Figures
Figure 1. A) A simplified example of how pyrodiversity is calculated using the functional dispersion metric (FDis), adapted from Laliberté & Lengendre (2010). x represents the location of j unique fire histories (“species”) in multidimensional traits-space, c is the multi-dimensional trait-space centroid of a landscape (“community”), zj is the trait distance of history j from c, and aj is the frequency (“abundance”) of history j within the landscape. FDis is calculated as the weighted mean distance from c. B) Fire trait surfaces used to calculate pyrodiversity for an example watershed. C) Conceptual model of the causes and effects of pyrodiversity. Solid lines represent direct effects and dashed lines represent mediated relationships.
Figure 2. Pyrodiversity of forested watersheds (HUC10s) in the western United States. Watersheds with less than 50% forest cover were not evaluated and are shown in white. The broader-scale HUC2 watersheds (clipped to the region of interest) are shown as black outlines.
Figure 3 . Correlations among watershed-level (HUC10) fire regime trait dispersion. Comparisons were made across a range of minimum fire numbers by sequentially removing watersheds with fewer recorded burns between 1985 and 2018.
Figure 4 . Drivers of watershed-scale pyrodiversity. A) cumulative percent of flammable area burned from 1985-2018, B) interacting climate effects of water deficit and actual evapotranspiration, C) interacting topographic effects of roughness and elevation, D) effect of wilderness designation, E) effect of human population density. B-E reflect both direct and burn activity-mediated effects. The effect of wilderness is modeled as a proportion of land area, but binary marginal effects are presented here for simplicity. Pyrodiversity is defined as the multivariate dispersion of fire frequency, severity, seasonality, and patch size.
Figure 5. Theoretical functional relationships between pyrodiversity and biodiversity. A positive and absolute relationship is shown as an orange dashed line and solid lines represent example ecosystems where the relationship is limited by the historic fire regime. a) Biodiversity may be greatest at high levels of pyrodiversity in mixed-severity fire regimes before habitat fragmentation or type conversion results in declines. b) In ecosystems with a history of infrequent fire or homogenous burn severities, biodiversity may benefit from some pyrodiversity, but high levels may result in unfilled ecological niches or type-conversion. c) Ecosystems with little natural wildfire may experience declines in biodiversity when any burning occurs due to a lack of fire-adapted traits in the regional species pool. Our ability to perceive the full functional form is limited by the environmental space sampled: i) when observing only low levels of pyrodiversity the instantaneous relationship would appear absolute and positive for all but the non-fire adapted fire regimes. ii) When observing only moderate levels of pyrodiversity, the functional relationship would appear flat or non-existent for b, but still linear for a. iii) When observing only high levels of pyrodiversity the relationship would appear weak or unrelated in many ecosystems but the magnitude of biodiversity would nevertheless depend on an ecosystem’s distance from its optimum level of pyrodiversity.
Supplementary Material. Ancillary methods and results in support of the manuscript.