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.