Climate and land use change can strongly affect vegetation in arid regions, including the southwestern US. Protected areas serve as a benchmark against which human-induced disturbance to vegetation can be evaluated. Knowledge about the spatial and temporal variability of vegetation in protected areas can also be used to manage these ecosystems more effectively. A long-term vegetation monitoring program was initiated in 1987 in national parks in southern Utah to measure canopy cover of plant species through time in different plant communities. We describe changes in canopy cover of plant species and functional types over the last 20 years and explore how climate, topography, soil characteristics, and biotic interactions contributed to vegetation dynamics.

Results/Conclusions

Perennial grasslands had a strong decline in perennial grasses, especially those with a C3 photosynthetic pathway, and gained shrubs and annual forbs. This change was most pronounced at lower elevations on shallow soils and was largely driven by increasing temperatures during the 20-year study period. There was a positive relationship between perennial grass canopy cover and soil nutrients. Annual grasslands were a dynamic plant community, sensitive to climatic conditions, and fluctuated between high annual grass and annual forb canopy cover. Shrublands had a decline in Atriplex spp. (saltbushes) and Sarcobatus vermiculatus (greasewood) after an extreme drought in 1989 but had increases in Coleogyne ramosissima (blackbrush) and Artemisia spp. (sagebrushes). Some of the variation in shrub canopy cover among sites was associated with soil texture. Pinyon-juniper woodlands had strong increases in Juniperus osteosperma (Utah juniper), while Pinus edulis (pinyon) decreased in canopy cover in years with high temperature. Riparian woodlands were the most species diverse plant community but saw significant declines in canopy cover of the dominant species Populus fremontii (Fremont cottonwood), Salix spp. (willows), and Juncus spp. (rushes). Annual grasslands and Artemisia spp./Sarcobatus vermiculatus-codominated shrublands had the highest canopy cover of introduced species, especially Bromus tectorum (cheatgrass).