Spatial and Temporal Variability of the Impacts of Pinyon and Juniper Reduction on Hydrologic and Erosion Processes Across Climatic Gradients in the Western US: A Regional Synthesis

Abstract

Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands are an important vegetation type in the Great Basin, Colorado Plateau, and southwestern desert regions of the western US that is undergoing substantial changes associated with land management, altered disturbance regimes, and climate change. We synthesized literature on the ecohydrologic impacts of pinyon and juniper tree reductions across plot to watershed scales, short- and long-term periods, and regional climatic gradients. We found that the initial plot- to hillslope-scale ecohydrologic and erosion impacts of tree reduction on pinyon and juniper woodlands by fire, mechanical tree removal, or drought depend largely on: (1) the degree to which these perturbations alter vegetation and ground cover structure, (2) initial conditions, and (3) inherent site attributes. Fire commonly imparts an initial increased risk for hillslope runoff and erosion that degrades over time with vegetation and ground cover recovery whereas tree reductions by mechanical means pose fewer initial negative ecohydrologic impacts. Tree reduction by either approach can enhance understory vegetation and improve site-level ecohydrologic function over time, particularly on sites with an initially favorable cover of native herbaceous vegetation and a cool-season precipitation regime. Understory vegetation and ground cover enhancements appear to increase ecohydrologic resilience of some woodland communities to disturbances such as drought, fire, and insect infestations. In contrast, intensive land use, prolonged drought or repeated burning associated with invasions of fire-prone grasses can propagate long-term site degradation through persistent elevated runoff and erosion rates. Our synthesis suggests the annual precipitation requirement for increases in plot- to hillslope-scale soil water availability for herbaceous enhancement through tree removal likely ranges from 200–400 mm for sites in the Great Basin and northern Colorado Plateau (cool-season precipitation regimes), and, although suggested with great uncertainty, likely exceeds 400 mm for woodlands with rain-dominated precipitation regimes in the southwestern US. Overall, literature is inconclusive regarding tree reduction impacts on watershed-scale changes in groundwater and streamflow. To date, there is little evidence that drought-related changes to vegetation in pinyon and juniper woodlands substantially affect watershed-scale water availability and streamflow at the annual time scale. Our synthesis identifies key knowledge gaps to overcome in improving understanding of the ecohydrologic and erosion impacts of broadly occurring pinyon and juniper tree reductions in the western US.

Understory Vegetation Change Following Woodland Reduction Varies by Plant Community Type and Seeding Status: A Region-Wide Assessment of Ecological Benefits and Risks

“…Similarly, the positive changes in cryptogams on drier black and Wyoming sites became statistically significant in response to mastication and cutting, whereas changes were not significant on mountain big sagebrush sites. Our explanation for the unexpected resilience on the drier sites supports broadscale studies illustrating that favorable changes in site ecohydrology are more pronounced in P–J woodlands that experience greater increases in understory vegetation, which in turn reduces runoff and increases water infiltration [ 76 , 201 , 202 ].…”

Overstory–Understory Vegetation Cover and Soil Water Content Observations in Western Juniper Woodlands: A Paired Watershed Study in Central Oregon, USA

“…The body of literature regarding large-scale understanding of ecohydrologic processes associated to the removal of juniper is still limited [21,22,37,38]. Soil water related studies in juniper woodlands have been conducted mostly at the plot-scale and there is an important need to understand soil-plant hydrological interactions at a larger scale [20].…”

Vegetation, ground cover, soil, rainfall simulation, and overland-flow experiments before and after tree removal in woodland-encroached sagebrush steppe: the hydrology component of the Sagebrush Steppe Treatment Evaluation Project (SageSTEP)

“…Sediment entrained by raindrops and shallow sheet flow in bare patches typically moves a limited distance downslope before deposition immediately upslope of and within vegetated areas (Emmett, 1970;Reid et al, 1999;Puigdefábregas, 2005;Pierson and Williams, 2016). Disturbances such as intensive land use, plant community transitions, and wildfire can alter this resource-conserving vegetation structure and thereby facilitate increases in runoff and soil loss through enhanced connectivity of overland-flow and sediment sources during rainfall events (Davenport et al, 1998;Wilcox et al, 2003;Pierson et al, 2011;Williams et al, 2014aWilliams et al, , 2014bWilliams et al, , 2018. The negative ramifications of woody plant encroachment and wildfire have been extensively studied on rangelands around the world, and this work has advanced understanding of runoff and erosion processes for these commonly occurring ecosystems (Schlesinger et al, 1990;Wainwright et al, 2000;Shakesby and Doerr, 2006;Shakesby, 2011;Pierson and Williams, 2016).…”

“…woodlands, invasions of fire-prone annual cheatgrass (Bromus tectorum L.), and altered fire regimes (Davies et al, 2011;Miller et al, 2011Miller et al, , 2019. Pinyon and juniper woodland encroachment of sagebrush vegetation can have negative hydrologic impacts (Miller et al, 2005;Petersen and Stringham, 2008;Pierson et al, 2007Pierson et al, , 2010Petersen et al, 2009;Williams et al, 2014aWilliams et al, , 2018. Encroaching trees outcompete understory sagebrush and herbaceous vegetation over time and thereby increase bare ground and connectivity of runoff and sediment sources (Miller et al, 2000;Bates et al, 2000Bates et al, , 2005Petersen et al, 2009;Pierson et al, 2010;Roundy et al, 2017).…”

“…Amplified soil loss over time perpetuates a woodland ecological state and long-term site degradation (Petersen et al, 2009). Land managers commonly employ various mechanical treatments and prescribed and natural fires to reduce tree cover and reestablish sagebrush vegetation and associated resource-conserving hydrologic function (Bates et al, 2000(Bates et al, , 2005(Bates et al, , 2014(Bates et al, , 2017Pierson et al, 2007;Miller et al, 2014;Roundy et al, 2014;Williams et al, 2018). However, managers are challenged with predicting potential vegetation and ecohydrologic effects of tree removal across diverse woodland landscapes and with determining the appropriate type and timing of available treatment options.…”