Submitted to: Trans American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: October 19, 2007
Publication Date: December 10, 2007
Citation: Pierson, F.B., Kormos, P.R. and Williams, C.J. 2007. The Influence of Woodland Encroachment on Runoff and Erosion in Sagebrush Steppe Systems, Great Basin, USA. EOS Transactions, American Geophysical Union, 88(52), Fall Meeting Supplement, Abstract H31G-0750. Technical Abstract: Pinyon and juniper woodlands have expanded 10 to 30% in the past 30 years and now occupy nearly 20 million hectares of sagebrush shrub steppe in the Great Basin Region and Colorado Plateau, USA. The conversion of sagebrush steppe to pinyon and juniper woodlands has been linked to changes in plant community structure and composition and respective increases in overland flow and erosion from these landscapes. The Sagebrush Steppe Treatment Evaluation Project (SageSTEP, www.sagestep.org) was implemented in 2005 as a 5 year interdisciplinary research study to evaluate restoration methodologies for sagebrush rangelands degraded by woodland and grassland encroachment over a six state area within the Great Basin. The hydrology component of SageSTEP focuses on the relationships between changes in vegetation and groundcover and runoff/erosion processes. In 2006, 140 small scale (0.5 m2) rainfall simulations were conducted at 2 locations within the Great Basin to determine whether critical thresholds exist in vegetation and ground cover that significantly influence infiltration, runoff, and erosion in pinyon and juniper woodlands. Simulation plots were distributed on interspaces (areas between shrub/tree canopies) and juniper, pinyon, and shrub coppices (areas underneath canopy). Water drop penetration times and litter depths were also collected for each plot to explore controls on soil hydrophobicity. Preliminary results suggest a positive correlation between litter depth and hydrophobicity, as soils under thick pinyon and juniper coppices are strongly water repellant and soils in interspaces and under shrub coppices are easily wettable. Interspace plots with varying amounts of grasses and forbs have the highest erosion and runoff rates due to higher percentages of bare ground and relatively low soil stability. Pinyon coppices have the least runoff and erosion due to very high litter depths and low bare ground cover, even though surface soils are hydrophobic. Juniper and shrub coppice plots produce a similar amount of sediment, but juniper coppices produce significantly more runoff. Lower sediment concentrations from juniper coppice plots are attributed to higher litter depths, higher surface soil stability, and a lower percent of bare ground, while high runoff rates are attributed to higher soil surface water repellency. Results suggest that runoff and erosion from woodland encroachment sites are highly influenced by the amount of ground cover and strength of water repellency.