Runoff and soil erosion on woodland-encroached sagebrush rangeland under simulated rainfall and concentrated flow conditions

The encroachment of sagebrush (Artemisia spp.) steppe by pinyon (Pinus spp.) and juniper (Juniperus spp.) (PJ) conifers modifies hydro-ecological functions of hillslopes and thereby alters runoff and erosion processes. This study evaluated structural-functional responses in a sagebrush-dominated community (Sagebrush site) and a PJ-dominated community (Woodland site) on an Upland Loam Mountain Big Sagebrush Ecological Site at Grand Staircase - Escalante National Monument in Kane County, Utah, USA. Fifty-four rainfall simulations and concentrated flow experiments were performed on eighteen 12 m2 plots spanning the Sagebrush and Woodland sites. Plots at the Sagebrush site (n=6) contained a mixture of sagebrush, vegetated interspace, and bare interspace microsites. Plots at the Woodland site were stratified to occur in litter-covered areas directly underneath tree canopies (Tree, 32% of site area, n=6) and in the sparsely vegetated intercanopy between trees (Intercanopy, 68% of site area, n=6). The goals of the study were to a) quantify soil, hydrological, and vegetation properties on the experimental sites, and b) evaluate how pinyon juniper encroachment into sagebrush affects runoff, infiltration, soil erosion, and concentrated flow characteristics. The percentage area of bare soil was similar across the Sagebrush and Woodland sites. However, the Woodland site contained less herbaceous and shrub cover than the Sagebrush site. Biological soil crust cover (37%) spanned continuous swaths of unvegetated areas in the Woodland site. Tree clusters and Intercanopy areas at the Woodland site served different hydrologic functions. The latter was spatially continuous providing efficient overland flow connectivity at the slope scale. The Sagebrush site had a finer vegetation spatial structure with shorter gap distances between plant bases (mean 1.1 m) and greater litter cover (48.1%), which increased resistance to overland flow and diminished formation of rills. Rill erosion rates measured at 15 L min-1 to 45 L min-1 inflow were ten times greater for the Woodland Intercanopy than Sagebrush plots. The total sediment yield from the plots was best correlated with cumulative runoff for all sites (0.77 < R2 < 0.96). The total amount of simulated rainfalls (223 mm) produced, on average, 0.7 t ha-1 and 3.5 t ha-1 of sediment yield from the Sagebrush and Woodland plots, respectively, a statistically significant 5-fold difference.