The global pyrogeography of
ecoregion flammability thresholds
Abstract. Anthropogenic climate change is creating a more
flammable future by increasing the number of days when vegetation is dry
enough to burn. Indices representing the percent moisture content of
dead fine fuels as derived from meteorological data have been used to
assess geographic patterns and temporal trends in vegetation
flammability. To date, this approach has assumed a single flammability
threshold, typically between 8 and 12%, controlling fire potential
regardless of the vegetation type or climate domain. Here we investigate
geographic variation in fuel moisture levels associated with observed
fire activity among ecoregions by analysing global reanalysis data and
remotely sensed burnt area products. This geospatial analysis identified
a wide range of ecoregion flammability thresholds (EFTs) associated with
fire activity, often well above or below the commonly used range of
values. Many boreal and temperate forests, for example, can burn with
much higher fuel moisture than previously identified; Mediterranean
forests, on the other hand, tend to burn with consistently low fuel
moisture. Bayesian modelling showed that EFTs are primarily driven by
eco-climatological variables, particularly precipitation and
temperature. Our analysis also identified complex associations between
vegetation structure, fuel types, and climatic conditions highlighting
the complexity in vegetation-climate-fire relationships globally. Our
study provides a critical, necessary step in understanding and
describing global pyrogeography and tracking changes in spatiotemporal
fire activity.