The relationship between fire regime and productivity/aridity in the western United States

Abstract:

Pyrogeographic theory suggested that global fire activity (fire frequency or area burned) changes along productivity/aridity gradients following a unimodal relationship, i.e. the intermediate fire–productivity hypothesis. However, less is known about whether this hypothesis applies to other components of fire regime, such as fire size distribution and severity. In this analysis, we summarized four key components of the fire regime, including fire frequency, area burned, median fire size, and percent area burned by high severity fires, between 1984 and 2010 within each Bailey’s section using data from monitoring trends in burned severity projects in the western United States (US). We then examined how these fire regime components varied along with gradients of net primary productivity (NPP) and climatic water deficit (WD) using generalized additive models. Results showed all these fire regime components had unimodal relationships with NPP at the Bailey’s section level. Statistical models with both NPP and WD explained significantly more variation of fire regime than NPP only model; suggesting fire regime was co-controlled by independent effects of productivity and fuel moisture in the western US. Results implied that fire frequency, area burned, and fire size in the western US could respond dramatically to productivity and WD change, but fire severity may be more resilient. Responses were different for forest and non-forest. For example, fire activity for non-forest was more sensitive to change in productivity, while fire activity for forest was more sensitive to change in WD. Fire size was the most sensitive fire regime component in non-forest areas, while fire severity was the most sensitive fire regime component in forest areas. This analysis extended the scope of global fire-productivity relationship, and provided insight on understanding how climate change may affect multiple components of the fire regime.