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.