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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Long-term effectiveness of tree removal to re-establish sagebrush steppe vegetation and associated spatial patterns in surface conditions and soil hydrologic properties

Author
item Williams, Christopher - Jason
item JOHNSON, J.C. - University Of Arizona
item Pierson, Fred
item BURLESON, C.S. - University Of Arizona
item Polyakov, Viktor
item KORMOS, P.R. - US Department Of Commerce
item Nouwakpo, Sayjro

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2020
Publication Date: 8/6/2020
Citation: Williams, C.J., Johnson, J., Pierson Jr, F.B., Burleson, C., Polyakov, V.O., Kormos, P., Nouwakpo, S.K. 2020. Long-term effectiveness of tree removal to re-establish sagebrush steppe vegetation and associated spatial patterns in surface conditions and soil hydrologic properties. Water. 12(8): 2213. https://doi.org/10.3390/w12082213.
DOI: https://doi.org/10.3390/w12082213

Interpretive Summary: The encroachment of sagebrush steppe plant communities by pinyon and juniper woodlands has substantially imperiled one of the most ecologically important rangeland ecosystems in North America. Woodland encroachment into sagebrush steppe can substantially degrade understory vegetation and thereby alter ecological function and delivery of ecosystem services. Reduction of understory vegetation commonly increases runoff rates and amplifies long-term soil loss of ecologically important surface soils. Public and private land managers throughout western North America are challenged with selecting and applying the most appropriate of various pinyon and juniper tree-removal practices to halt and/or reverse the negative ramifications of woodland encroachment. This study evaluated long-term effectiveness of prescribed fire and various mechanical treatments to remove pinyon and juniper, restore sagebrush steppe vegetation, and improve hydrologic function at two woodland encroached sagebrush sites in the western US. As assessed 13 years after tree removal, the treatments effectively enhanced sagebrush steppe vegetation, ground surface conditions, and soil hydrologic properties that promote infiltration and limit hillslope runoff. The results demonstrate the magnitude of improvements in vegetation and hydrologic function vary with treatment type and illustrate the complexities in selecting, implementing, and assessing tree removal practices on woodland encroaches sites. The findings advance ecological understanding regarding the effectiveness of multiple tree removal practices to restore vegetation and hydrologic function on woodland-encroached sagebrush steppe. The long-term nature of the study provides unique insight regarding treatment selection and implementation for improved management of this ecologically important ecosystem.

Technical Abstract: Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland encroachment into sagebrush steppe communities throughout western North America has substantially imperiled one of the most ecologically important rangeland ecosystems in the world. Pinyon and juniper encroachment degrades sagebrush steppe vegetation, negatively affects ecological function, and limits delivery of ecosystem services. Loss of critical understory vegetation with woodland succession increases hillslope runoff, and, over time, amplified runoff and soil erosion rates can perpetuate a woodland state and transition a site beyond an irreversible ecological threshold. Various tree removal practices are employed to re-establish sagebrush steppe vegetation and an associated resource-conserving ecohydrologic function. Effectiveness of these practices is highly variable owing to the vast domain in which woodland encroachment occurs, differences in treatment applications, and the myriad of possible pre-treatment conditions and post-treatment land uses and climate trends. This study evaluated the long-term (13 yr post-treatment) effectiveness of prescribed fire and mechanical tree removal to re-establish sagebrush steppe vegetation and associated spatial patterns in ground surface conditions and soil hydrologic properties at two woodland-encroached sites. Specifically, we assessed effects of tree-removal on: (1) vegetation and ground cover at the hillslope-scale (990 m2 plots) and (2) associated spatial patterns in point-scale ground surface conditions and soil hydrologic properties along transects extending from tree bases and into intercanopy areas between trees. Both sites were in mid to late stages of woodland encroachment with extensive bare conditions (~60-80% bare ground) throughout a degraded intercanopy area (75% of domain) surrounding tree islands (25% of domain, subcanopy areas). All tree removal treatments effectively removed mature tree cover and enhanced hillslope vegetation attributes typical of sagebrush steppe. Enhanced herbaceous (4-15-fold increases) cover in burned areas reduced bare interspace (bare area between plants) by at least 4-fold and improved hydraulic conductivity (> than 2-fold) and overall ecohydrologic function. Mechanical treatments retained or increased sagebrush and generally increased intercanopy herbaceous vegetation. Intercanopy ground surface conditions and soil hydrologic properties in mechanical treatments were generally similar to those in burned areas, but were also statistically similar to the same measures in untreated areas in most cases. This indicates that vegetation and ground surface conditions in mechanical treatments are trending toward significant improved hydrologic function over time. Treatments had limited impact on soil hydrologic properties within subcanopy areas, but burning did reduce soil water repellency strength (by 4-fold) and occurrence underneath trees (by 3-fold). Overall, the treatments over a 13 yr period enhanced vegetation, ground surface conditions, and soil hydrologic properties that promote infiltration and limit runoff generation for intercanopy areas representing about 75% of area at the sites. Tradeoffs in treatment alternatives were evident. Burning removed already limited sagebrush cover from the sites and re-establishment to pre-burn or greater levels may take 30-50+ yr depending on the site. Burning also facilitated transition of subcanopy areas from hydrologically stable litter-covered islands to patches of the invasive annual cheatgrass (Bromus tectorum L.). Cheatgrass increases were minimal in subcanopy areas in mechanical treatments, but intercanopies within these treatments also exhibited lesser increases in desired perennial grasses and delayed hydrologic improvement. The variations in woodland responses across sites, treatments, and measurement scales in this long-term study illustr