3.2 Gamma Diversity (γ)
We recorded 36 species in the 138 plots in 2002-2003 and 48 in
2017-2018, 6-7 years after the 2011 Horseshoe Two Fire. All species were
native plants. The rarefaction estimate was 37 and 49 species,
respectively, indicating a significant temporal increase in landscape
woody plant richness (P <0.01; Figure 2). Rarefied γ
richness increased over time for juniper and piñon woodlands but not for
pine-oak forest (Figure 3). If the temporal change in γ richness was
tied to fire, then we would expect the increase to occur in burned plots
only—a prediction supported by the results, with γ increasing
significantly at low fire severity and marginally insignificantly at
moderate severity (Figure 4).
Only three species were unique to the pre-fire data, whereas 13 were
unique to the post-fire sample. Species new to the 138 plots during the
second sampling were cacti and shrub growth forms (e.g., Amorpha
fruticose L., Bouvardia ternifolia (Cav.) Schltdl,
Philadelphus microphyllus A. Gray, and Toxicodendron
rydbergii (Small ex Rydb.) Green)), species typically found in open
areas. In contrast, two of the three species disappearing from the plots
during this time span—Pinus arizonica and Pseudotsuga
menziesii —were coniferous tree growth forms that regenerate from
seed. These were present elsewhere in the study area during the
post-fire sample, but no longer in the sample plots.