4 DISCUSSION
Anthropocene fire regimes are catalyzing major shifts in plant community
structure, function, and diversity across the world (Bowman, 2020). We
detected changes in woody plant species diversity in Chiricahua National
Monument from 2002-2003 to 2017-2018, a time span intersected by the
Horseshoe Two Megafire of 2011 and characterized by increasing
aridification throughout the southwestern United States. The key results
were that gamma species richness (γ) increased by nearly 1/3, alpha
richness (α) declined by 1/5, and beta diversity (β) increased nearly
two-fold. As a result, the contributions of α and β to γ reversed their
order from the first sample (higher for α) to the second (higher for β).
Several lines of evidence tie these temporal changes directly to the
Horseshoe Two Fire for all three scales of species diversity. γ, α
species richness, Inverse Simpson, total β, and β turnover all shifted
from before to after the fire in burned but not unburned plots. The
increase in γ, a common result for ecosystems with fire-adapted species
(Perry et al., 2011; Romme et al., 2016; Pausas & Ribiero, 2017; He et
al., 2019), is not surprising for Chiricahua National Monument, given
that wildfires have been largely excluded for more than a century in
vegetation that historically experienced frequent fire. In our 138
plots, only three species disappeared, while 13 newcomers
appeared—primarily cacti indicative of drier conditions and shrub
species that thrive in exposed areas, both of which became more
prevalent in the wake of the wildfire. The negative impact of the
Horseshoe Two Fire on α species richness differs from most studies of
plants, which have usually detected positive effects of fire (e.g.,
Burkle et al., 2015 [forbs and graminoids]; He et al., 2019; Miller
& Safford, 2020; but see Collins et al., 2007; Burkle et al., 2015
[woody plants]). The significant increase in the Inverse Simpson
Index aligns more closely with the bulk of those studies, but that
result is difficult to interpret given that this index incorporates not
just richness but also evenness, which increased in both burned and
unburned plots (i.e., independent of the fire).
The generally positive effect of the Horseshoe Two Fire on β is similar
to results from studies across a wide range of taxa (Myers et al., 2015;
He et al., 2019; Miller & Safford, 2020), although neutral or negative
outcomes are common as well (Reilly et al., 2006; He et al., 2019;
Richter et al., 2019; Miller & Safford, 2020). β in our study stemmed
chiefly from species turnover rather than nestedness, as did significant
spatial and temporal patterns. In other words, along fire and
topographic gradients, as well as across the two sample periods, woody
plant communities tended to exhibit wholesale replacement of species
(turnover) rather than shifts in which smaller local assemblages were
subsets of larger ones (nestedness). The role of species turnover across
spatial gradients accords well with the famously high level of terrain
ruggedness and tightly packed communities of Chiricahua National
Monument (Poulos et al., 2007) and throughout the Sky Islands (Whittaker
et al., 1968 Niering & Lowe, 1984; Barton, 1994; Coblentz & Ritters,
2004; Villarreal et al., 2019). That the Horseshoe Two Fire depressed α
species richness but amplified β suggests that fire did not act
uniformly in decreasing local richness, but instead filtered out species
differentially among plots, promoting variation in post-fire
assemblages, nearly entirely through the enhancement of species
turnover.
The intermediate disturbance hypothesis (IDH) in the context of fire has
focused largely on α species richness (Connell, 1978; Sousa, 1979;
Huston, 2014), but has also been proposed for β and γ (He et al., 2019).
We found support for the IDH for fire severity but only for γ, which
peaked at low severity. This pattern appears to have stemmed from both
higher species extirpation and lower establishment of novel species
associated with higher severity fire, probably a result of the extreme
conditions in those plots during and after the Horseshoe Two Fire.
Outcomes vary widely regarding γ and the IDH, with Richter et al. (2019)
supporting the IDH for understory plant communities in the Sierra
Nevada, some studies finding no relationship (Miller & Safford, 2020),
and others arguing that the positive effect of fire on landscape-scale
plant diversity generally increases linearly along fire gradients
(Pausas & Ribiero, 2017; He et al., 2019).
In contrast to γ, neither α nor β exhibited unambiguous support for the
IDH. β increased for all fire severity classes, although the continuous
relationship with dNBR exhibited a slight decline at very high severity.
Figure 5 appears to reveal a hump-shaped relationship between α and fire
severity, but the same pattern occurs for the results for before the
fire. These parallel relationships stem not from any impact of fire, of
course, but instead from the sharp decline in fire severity with
elevation (r=0.43) and an underlying hump-shaped relationship of
diversity with elevation. Only by examining changes in diversity from
pre- to post-fire in burned vs. unburned plots were we able to establish
that alpha diversity and fire severity exhibited no clear relationship.
