3.3 Alpha Diversity (α)
The mean number of species per plot was 6.70 (+ 0.02 1SE) before
and 5.36 (+ 0.03 1SE) after the fire. The Shannon-Weiner, Simpson,
and Unbiased Simpson indices did not change from pre- to post-fire, and
temporal change for these metrics was not associated with vegetation
type, fire severity, or topographic position
(P >0.05; results not shown). Figures 2-6 and Table 1
display the results for species richness, evenness, and the inverse
Simpson index, all of which exhibited significant patterns across space
and time.
Mean species richness per plot decreased significantly from the first to
the second sample period for all plots combined (Figure 2) and for each
vegetation type separately (Figure 3; P >0.05). If
this temporal change were caused by the Horseshoe Two Fire, we would
expect shifts only for burned plots—a prediction supported by the
results, with mean species richness decreasing significantly at low and
high severity and marginally insignificantly at moderate severity
(Figure 4). Richness exhibited a hump-shaped relationship with dNBR, but
this was, unexpectedly, true for both the pre-fire (i.e., before any
fire at all) and the post-fire data, with no significant interaction
with timestep (Table 1, Figure 5). The change in the number of species
for each plot over time (pre-fire minus post-fire) was not significantly
related to dNBR for all plots combined (P>0.10 data not
shown) or for any of the vegetation types analyzed separately, although
a marginally insignificant linear decline was found for piñon woodlands
(Figure 6). Species richness was significantly shaped by elevation
(humped-shaped polynomial) and TRI (curvilinear increase) but not by
TRMI (Table 1, Figure 5)—a relationship that was true for both
pre-fire and post-fire, with no interaction of timestep and topography.
Mean evenness increased from the first to the second sample period for
all plots combined (Figure 3). This increase occurred for pine-oak
forest and piñon woodland but not for juniper woodland (Figure 4).
Evenness increased for all fire severity levels, including unburned
plots, although the relationship was marginally insignificant for
moderate severity (Figure 5). The relationship of evenness to dNBR
changed significantly over the two sample periods, exhibiting a
hump-shaped relationship for pre-fire and a curvilinear increase for
post-fire (Table 1, Figure 6). For both sample periods, evenness was
significantly shaped in a curvilinear fashion by elevation and TRI but
not by TRMI (Table 1, Figure 6).
Mean inverse Simpson index increased from the first to the second sample
period for all plots combined (Figure 3). This increase occurred for
pine-oak forest and juniper woodland but not for piñon woodland (Figure
4). This index increased significantly only for high severity plots,
although an increase for moderate severity was marginally insignificant
(Figure 5). The Inverse Simpson index exhibited a hump-shaped
relationship with dNBR, but this was unexpectedly true both for the
pre-fire (i.e., before any fire at all) and the post-fire data, with no
significant interaction with timestep (Table 1, Figure 6). The Simpson
Index changed in a curvilinear manner with elevation (hump-shaped) and
TRI (increasing) but was not significantly related to TRMI (Table 1,
Figure 6).