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Title: Can wildfire serve as an ecohydrologic threshold-reversal mechanism on juniper-encroached shrublands

item Williams, Christopher - Jason
item Pierson Jr, Frederick
item AL-HAMDAN, OSAMA - University Of Idaho
item KORMOS, PATRICK - Boise State University
item Hardegree, Stuart
item Clark, Pat

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2012
Publication Date: 1/24/2014
Citation: Williams, C.J., Pierson Jr, F.B., Al-Hamdan, O.Z., Kormos, P.R., Hardegree, S.P., Clark, P. 2014. Can wildfire serve as an ecohydrologic threshold-reversal mechanism on juniper-encroached shrublands. Ecohydrology. 7:453-477. DOI: 10.1002/eco.1364.

Interpretive Summary: Millions of hectares of sagebrush steppe in the Great Basin, USA, have been degraded by woodland encroachment. The expansion of conifers into sagebrush steppe commonly results in decreased understory vegetation and increased runoff and erosion. Land owners and management agencies across the Great Basin are actively seeking sagebrush steppe restoration guidelines and quantification of restoration treatment effects. This study used rainfall simulation and concentrated flow simulations to investigate the impacts of western juniper encroachment on runoff and erosion and to evaluate short-term effects of wildfire on woodland hydrologic and erosional processes. Runoff and erosion were significantly greater from degraded and bare intercanopy areas between trees than in areas underneath juniper canopies. The primary effects of woodland encroachment were a reduction in understory cover and formation of well-connected and highly-erodible intercanopy area. Severe burning amplified runoff and erosion from areas underneath trees and the effects persisted for two years following the fire. In contrast, burning improved infiltration and decreased erosion in intercanopy areas within the first two years after fire. Improved hydrologic and erosion function in intercanopy areas occurred due to fire-induced increases in grasses and forbs. Results indicate fire may provide a restoration pathway for woodland-encroached sagebrush steppe by increasing intercanopy cover within the first several years following burning. Short-term increased soil loss from tree canopy areas is potentially more favorable than long-term soil loss with continued woodland degradation. The study results provide land managers insight into potential responses of restoration treatments on sagebrush steppe rangelands encroached by western juniper.

Technical Abstract: Woody plant encroachment on water-limited lands can induce a shift from biotic (plant)-controlled resource retention to abiotic (physical)-driven losses of critical soil resources. The biotic-to-abiotic shift occurs where encroachment propagates connectivity of runoff processes and amplified cross-scale erosion, that in-turn, promote ecohydrologic resilience of the post-encroachment community. We investigated these relationships for woodland-encroached sagebrush steppe in the Great Basin, USA and evaluated wildfire as a mechanism to reverse the post-encroachment soil erosion feedback. We measured vegetation, soils properties, and runoff/erosion from experimental plots on burned and unburned areas of a late-succession woodland one and two years post-fire. Our findings suggest the biotic-to-abiotic shift and amplified cross-scale erosion occur where encroachment-induced bare ground exceeds 50-60% and bare gaps between plant bases frequency extend beyond 100 cm. The trigger for amplified cross-scale erosion is formation of concentrated flow within the degraded intercanopy between trees. Burning in this study decreased ecohydrologic resilience of the late-succession woodland through herbaceous recruitment two years post-fire. Increased intercanopy herbaceous productivity decreased connectivity of bare ground, improved infiltration, and reduced erosion, but the study site remained vulnerable to runoff and erosion from high-intensity rainfall. We therefore opine burning can reduce woodland ecohydrologic resilience, but that woodland structural and functional ecohydrologic attributes may remain enhanced by intense rainfall for an undetermined period post-fire. We cannot conclude whether wildfire reverses the woodland-induced soil erosion feedback on sagebrush rangelands. However, our results do suggest wildfire may provide a restoration pathway for sagebrush-steppe by reducing woodland ecohydrologic resilience over time.