Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2010
Publication Date: July 1, 2010
Citation: Cline, N.L., Roundy, B.A., Pierson, F.B., Kormos, P., and Williams, C.J. 2010. Hydrologic Response to Mechanical Shredding in a Juniper Woodland. Rangeland Ecology and Management. 63:467-477. Interpretive Summary: Tree encroachment into shrub steppe and increases in tree cover on established pinyon-juniper woodlands in the Great Basin, USA, have amplified fuel densities in these ecosystems. The altered fuel densities are thought to have increased the occurrence and extent of catastrophic wildfires and have prompted land managers to search for effective fuel control methods. Recently, mechanical shredding (Bull Hog ®) has been used to reduce juniper trees to a residue on or around juniper mounds. This study used small-plot rainfall simulation (0.5 m2) to investigate soil compaction and hydrologic responses from mechanical tree shredding of Utah juniper (Juniperus ostesperma) in a sagebrush/bunchgrass plant community (Artemisia nova, Artemisia tridentata ssp. Wyomingensis/Pseudoroegneria spicata, Poa secunda). Minor compaction from the mechanized shredders was observed, but shredded tree material mitigated these impacts. The results suggest runoff and erosion are potentially reduced where shredding spreads tree residue into adjacent areas beyond original canopy drip lines. Limited adverse and some positive hydrologic responses to shredding at the small plot scale indicate that shredding is a potentially hydrologically-viable fuel control method. Because hydrologic responses to treatments are site and scale-dependent, determination of shredding effects on other sites and at hillslope or larger scales requires further investigation.
Technical Abstract: Juniper (Juniperus spp.) woodland expansion in the western United States is thought to result in increased catastrophic wildfires throughout its range and has prompted land managers to search for effective fuel control methods. Recently, mechanical shredding (Bull Hog ®) has been used to reduce juniper trees to a residue on or around juniper mounds. On hillslopes, compaction from rubber tires or steel tracks could potentially increase runoff and sediment yield while shredded tree residue could decrease them. We investigated soil compaction and hydrologic responses from mechanical shredding of Utah juniper (Juniperus ostesperma) in a sagebrush/bunchgrass plant community (Artemisia nova, Artemisia tridentata ssp. Wyomingensis/Pseudoroegneria spicata, Poa secunda) on a gravelly loam soil with a 15% slope in the Onaqui Mountains of Utah. Rain simulations were applied on 0.5 m2 runoff plots at two rates: 64 mm•h-1 (dry run – soil initially dry) and 102 mm•h-1 (wet run – soil initially wet). Runoff and sediment were collected from runoff plots placed in four blocks each containing four microsites (juniper mound, shrub mound, vegetation-free or bare interspace and grass interspace) with undisturbed or tracked treatments for each microsite type and a residue-covered treatment for grass and bare interspace microsites. Soil penetration resistance was measured at the hillslope scale; canopy cover and ground cover were measured at the hillslope and runoff plot scale; and surface roughness was measured at the runoff plot scale. After shredding trees at a density of 453 trees ha-1, tire tracks covered 15% of the hillslope. Tracked soils on interspaces and shrub mounds had significantly (P < 0.05) more penetration resistance than untracked soils. For the wet run, infiltration rates of grass interspaces were significantly decreased (P < 0.05) by tire tracks but infiltration rates on juniper mounds and bare interspaces were unchanged. Bare interspace plots covered with residue had significantly higher (P < 0.05) infiltration rates and lower sediment yields than those without residue. This suggests that greatest hydrologic benefits will occur when shredding spreads tree residue beyond tree mounds to cover adjacent interspaces. Limited adverse and some positive hydrologic responses to shredding at the microsite scale suggest that shredding is a hydrologically-viable fuel control method on similar sites. Because hydrologic responses to treatments are site and scale-dependent, determination of shredding effects on other sites and at hillslope or larger scales will more fully guide management actions.