Missing Climate Feedbacks in Fire Models: Limitations and Uncertainties in Fuel Loadings and the Role of Decomposition in Fine Fuel Accumulation

Abstract

Climate change has lengthened wildfire seasons and transformed fire regimes throughout the world. Thus, capturing fuel and fire dynamics is critical for projecting Earth system processes in warmer and drier future. Recent advances in fire regime modeling have linked land surface models with fire behavior models. Such models often rely on fine surface fuels to drive fire behavior and effects, and while many models can simulate processes that control how these fuels change through time (i.e., fine fuel accumulation), fuel loading estimates remain highly uncertain, largely due to uncertainties in the algorithms controlling decomposition. Uncertainties are often amplified in climate change forecasts when initial conditions and feedbacks are not well represented. The goal of this review is to highlight fine fuel decomposition as a key uncertainty in model systems. We review the current understanding of mechanisms controlling decomposition, describe how they are incorporated into models, and evaluate the uncertainties associated with different approaches. We also use three state‐of‐the‐art land surface fire regime models to demonstrate the sensitivity of decomposition and subsequent wildfire projections to both parameter and model structure uncertainty and show that sensitivity can increase substantially under future climate warming. Given that many of the governing decomposition equations are based on individual case studies from a single location, and because key parameters are often hard coded, critical uncertainties are currently ignored. It is essential to be transparent about these uncertainties as the domain of land surface models is expanded to include the evaluation of future wildfire regimes.

Projecting Future Fire Regimes in a Semiarid Watershed of the Inland Northwestern United States: Interactions Among Climate Change, Vegetation Productivity, and Fuel Dynamics

“…For example, spatial partitioning of moisture and nitrogen in litter stores can accelerate or decelerate decomposition during drying and rewetting cycles (Birch, 1959). In addition, modeled decomposition rates can be highly sensitive to parameter and model structure uncertainties and this sensitivity increases with climate warming (Hanan et al, 2022). This can be problematic when projecting the future fuel loading under climate change, particularly over long timescales such as when extending projections from the 2040s to the 2070s.…”

“…This non-monotonic response occurred because decreases in fuel loading dominated over the increases in fuel aridity. However, the decreases in fuel loading (caused by higher litter decomposition rates) outstripped changes in fire weather and caused a dramatic reduction in wildfire under the climate change in the far future (2070s)-this process needs to be explored further given the uncertainties in modeling future decomposition (Hanan et al, 2022).…”

Multitemporal lidar captures heterogeneity in fuel loads and consumption on the Kaibab Plateau

“…Likewise, previous field observation-based studies in ponderosa pine forests (Roccaforte et al 2012 ) and mixed conifer forests (Dunn and Bailey 2015 ; Eskelson and Monleon, 2018 ; Stevens-Rumann et al 2020 ) have documented similar asymptotic temporal trends in post-fire surface fuel loads that reached a steady state at 6–20 years post fire. Fine fuel accumulation is the balance between the input and the removal of fuels, mainly driven by litterfall and decomposition (Hanan et al 2022 ). Litter accumulates on the soil until litterfall equals decomposition and accumulation stabilizes around a mean steady state (Ewel et al 1976 ).…”

Reimagine fire science for the anthropocene

“…Models of fire behavior and effects span spatial and temporal scales, but fundamental to each is the consideration of fuels, vegetation, and emissions. We must work to capture fuel heterogeneity, including the physiological dynamics that influence vegetation fuel loading (157), fuel moisture (158,159), and the flammability of live and dead vegetation (160,161). Fuel moisture and its variation in space and time have the capacity to alter fire behavior (162) and ecosystem vulnerability to wildfire (163).…”