This pattern demonstrates the importance of coupling pre- and post-fire
sampling, but also reveals a weakness of our study: the lack of
experimental treatments in which factors such as fire severity and
elevation can be investigated independently.
Although the IDH has been rarely tested for β (see Burkle et al., 2015;
Richter et al., 2019 for mixed results), many studies support the
proposition for α (DeSiervo et al., 2015; Morgan et al., 2015; Stevens
et al., 2015 Heydari et al., 2017; He et al., 2019; Richter et al.,
2019; Strand et al., 2019; Miller & Safford, 2020; but see Schwilk et
al., 1997). Three factors might explain the negative impact of fire on α
and lack of support for the IDH in our results. First, Huston (1979,
2014; see also Burkle et al., 2015) argued on theoretical grounds that a
hump-shaped diversity-disturbance relationship should occur primarily in
more productive sites, where disturbance alleviates interspecific
competition and promotes species co-existence, rather than in
unproductive areas, such as the dry, hot sites in our study area, where
interspecific competition may be less pronounced to begin with. The few
tests of this IDH-productivity hypothesis for fire have produced mixed
results (Burkle et al., 2015 Strand et al., 2019; Miller & Safford,
2020). Second, more than a century of fire exclusion in Chiricahua
National Monument may have reduced the population sizes of less common,
fire-associated species, resulting in lower and more variable seed rain
of these species at the local scale after the fire. Moreover,
regeneration of these species after the Horseshoe Two Fire, especially
from seed, might have been constrained by the long-term drought that
started in the 1990s and was especially extreme post-fire in 2011
(Williams et al., 2014). These three scenarios together could translate
into the input of relatively few seeds with low establishment
probabilities, leading to deficient replacement of species extirpated
from local assemblages by the Horseshoe Two Fire, and a lack of positive
response to the reduction of local competition assumed by the IDH.
We found mixed support for the hypothesis that pyrodiversity promotes β,
a relationship confirmed in many studies of plants (Perry et al, 2011;
Burkle et al., 2015; Myers et al., 2015 Heydari et al., 2017; Freeman et
al., 2019; He et al., 2019)—but not all (Reilly et al., 2006; Masunga
et al., 2013). While our results do not explain whether variation in
fire severity led to higher β because of differential fire-induced
mortality or post-fire regeneration, the number of novel species after
the fire far outstripped the number of extirpated species, suggesting a
role for newfound species regeneration in the wake of fire. The decline
in α species richness, however, points to the potential importance of
differential species mortality across plots. Plots subject to high fire
severity, in particular, were markedly transformed by fire-induced
mortality of nearly all stems, massive resprouting of oaks and shrubs,
and minimal regeneration of fire-resistant conifers that rely on
establishment by seed. These plots diverged significantly from those
experiencing lower fire severities. In Cave Creek Canyon, on the east
side of the Chiricahua Mountains, Barton and Poulos (2018; see also
Minor et al. 2017) documented such conversion of structurally complex
Madrean pine-oak forests to sprouting shrublands after high severity
fire—a pattern found increasingly across the Southwest (Falk, 2013;
Coop et al., 2020). The same process appears in part to have
significantly shifted species diversity patterns in Chiricahua National
Monument after the Horseshoe Two Fire.
A recent review by Miller & Safford (2020) demonstrated that both α
species richness and β are generally highest at fire severities typical
of the historic fire regimes of plant communities rather than strictly
following the IDH. Ecosystems characterized historically by frequent,
surface fire tended to exhibit the highest plant diversity at low to
moderate fire severity, whereas diversity for areas typically
experiencing infrequent, stand replacing fires often peaked at higher
fire severity. Ecological filtering for compatibility of species to fire
regime is the assumed driving force underlying these patterns. In our
study, pine-oak forest was historically characterized by frequent, low
severity surface fires (Swetnam et al., 1989; Swetnam et al. 1996;
Barton et al., 2001), whereas juniper and piñon woodlands experienced a
mixed-severity fire regime (Baisan & Morino, 2000). If historic fire
regime shaped the responses of species diversity to fire, we would thus
expect divergent responses to fire between pine-oak forest and the other
two vegetation types. The results do not support this prediction: the
three vegetation types exhibited similar decreases in α species richness
and increases in total β over time when burned plots were analyzed
separately (P <0.01; data not shown). Moreover, species
richness and pre-/post-fire dissimilarities showed significant
continuous relationship with dNBR only for pine-oak forest, for which β
peaked at nearly the highest fire severity, contrary to what would have
been predicted given its historic low severity fire regime.
