3.4 Beta Diversity (β)
For “pairwise plots” (see Methods), adonis2 tests revealed that dissimilarity increased significantly from the pre-fire to the post-fire sample for total β, species turnover, and nestedness (Figure 3). Species turnover contributed much more to total β than did nestedness: mean distance from centroids were 0.387 for turnover and 0.070 for nestedness for pre-fire data and 0.445 and 0.093, respectively, for post-fire data. Total β and turnover both increased for juniper woodlands and piñon woodlands tested separately but not for pine-oak forest (Figure 4). Nestedness increased for juniper woodlands and pine-oak forest but not for piñon woodlands. Total β and turnover exhibited significant temporal and spatial trends, more so than did nestedness. If fire were associated with the temporal increase in β, we would expect β to increase over time for burned but not for unburned plots—a hypothesis supported for total and turnover results, but not for nestedness, which increased for burned and unburned plots alike (Figure 5). For both pre-fire and post-fire data, total β declined in a curvilinear manner with elevation, increased linearly with TRMI, and declined moderately with TRI (Table 2, Figure 7).
For “matched plots,” the timestep component is embedded in the individual plot pre-fire/post-fire dissimilarities. Mean dissimilarity differed among vegetation types for total β and turnover, but not for nestedness (Figure 3). For total and turnover β, juniper woodland plots changed more than those in piñon woodland, but other pairwise vegetation type comparisons were not significant. If fire were a driver of temporal changes in β, we would expect higher pre-fire/post-fire dissimilarity for burned than unburned plots for β—a hypothesis supported for total β (Figure 4). In fact, matched plot beta diversity increased with dNBR up to high severity, at which point it declined slightly (Figure 7). When plot dissimilarity vs. dNBR was analyzed separately for each vegetation type, this relationship was significant only for pine-oak forest, which exhibited the same curvilinear relationship of β with dNBR as found for all plots (Figure 6). Finally, plot dissimilarity decreased from low to high elevation for all β components, increased with TRI for total only, and was unrelated to TRMI (Table 2, Figure 7).
For the “mantel test”, one analysis supported the hypothesis that pyrodiversity begets β among plant communities, but another analysis did not. As dissimilarity in fire severity between two plots increased so did the amount of change in woody plant community dissimilarity from before to after the Horseshoe Two Fire (mantel r=0.12, P =0.001). These same pairwise plot contrasts in fire severity were not, however, correlated with post-fire plot dissimilarities in communities alone (mantel r=0.04, P =0.12). We rejected the null hypothesis that these matrix relationships were artifacts of correlated effects of physical distances among plots (mantel r=0.02, P =0.16). Including distance as a second variable in partial mantel tests also did not change the outcomes of the original analyses.