Changing fire regimes
Humans have altered fire regimes directly through management and indirectly by altering the earth’s climate, and such shifts are almost certainly also changing pyrodiversity. Perhaps the most clear effects of altered fire regimes on pyrodiversity are seen where fire exclusion and suppression policies have dramatically reduced burned area and shifted fire severity patterns in fire-adapted forests (Mallek et al.2013). Conversely, climate change is increasing fire activity by lengthening fire seasons and increasing water deficits (Flanniganet al. 2009; Abatzoglou & Williams 2016; Westerling 2016; Hessburg et al. 2019). Given the strong link between climatic water deficit, fire activity and pyrodiversity, these changes may increase pyrodiversity in the short term where deficits of fire activity currently exist but could result in lower levels of pyrodiversity for areas with high levels of contemporary burn activity (Fig. 4a). Additionally, in many areas larger fires are increasingly accompanied by ever larger and simpler shaped patches of high-severity effects (Stevenset al. 2017; Steel et al. 2018), which could result in lower pyrodiversity at fine scales. Where the frequency of high-severity fire exceeds the natural range of variation of an ecosystem, higher rates of type-conversion (e.g. from forests to shrubland) may occur (Coppoletta et al. 2016; Welch et al. 2016). This may be particularly problematic in dry areas where a further increase in water deficit can lead to a consistent loss in productivity (Hessburg et al. 2019) or when wildfires interact with other climate-exacerbated disturbances such as periodic drought and beetle infestations (Coopet al. In Press). Ultimately, climate-related shifts in pyrodiversity are likely to be uneven across the western United States and globally. How these changes impact biodiversity and ecosystem process may depend on whether emerging pyrodiversity patterns result in a dramatic departure from historic fire regimes (Fig. 5).