Drivers of pyrodiversity
Climate exerts strong controls on biome distribution and fire regimes
globally, while topography is often omitted from or considered less
important in assessments at the fire regime level (Whittaker 1975;
Stephenson 1990; Archibald et al. 2013). Here we establish that
climatic control extends to variation in current fire patterns both
directly and indirectly as mediated by burn activity (Fig. 4b). Relative
to climate, we found elevation and topographic roughness to have small
but meaningful effects on proportion area burned and pyrodiversity (Fig.
4c; Table S1). Hempson et al. (2018) found a negative relationship
between pyrodiversity and precipitation with a pyrodiversity peak in dry
areas of Africa, but no discernible effect of topographic roughness.
This observed relationship with precipitation is consistent with our
finding that pyrodiversity increases with climatic water deficit but is
somewhat at odds with our finding of a positive relationship with actual
evapotranspiration, which is related to precipitation. Together these
assessments indicate pyrodiversity is dependent both on the production
of vegetative biomass and its seasonal availability to burn as fuel.
Topographic roughness may be important in supporting intra-fire
variability if rapid changes in terrain disrupt fire behavior and break
up patches of fire severity (Estes et al. 2017; Povak et
al. 2018). However, topography may exert little control on variability
of fire-level metrics such as fire size and maximum burn intensity
(Hempson et al. 2018).
We interpret the negative relationship between human population density
and pyrodiversity to reflect highly successful fire exclusion and
suppression efforts across much of North America (Marlon et al.2012). Changes in vegetative structure and fire patterns attributable to
fire suppression have already been documented in fire-adapted ecosystems
(Hessburg et al. 2005; Steel et al. 2015; Lydersen &
Collins 2018), and these findings indicate pyrodiversity is almost
certainly lower in such systems than historic levels. The strong
positive effect of proportion wilderness likely reflects the fire policy
of many US wilderness areas, which strive to restore pre-suppression era
fire regimes (Stephens et al. 2016). Wilderness areas that
explicitly allow lightning-caused wildfires to be used for resource
objectives (van Wagtendonk 2007) appear to contain greater levels of
pyrodiversity. However, the benefit of wilderness is likely highly
context dependent. Some of the most pyrodiverse areas in the western
United States fall within wilderness areas such as Yosemite National
Park (Collins et al. 2007), Frank Church-River of No Return
Wilderness, Bob Marshall Wilderness and the Gila Wilderness (Parkset al. 2014), but not in the wildness areas of Olympic National
Park characterized by a very wet climate. Interestingly, Sequoia-Kings
Canyon National Park in the southern Sierra Nevada of California was an
early pioneer in the use of both prescribed and managed natural fire
(van Wagtendonk 2007; Stevens et al. 2020) but does not appear
particularly pyrodiverse, while an area just to its south (Kern Plateau,
Sequoia National Forest) does (Fig. 2).
The full nature of human influence on pyrodiversity is likely more
complex than can be captured by the necessarily coarse measures of
population density and wilderness designation. At sub-watershed scales,
the use of prescribed and cultural burning are likely important
contributors to pyrodiversity in some areas (Lewis 1973; Bird et
al. 2018). Tribal burning in California serves an array of cultural
purposes and creates diverse habitat mosaics that sustained meadows,
woodlands, wetlands, coastal prairies, and grasslands (Lewis 1973;
Anderson 2013). Many Tribes used a system of patch burning that
manipulated vegetation at fine spatial scale to meet their management
objectives. How these cultural fire regimes impact pyrodiversity
deserves continued evaluation where fire histories exist at finer scales
than the national MTBS dataset.