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.