Long‐term evidence for fire as an ecohydrologic threshold‐reversal mechanism on woodland‐encroached sagebrush shrublands

Abstract

Encroachment of sagebrush (Artemisia spp.) shrublands by pinyon (Pinus spp.) and juniper (Juniperus spp.) conifers (woodland encroachment) induces a shift from biotic-controlled resource retention to abiotic-driven loss of soil resources. This shift is driven by a coarsening of the vegetation structure with increasing dominance of site resources by trees. Competition between the encroaching trees and understory vegetation for limited soil and water resources facilitates extensive bare intercanopy area between trees and concomitant increases in run-off and erosion that, over time,propagate persistence of the shrubland-to-woodland conversion. We evaluated /journal/eco 1 of 28 plant community physiognomy and thereby can reverse the soil erosion feedback on sagebrush shrublands in the later stages of woodland encroachment.

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)

“…This additional treatment was applied in 2007 and 2015 to some plots in cut treatment areas at Marking Corral and Onaqui and in 2008 and 2009 in unburned areas at Castlehead. Treatment applications and descriptions and the study experimental design are explained in earlier papers by Pierson et al (2010Pierson et al ( , 2013Pierson et al ( , 2014Pierson et al ( , 2015 and by Williams et al (2014aWilliams et al ( , 2019aWilliams et al ( , 2020, and all treatments for each site each year are provided in Table 2. A suite of biological and physical attributes at each site were measured at point, small rainfall plot (0.5 m 2 ), overland-flow plot (∼ 9 m 2 ), large rainfall plot (13 m 2 ), and hillslope plot (990 m 2 ) scales. Soil bulk density of the near surface (0-5 cm depth) was sampled as a point measure in interspace microsites between plants, shrub coppice microsites underneath shrub canopies, and tree coppice microsites underneath tree canopies.…”

“…Overland-flow simulations run independent of rainfall-simulation experiments were conducted on borderless plots but contained a runoff collection trough at the downslope plot base ( Fig. 2d-e; Pierson et al, 2013Pierson et al, , 2015Williams et al, 2014aWilliams et al, , 2019aWilliams et al, , 2020.…”

“…Rainfall was applied to small rainfall plots by a Meyer and Harmon-type portable oscillating-arm rainfall simulator fitted with 80-100 Veejet nozzles ( Fig. 1a; Meyer and Harmon, 1979;Pierson et al 2010Pierson et al , 2013Pierson et al , 2014Williams et al, 2014aWilliams et al, , 2019aWilliams et al, , 2020. The applied rainfall kinetic energy (200 kJ ha −1 mm −1 ) and raindrop size (2 mm) were within approximately 70 kJ ha −1 mm −1 and 1 mm respectively of values reported for natural convective rainfall (Meyer and Harmon, 1979).…”

“…were derived for each transect and averaged across the transects on each plot to determine gap-class plot means, similar to large rainfall plots. Datalogger-controlled flow regulators (see Pierson et al, 2010Pierson et al, , 2013Pierson et al, , 2015Williams et al, 2014aWilliams et al, , 2019aWilliams et al, , 2020 were used to apply concentrated flow release rates of 15, 30, and 45 L min −1 to each overland-flow plot. Flow was routed into and through a metal box filled with Styrofoam pellets and was released through a 10 cm wide mesh-screened opening at the box base ( Fig.…”

“…The resulting 8 min runoff and sediment variables were derived as explained for the 45 min rainfall simulations. The velocity of overland flow was measured using a concentrated salt tracer applied into the flow and electrical conductivity probes to track the mean transit time of the tracer over a set flow path length (usually 2 m; Pierson et al, 2010Pierson et al, , 2013Pierson et al, , 2015Williams et al, 2014aWilliams et al, , 2019aWilliams et al, , 2020. The width, depth, and a total rill area width (TRAW) of overland flow were measured along flow cross sections 1, 2, and 3 m downslope from the flow release point (Pierson et al, 2010).…”