Modeling the Interconnected Effects of Fuel Treatments on Forests, Water, and Fire
Fuel treatments, the reduction of forest biomass through mechanical removal or burning, are a flexible forest management tool used to address a variety of human and environmental concerns. Treatments can be used to reduce high severity fire, improve forest productivity and drought resilience, and increase streamflow. However, the effects of fuel treatments can be inconsistent and uncertain. Fuel treatment effects are sensitive to the treatment, the biophysical environment in which the treatment is done, and can be further altered by climate change. Climate change is already increasing wildfire size and frequency, drought, and straining water supplies in much of the Western US. Given that fuel treatments are likely to play a key role in current and future forest management, it is critical that we understand the full range of fuel treatment interactions with climate and effects on forests, water, and fire. Existing ecohydrologic models are limited in their ability to model fuel treatment effects because they do not account for both the within forest stand ecohydrologic effects of changes in forest structure and the hillslope-scale variation in radiation, water availability, and other meteorologic drivers. The ability to simulate heterogenous vegetation cover at stand scales is key to implementing fuel treatments like forest thinning. To address this gap, I adapted the Regional Hydro-Ecological Simulation System (RHESSYs) to include a new multiscale routing (MSR) approach, RHESSys-MSR. In addition to modeling within forest stand heterogeneity, MSR enables an additional layer of hydrologic routing that is included within existing topographic hillslope routing. Chapter 1 of this thesis describes the implementation of RHESSys-MSR, and Chapters 2 & 3 apply this model to investigate fuel treatment effects in a changing climate. In the first application of these methods (Chapter 2), I simulate a large set (13,500) of model scenarios varying treatment type, biophysical and climatic conditions for a Central California Sierra forest stand. Results show that plant accessible water storage capacity and vegetation type are dominant environmental controls that alter the effects of fuel treatments. More broadly I find that estimating the effect of fuel treatments based on only a single biophysical variable fails to capture the extent of possible treatment effects. In the second application (Chapter 3), I investigate the interactions between projected climate change and fuel treatment area on the effects of treatments on forest health, fire risk, and streamflow at the watershed scale. Results show that while fuel treatment effects can be noteworthy in the short term (< 5 years), projected climate change is likely to mask long term (20-year) fuel treatment effects. Fuel treatments and their effects are complex, spanning the environmental domains of forests, water, fire, and climate. Treatments are further complicated by the wide variety in the treatment itself, varying in how forest structure is affected, where it’s implemented, and how often. Model applications like RHESSys-MSR are critical to reducing this uncertainty and developing place-based estimates of fuel treatment effects that can support forest managers. The persistent challenges in understanding fuel treatments and their effects make any progress all the more essential, and as this research and more contributes to this understanding, we can make more informed forest management decisions for the future.