Results
Watersheds experienced a wide range of fire activity during the study
period, with a median of 2 fires (mean = 3; range: 0-48). These fires
resulted in a median of 3.6% (mean = 16%; range = 0-250%) of the
flammable area burned. The median watershed had a pyrodiversity value of
0.04 (mean = 0.09; range = 0-0.35). Hotspots of pyrodiversity include
watersheds in the North Cascades of Washington state, the Northern Rocky
Mountains within and around the Frank Church-River of No Return
Wilderness, Yellowstone National Park, the Mogollon Rim region of
Arizona and New Mexico, and the mountainous regions of California
especially the Klamath Mountains, and parts the Sierra Nevada (Fig. 2).
When including watersheds with no recent fire history, variation in burn
frequency, patch size, and severity are highly correlated. However, when
sequentially excluding areas with less active fire histories, theses
correlations quickly dissipate. The correlation between frequency and
patch size approximates 0.5 when considering watersheds with 14 or more
fires since 1985. The frequency-severity correlation drops below 0.5
once watersheds with fewer than 8 fires are excluded. Patch size and
severity plateau at approximately 0.65 when considering watersheds with
10 or more fires. Seasonality is largely uncorrelated with the other
three fire regime traits, starting between 0.13 and 0.23 when watersheds
with at least one fire are included, and dropping below or near zero
when restricting correlations to areas with more active fire histories
(Fig. 3).
Climate, topography, and human influence metrics show clear effects on
proportion of flammable area burned between 1985 and 2018. Proportion
wilderness followed by climatic variables showed the strongest relative
effects. Proportion burned area increased with proportion wilderness
with a scaled effect (\(\beta_{\text{wild}}\)) of 0.70 (90% confidence
interval [CI] = 0.53, 0.86). Climatic water deficit (CWD) also had a
strong positive effect (\(\beta_{\text{CWD}}\) = 0.53; CI = 0.47, 0.59),
as did actual evapotranspiration (AET; \(\beta_{\text{AET}}\) = 0.10; CI
= 0.048, 0.16), and the interaction of CWD and AET (\(\beta_{AET*CWD}\)= 0.27; CI = 0.23, 0.31). Both topographic roughness
(\(\beta_{\text{rough}}\) = 0.15; CI = 0.11, 0.19) and elevation
(\(\beta_{\text{elev}}\) = 0.084; CI = 0.011, 0.16) are positively
associated with burn area, but these variables interact negatively
(\(\beta_{elev*rough}\) = -0.10; CI = -0.14, -0.068). Human population
density was negatively associated with proportion burned area with an
effect estimate (\(\beta_{\text{pop.den}}\)) of -0.15 (CI = -0.183,
-0.109) (Table S1).
When proportion of flammable area burned is included as a predictor of
pyrodiversity it has by far the greatest effect, with much of the
ultimate effects of climate, topography and human influence being
mediated by this variable. The proportion burned area is strongly
positively associated with pyrodiversity (\(\beta_{\text{prop.burn}}\) =
2.5; CI = 2.4, 2.5), with a negative quadratic term
(\(\beta_{\text{prop.bur}n^{2}}\) = -0.78; CI = -0.80, -0.77). These
parameter estimates indicate a pyrodiversity peak when an average of
63% (CI = 61%, 65%) of a watershed has burned between 1985 and 2018
(Table S1; Fig. 4a). This apparent maximum equates to a 53-year fire
rotation (CI = 51, 54 years), a measure of the time required to burn an
area equivalent to the size of a landscape (Heinselman 1973). In some
cases, the combined direct and indirect effects on pyrodiversity are
reinforcing (e.g. topography) while others dampen their ultimate
influence (e.g. climate). For a given level of fire activity,
pyrodiversity is negatively associated with CWD (\(\beta_{\text{CWD}}\)= -0.048; -0.068, -0.029) and AET (\(\beta_{\text{AET}}\) = -0.019; CI =
-0.035, -0.003) with a positive interaction (\(\beta_{AET*CWD}\) =
0.014; CI = 0.001, 0.027) between the two climate variables. The
combined marginal indirect and direct effects show CWD and AET interact
to produce low pyrodiversity when watersheds lack an annual dry period
but high pyrodiversity in productive areas coupled with dry periods
(Fig. 4b). Similar to the burn activity model, elevation
(\(\beta_{\text{elev}}\) = 0.035; CI = 0.013, 0.057) and topographic
roughness (\(\beta_{\text{rough}}\) = 0.016; CI = 0.004, 0.028) are
positively associated with pyrodiversity, with a likely slight negative
interaction between the two (\(\beta_{elev*rough}\) = -0.013; CI =
-0.026, 0.00). Consequently, pyrodiversity is maximized either at higher
elevations or relatively low elevations with variable topography (Fig.
4c). When accounting for the level of burn activity, the direct effect
of human population density on pyrodiversity is positive
(\(\beta_{\text{pop.den}}\) = 0.029; CI = 0.017, 0.04) and proportion
wilderness shows no clear direct effect (\(\beta_{\text{wild}}\) =
0.022; CI = -0.022, 0.066). Combined with a clearly positive indirect
effect of proportion wilderness, and negative indirect effect of
population density, watersheds in designated wilderness have higher
pyrodiversity on average, while more populated areas have marginally
lower pyrodiversity (Fig. 4d & e).