At least two factors might have mitigated the role of historic fire
regime in the responses of these vegetation types. First, although
Miller & Safford (2020) cite evidence for the homogenizing effect of
high severity fire, a century without fire may well have had the same
impact on the vegetation of Chiricahua National Monument, especially
pine-oak forest, which was transformed over a century of fire exclusion
from an open woodland to a much denser, even light-limited, forest. It
is not surprising that fire of any severity may have injected
heterogeneity onto this vegetative canvas, promoting β. Second, the
species pool governing pine-oak forest includes typical fire-resistant
species that regenerate only from seed (e.g., Pseudotsuga
menziesii and P. arizonica ), and thus, as assumed by Miller &
Safford (2020), respond poorly to high severity fires (Barton, 2002;
Barton & Poulos, 2018). Other species common in that vegetation type,
however, exhibit multiple traits, such as thick bark, resprouting
capacity, and serotinous cones (e.g., P. leiophylla ), that are
adaptive in the context of multiple fire severities. Poulos et al.
(2018) argued that such multiple strategies might indicate a complex
evolutionary history with respect to the fire regimes experienced by
these species. An important assumption of the historic fire regime
hypothesis that bears further inspection, therefore, is the extent to
which species traits are the legacy of recent historic fire regime
versus a longer, more complex evolutionary heritage.
The evenness component of α and the nestedness component of β exhibited
temporal changes that were independent of fire, that is, these metrics
changed in both burned and unburned plots over the 15-year period. While
this study documents the effects of wildfire on woody plant diversity in
Chiricahua National Monument, long-term drought stress may also be
influencing contemporary woody plant diversity dynamics at this site.
The region has experienced severe moisture deficits since the 1990s
(Cook et al, 2014; Cook et al., 2015; Ault et al. 2016). Restricted soil
moisture and pronounced vapor pressure deficit have caused a wide range
of recent ecological changes in the region, including larger, more
intense wildfires (Abatzoglou & Kolden, 2013; Abatzoglou & Williams,
2016; Singleton et al, 2016; Williams et al., 2019), pronounced tree
mortality (Allen et al., 2010; Williams et al., 2013), and shifts in
community composition (Falk, 2013; Coop et al., 2020). We cannot rule
out the possibility that moisture stress amplified fire-associated
impacts even in burned plots (Barton and Poulos, 2018 Poulos et al.,
2020; Poulos et al., in press). The diversity effects of fire comprise
the sum of mortality imposed by burning and post-fire plant regeneration
from seed and resprouting. Intensified moisture stress acting especially
on post-fire seed germination and establishment may well have acted
synergistically with fire in shaping the diversity patterns described in
this paper, an argument made also for changes in species composition
after the Horseshoe Two Fire (Barton & Poulos, 2018; Poulos et al.,
2020). The possible mechanisms connecting fire, drought, and diversity
are unclear at this point and deserve further investigation.
Independent of time and fire, topography strongly influenced species
diversity in Chiricahua National Monument. In fact, the relationships of
diversity indices with topographic variables changed little from before
to after the fire, suggesting that topography is an intrinsic regulator
of diversity at this site, regardless of the effects of fire. Elevation
is a complex master environmental variable controlling the structure,
composition, and processes of Sky Island ecosystems (Shreve, 1915;
Marshall, 1957; Whittaker & Niering, 1975; Sawyer & Kinraide, 1980;
Niering & Lowe, 1984; Barton, 1994; Poulos et al., 2007, 2010). From
lower to higher elevation, temperature decreases and moisture increases;
other key environmental variables change as well along this gradient
(Shreve, 1915; Whittaker et al., 1968; Barton, 1994; Vivoni et al.,
2007). α peaked at intermediate elevations and exhibited sharp
reductions towards both lower and higher elevation in our study, which
is similar to past studies of Chiricahua National Monument (Poulos et
al., 2007) and other Sky Island ranges (Whittaker & Niering, 1975). In
contrast, β decreased with elevation, suggesting higher levels of
habitat heterogeneity at lower elevations. All three α metrics increased
with increasing terrain ruggedness (TRI), which measures the degree of
topographic complexity at the plot scale. Higher levels of α in more
rugged plots likely arises from increased microhabitat heterogeneity and
favorable conditions for a wider array of species than in more
homogeneous terrain. Coblentz and Riitters (2004) found a similar
relationship at the regional scale, attributing the pronounced
biodiversity of the Sky Island ranges of the Southwest USA and northern
Mexico to the physical complexity of the mountains (see also Felger &
Wilson, 1995)