The capacity to quantify changes in plant communities in response to shifting environmental conditions is crucial to predicting future composition. In the arid Southwest, the effects of climate change are already being felt as annual mean temperatures increase and annual precipitation levels decrease. Utilizing permanent 1-m2 quadrats put in place over a century ago, we analyzed rates of change of understory species in two sites at two elevations in Northern Arizona to ask two questions. 1) Does the magnitude of change in community composition differ between the two sites? And, 2) Are there changes in species richness at the two sites, and do patterns of change differ? Plant cover by species was mapped on 1-m2 quadrats at two study locations, the Sierra Ancha Experimental Forest (1100m), and the “Hill and Wild Bill” plots surrounding Flagstaff, Arizona (2000m); cover data was then digitized using ArcMap. The data for this study were collected over 82 years (1935 to 2017) at Sierra Ancha, and 95 years at the Hill and Wild Bill plots (1912-1938 to 2007). Magnitude of change in understory communities was quantified using Bray-Curtis dissimilarity and Welch’s T-test. Change in species richness were also assessed using Welch’s T-test.
Results/Conclusions
There were significant differences in the magnitude of compositional change between the two sites. At Sierra Ancha, there was a significantly greater change in species composition between the two measurement periods, with an average Bray-Curtis distance of 0.88 between measurement periods (min = 0.51, max = 1). Species richness also differed significantly between the two measurement periods, and was substantially higher in 2017 vs. 1935 (mean of 4.85 vs. 7.9 species per plot, P< 0.001). The higher elevation Hill and Wild Bill plots showed a smaller magnitude of change (Bray-Curtis distance = 0.76, min = 0.31, max = 1, P < 0.001). In addition, there was not a significant change in species richness across the two measurement periods (mean of 6.8 vs. 6.6 species per plot, P = 0.71). These results indicate that plant communities at the lower elevation have significantly altered composition, with an increase in species richness over time. These communities appear to be shifting more rapidly and demonstrating lower stability in understory composition compared to higher elevation communities. This denotes a lower magnitude of change over the study period, and no significant difference in species richness; further analysis is needed to assess drivers of these changes.