ARS | Publication request: Wildfire as a mechanism to reverse ecohydrologic thresholds in juniper-encroached shrublands

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: September 28, 2012
Publication Date: December 3, 2012
Citation: Williams, C.J., Pierson, F.B., Al-Hamdan, O.Z., Hardegree, S.P., Clark, P.E., and Kormos, P.R. 2012. Wildfire as a mechanism to reverse ecohydrologic thresholds in juniper-encroached shrublands. Presented at the American Geophysical Union Annual Fall Meeting, December 3-7, 2012, San Francisco, CA.

Technical Abstract: Woodland encroachment into Great Basin, USA, sagebrush steppe potentially educes a shift from biotic-controlled resource retention to abiotic-driven losses of critical soil resources. The biotic-to-abiotic shift occurs where encroachment propagates runoff connectivity and amplified cross-scale erosion, that in-turn, promote woodland ecohydrologic resilience. Our research objective was to evaluate wildfire as a threshold-reversal mechanism for restoration of sagebrush steppe ecohydrologic resilience. We examined vegetation, soils, and runoff and erosion from artificial rainfall and concentrated flow experiments across multiple scales on burned and unburned areas of a late succession woodland to address three research questions: 1) Are there key vegetation and soil indicators that a former sagebrush community has experienced the biotic-to-abiotic shift?, 2) Can fire decrease late-succession woodland ecohydrologic resilience by increasing vegetation and ground cover within the first two years post-fire?, and 3) Is the soil erosion feedback reversible by burning in the later stages of woodland encroachment? Our findings suggest encroachment-induced increases in bare ground beyond 60% and extensive bare ground gaps (> 100 cm) between plant bases amplify erosion across spatial scales, indicative of the biotic-to-abiotic shift. The primary trigger for increased cross-scale erosion is concentrated flow formation within bare intercanopy areas. Burning in this study decreased late-succession woodland ecohydrologic resilience through herbaceous recruitment two years post-fire. The increased plant productivity decreased the structural connectivity of bare ground, improved infiltration of moderate intensity rainfall, and reduced erosion from simulated concentrated flow. However, the research site remained vulnerable to runoff and erosion from high intensity rainfall. We therefore opine burning can reduce woodland ecohydrologic resilience via post-fire plant recruitment, and that the structural and functional attributes of woodlands may persist during intense rainfall within the first two years post-fire. Our results do not conclusively indicate burning reverses the soil erosion feedback on woodland encroached sites. Increased intercanopy infiltration and decreased concentrated flow erosion two years post-fire do however suggest continued post-fire vegetation recruitment may restore the sagebrush steppe structural-functional state over time. We conclude that fire, through a hysteresis effect, may act as a threshold-reversal mechanism to restore sagebrush steppe ecohydrologic resilience.