Trends in western USA fire fuels using historical data and modeling

Background: Recent increases in wildfire activity in the Western USA are commonly attributed to a confuence of factors including climate change, human activity, and the accumulation of fuels due to fire suppression. However, a shortage of long-term forestry measurements makes it difficult to quantify regional changes in fuel loads over the past century. A better understanding of fuel accumulation is vital for managing forests to increase wildfire resistance and resilience. Numerical models provide one means of estimating changes in fuel loads, but validating these models over long timescales and large geographic extents is made difficult by the scarcity of sufficient data. One such model, MC2, provides estimates of multiple types of fuel loads and simulates fire activity according to fuel and climate conditions. We used the Forest Inventory and Analysis Database (FIADB) observed data to validate MC2 estimates of fuel load change over time where possible. Results: We found that the MC2 model’s accuracy varied geographically, but at a regional scale the distributions of changes in fuel loads were similar to distributions of FIADB values. While FIADB data provided consistent measurement types across a wide geographic area, usable data only spanned approximately 30 years. We therefore supplemented this quantitative validation with a qualitative comparison to data that covered less area, but for much longer time spans: long-term forestry plots outside of the FIA plot network and repeat photography studies. Both model results and long-term studies show increases in fuel loads over the past century across much of the western USA, with exceptions in the Pacific Northwest and other areas. Model results also suggest that not all of the increases are due to fire suppression. Conclusions: This model validation and aggregation of information from long-term studies not only demonstrate that there have been extensive fuel increases in the western USA but also provide insights into the level of uncertainty regarding fire suppression’s impact on fuel loads. A fuller understanding of changing fuel loads and their impact on fire behavior will require an increase in the number of long-term observational forestry studies.