Understanding How Fuel Treatments Interact With Climate and Biophysical Setting to Affect Fire, Water, and Forest Health: A Process-Based Modeling Approach

William D Burke,Max A Moritz,Maureen C Kennedy,Christina Tague

doi:10.3389/ffgc.2020.591162

Abstract

Fuel treatments are a key forest management practice used to reduce fire severity, increase water yield, and mitigate drought vulnerability. Climate change exacerbates the need for fuel treatments, with larger and more frequent wildfires, increasing water demand, and more severe drought. The effects of fuel treatments can be inconsistent and uncertain and can be altered by a variety of factors including the type of treatment, the biophysical features of the landscape, and climate. Variation in fuel treatment effects can occur even within forest stands and small watershed management units. Quantifying the likely magnitude of variation in treatment effects and identifying the dominant controls on those effects is needed to support fuel treatment planning directed at achieving specific fire, water, and forest health goals. This research aims to quantify and better understand how local differences in treatment, landscape features, and climate alter those fuel treatment effects. We address these questions using a mechanistic coupled ecohydrologic model—the Regional Hydro-Ecological Simulation System (RHESSys). We ran 13,500 scenarios covering a range of fuel treatment, biophysical, and climate conditions, for the Southern Sierra Nevada of California. Across fuel treatment type, biophysical, and climate parameters, we find nontrivial variation in fuel treatment effects on stand carbon, net primary productivity, evapotranspiration, and fire-related canopy structure variables. Response variable estimates range substantially, from increases (1–48%) to decreases (−13 to −175%) compared to untreated scenarios. The relative importance of parameters differs by response variable; however, fuel treatment method and intensity, plant accessible water storage capacity (PAWSC), and vegetation type consistently demonstrate a large influence across response variables. These parameters interact to produce non-linear effects. Results show that projections of fuel treatment effects based on singular mean parameter values (such as mean PAWSC) provide a limited picture of potential responses. Our findings emphasize the need for a more complete perspective when assessing expected fuel treatment outcomes, both in their effects and in the interacting biophysical and climatic parameters that drive them. This research also serves as a demonstration of methodology to assess the likely variation in potential effects of fuel treatments for a given planning unit.

Highlights

Informed forest and vegetation management is progressively more important as both severe drought and wildfire activity are predicted to increase in the Western US (Moritz et al, 2012; Clark et al, 2016)
The 13,500 scenarios produced by the varied input parameters result in noteworthy range and variability in effects on forests, water (ET), and fire (FSP, shrub fuel height, and conifer canopy fuel gap)
Most treatment effect distributions are roughly normally distributed, some variables including ET, shrub fuel height, and conifer canopy fuel gap (Figures 4C,E,F) show left tailed skews. The result of this is that, despite fuel treatment effects broadly conforming to expected outcomes (H1–H4), some subset of scenarios will diverge from those expectations

Summary

Introduction

Informed forest and vegetation management is progressively more important as both severe drought and wildfire activity are predicted to increase in the Western US (Moritz et al, 2012; Clark et al, 2016). At the same time, increasing fire severity in many of these regions has led to unprecedented social and economic costs (Moritz et al, 2014) Given these ecologic and socio-economic costs, fuel treatments are increasingly proposed as a way to reduce risks associated with both droughts and fires. Fuel treatments have a variety of purposes, from timber harvestoriented practices to increase productivity, to the restoration of historic forest structures and associated habitat. Key among these purposes is the role that fuel treatments can play in reducing wildfire severity (Hessburg et al, 2016; Barros et al, 2019) and mitigating drought impacts on vegetation (Tague et al, 2019). We need to understand more broadly how those treatments are altering our landscapes and affecting resources we care about, both directly and indirectly

Objectives

Methods

Results

Discussion

Conclusion