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.