Introduction
Fire is a fundamental ecological process (McLauchlan et al. 2020) that plays a central role in biome distribution and biodiversity globally (Bond et al. 2005; He et al. 2019). Fire patterns and their ecological consequences differ according to a number of important fire regime characteristics including burn frequency, severity, seasonality and spatial pattern (Keeley et al. 2011; van Wagtendonk et al. 2018). Much effort has gone into quantifying the central tendencies of these fire regime characteristics (e.g. mean fire return interval) and their underlying drivers (Agee 1996; Krawchuk & Moritz 2011; Archibald et al. 2013), but until recently what determines the inherent variation of fire regime characteristics, known as pyrodiversity, has received little attention. Martin and Sapsis (1992) first proposed pyrodiversity begets biodiversity by creating heterogeneous landscapes composed of dissimilar habitats and ecological niches. Since the theory was formalized, the potential importance of heterogeneity in fire regimes for ecosystem pattern and function has gained increasing attention both in research and ecosystem management (Parr & Andersen 2006; He et al. 2019). However, the expanded scrutiny has come with little consistency in definition or application of the pyrodiversity concept. A generalizable approach for quantifying pyrodiversity and an improved characterization of the phenomena’s socioecological drivers is necessary for advancing understanding of its ecological importance.
The presumed link between pyrodiversity and biodiversity has influenced conservation efforts, particularly where prescribed burning or “patch mosaic burning” is used to diversify fire histories across a managed landscape (Parr & Andersen 2006). However, the development of robust ecological linkages to pyrodiversity has been hampered by our limited ability to fully capture relevant fire history components with sufficient spatial resolution, and temporal extent. This limitation may no longer apply as computing capabilities and spatial data availability have advanced considerably in recent years (e.g. Parks et al.2019). For example, the “visible mosaic” represented by the landscape pattern created by the most recent wildfire and subsequent successional processes can be easily observed (Minnich 1983; Turner & Romme 1994). However, observing the “invisible mosaic” that includes components of fire history such as the timing and severity of previous fire events requires access to remotely sensed fire histories. Ecological legacies attributable to this invisible mosaic nevertheless can influence biodiversity, and assessing its relative importance may be necessary for effective conservation in fire-prone ecosystems (Parr & Andersen 2006; Brown & York 2017).
The complexity associated with distilling relevant fire regime components (subsequently referred to as “traits”) into a measure of pyrodiversity has resulted in varied approaches. Often these methods have focused a single fire regime trait such as burn severity (Tingleyet al. 2016; Steel et al. 2019) or frequency (Tayloret al. 2012; Brown & York 2017). Such approaches implicitly assume a single trait serves as a surrogate for other fire regime characteristics and captures the most relevant aspects of pyrodiversity (He et al. 2019). This is likely a valid assumption in some cases, but without an understanding of how fire regime traits covary this can result in misleading conclusions (Keeley et al. 2011). Other studies have incorporated multiple traits and treated unique combinations as distinct “species” when applying biodiversity metrics such as Simpson’s diversity index (Ponisio et al. 2016). However, traditional diversity metrics do not account for the trait-distance between species and in the case of fire histories, definitions of species are sensitive to how continuous measures are classified into levels (e.g. four or more classes of burn severity). Hempson et al. (2018) proposed perhaps the most generalizable method of assessing multiple dimensions of pyrodiversity by quantifying pyrodiversity as the multivariate range (convex hull) of four fire traits. However, their method is limited to coarse-scale analyses and is not able to capture critical within-fire traits, such as variation in burn severity and spatial pattern (i.e. patch size). Together these assessments and others provide valuable contributions to our understanding of pyrodiversity’s ecological role, but a more comprehensive and consistent approach is necessary to test whether pyrodiversity promotes biodiversity absolutely or if the relationship varies among ecosystems and taxa.
Fire regime central tendencies are controlled by climate, topography and human influence (Agee 1996; Archibald et al. 2013), and are reciprocally dependent on the structure and flammability of extant vegetation (Bond et al. 2005). Through the annual and seasonal availability of solar energy and water balance, climate determines distributions of vegetation types, primary productivity and fuel flammability (Stephenson 1998; Krawchuk & Moritz 2011). Topography also influences water balance, but can further exert direct control on fire behavior (van Wagtendonk et al. 2018), which in the aggerate likely influences fire patterns across landscapes (Povak et al.2018; Hessburg et al. 2019). Humans have influenced wildland fire for millennia either through direct management, accidental ignitions, or indirectly through alterations of vegetation via land-use change (Marlonet al. 2008; Bowman et al. 2011; Archibald et al.2013), but many areas have shifted from historic fire use that was locally driven and variable across landscapes to contemporary broad-scale fire management (dominated by suppression) that has homogenized landscapes (Hessburg et al. 2005; Marlon et al. 2012). These underlying drivers likely influence variation in fire patterns as well, either directly or as mediated by total burn activity.
Here we develop a comprehensive method for quantifying pyrodiversity using four fire regime traits within a functional diversity framework. We apply this measure of pyrodiversity broadly across all forested areas in the western United States and assess how pyrodiversity varies with climate, topography and human influence. This approach is fully reproducible, can be applied from fine- to broad-scales using associated R code, and can help advance our understanding of the role of pyrodiversity in the maintenance of biodiversity and ecological function.