Successional trajectories of a grazed salt desert shrubland

Abstract

Successional trajectories through the statistical space of ordinations were used to examine response to grazing in salt desert shrub communities of western Utah, USA. Relative cover data were periodically collected over a 53 year period from grazing exclosures and pastures grazed with light or heavy stocking rates in fall or spring (4 grazing treatments). Two-way indicator species analysis was used to select the 98 most similar plots (24 m2) from among 358 plots in 1935. Foliar cover of 23 species on those 98 plots was followed at irregular intervals over 53 years, resulting in 709 plot-time samples. Successional trajectories were developed for each grazing treatment with ordinations from detrended correspondence analysis. Grazing season had a more pronounced influence on floristic trajectories than did grazing intensity. These ordinations suggest grazing season was an important factor regulating response to grazing and identify annual March-April grazing as an important cause of retrogression in the salt desert shrub ecosystem.