Discussion
The concept of pyrodiversity has received considerable attention in
recent years as the inevitability of wildfire and its fundamental role
in many ecosystems is increasingly recognized. While pyrodiversity
clearly has appeal and applicability to many ecological disciplines, to
date the concept remains nebulous with varied and often narrow
definitions. Here we present a generalizable functional diversity
approach to quantifying pyrodiversity and tested its drivers across
forested watersheds of the western United States. At the HUC10 watershed
scale, pyrodiversity was strongly but non-linearly related to fire
activity with an observed pyrodiversity peak when approximately 63% of
the flammable land area burned over the 33-year study period (equivalent
to a 53-year fire rotation). Of the ultimate drivers tested, climate and
proportion wilderness showed the strongest controls on pyrodiversity
with productive but seasonally dry watersheds in wilderness areas most
often characterized by variable fire histories. Areas with high
topographic roughness or high elevation as well as areas with low human
population density also tended to be more pyrodiverse. Correlations
among individual pyrodiversity traits declined with the number of fires
observed in a given watershed, suggesting the use of a single fire
regime trait (e.g. severity) may be appropriate for describing
pyrodiversity following isolated fire events but is insufficient for
characterizing landscapes with active fire regimes. A multi-dimensional
approach supported by moderate- to high-resolution spatial data is
likely necessary to capture the inherent complexity of fire across
landscapes and bioregions.