2010, AndersonTeixeira et al 2013, Tarancón et al 2014, Fensham et al 2015. Additionally, hotter droughts drive shorter lifespans for surviving and establishing plants (Adams et al 2009) and can be expected to favor: smaller-statured trees; transition of resprouting tree species toward more shrub-like morphologies; and non-tree life forms such as resprouting shrubs and grasses in many areas (Zeppel et al 2014, McDowell andAllen 2015).…”

“…Thinned forests can still die from sufficiently extreme drought and heat stress (Tarancón et al 2014, Elkin et al 2015. On-site water retention (e.g., mulching) or enhancement (irrigation) are practical only on small selected locations.…”

“…As considered throughout this article, the diverse impacts of hotter drought could overwhelm the effects of forest management actions. For example, one key management strategy, forest thinning, likely becomes insufficient to buffer trees against mortality when droughts become severe enough ), just as it may be insufficient to prevent wildfire spread under hotter drought conditions (Tarancón et al 2014). As another example, sustaining historical forests through management of the genetics of native tree populations depends on genetic variation being sufficiently large to buffer against hotter drought events, which generally is uncertain currently.…”