Long-term evidence for fire as an ecohydrologic thresholdreversal mechanism on woodland-encroached sagebrush shrublands

Authors: Williams, Christopher - Jason; PIERSON JR, FREDERICK; NOUWAKPO, S.K. - UNIVERSITY OF NEVADA; KORMOS, P.R. - NATIONAL OCEANIC & ATMOSPHERIC ADMINISTRATION (NOAA); AL-HAMDAB, O.Z. - TEXAS A&M UNIVERSITY; Weltz, Mark

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2018
Publication Date: 2/28/2019
Publication URL: https://handle.nal.usda.gov/10113/6472488
Citation: Williams, C.J., Pierson, F.B., Nouwakpo, S., Kormos, P., Al-Hamdab, O.Z., Weltz, M.A. 2019. Long-term evidence for fire as an ecohydrologic threshold reversal mechanism on woodland-encroached sagebrush shrublands. Ecohydrology. 12(4):e2086. https://doi.org/10.1002/eco.2086.
DOI: https://doi.org/10.1002/eco.2086

Interpretive Summary: The conversion of sagebrush rangelands to pinyon and juniper woodlands has significantly altered a vast expanse of the western United States. This woodland encroachment of sagebrush rangelands poses a host of negative ramifications to ecosystem services, including degradation of understory vegetation and wildlife habitat, limited forage for wild and domestic animals, and high rates of runoff and soil loss. Prescribed fire is commonly implemented on woodland-encroached sites to remove trees and re-establish sagebrush vegetation and associated low rates of runoff and soil loss. However, knowledge of the long-term benefits of such practices is limited. This study used rainfall simulation and overland flow experiments to investigate the long-term impacts of prescribed fire on vegetation, hydrologic function, and erosion at two sagebrush sites in the later stages of woodland-encroachment. Prescribed burning initially increased bare conditions at both sites and increased runoff and erosion rates. However, tree reduction by burning facilitated dramatic increases in grass cover over nine growing seasons after fire that improved infiltration, limited runoff and soil loss, and re-established a trajectory toward overall improved delivery of ecosystems services. The study is unique in that it spans a 10 yr period and the results provide land managers insight into potential long-term benefits of prescribed fire restoration treatments on sagebrush steppe rangelands encroached by pinyon and juniper trees.

Technical 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 runoff and erosion that, over time, propagate persistence of the shrubland-to-woodland conversion. We evaluated whether tree removal by burning can decrease late-succession woodland ecohydrologic resilience by increasing vegetation and ground cover over a 9 yr period after fire, and whether the soil erosion feedback on late-succession woodlands is reversible by burning. To address these questions, we employed a suite of vegetation and soil measurements and rainfall simulation and concentrated overland flow experiments across multiple plot scales on unburned and burned areas at two sagebrush sites in the later stages of woodland succession. Prior to burning, tree cover was approximately 28% at the sites and more than 70% of the area at the sites was intercanopy with depauperate understory vegetation and extensive bare ground (52-60% bare soil and rock). Burning initially increased bare ground across fine (<1 m2) to patch (tens of meters) scales, resulting in enhanced sediment availability at the fine-scale, sustained high runoff and erosion within degraded intercanopies, and amplified runoff and erosion from tree canopy areas. However, fire-induced increases in grass cover over nine growing seasons improved infiltration, limited runoff and sediment delivery from the fine-spatial scale, and reduced intercanopy runoff and erosion at the patch-scale. These changes reflect a switch in vegetation structure, triggered by burning and subsequent vegetation re-establishment, and a shift to biotic control on runoff and erosion across spatial scales. The responses and persistence over the 9-yr period post-fire at the two sites demonstrate that fire can decrease woodland ecohydrologic resilience by altering plant community physiognomy and thereby can reverse the soil erosion feedback on sagebrush shrublands in the later stages of woodland encroachment.