Post-Treatment Hydrologic Response to Mechanical Shredding in a Juniper Woodland

Date of Award

10-2008

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Plants, Soils, and Climate

First Advisor

Bruce A. Roundy

Second Advisor

Richard E. Terry

Third Advisor

Steven L. Petersen

Abstract

Juniper (Juniperus spp.) woodland expansion in the western United States is thought to result in increased 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 mulch residue on or around the juniper mound. On hillslopes, tracking from rubber tires or steel tracks could potentially increase runoff and sediment yield while the tree mulch residue could decrease them. We investigated soil compaction and hydrologic responses from mechanical shredding on a gravelly loam soil with a 15% slope in the Onaqui Mountains of Utah. Rain simulations were applied on 0.5 m2 plots at two rates: 64 mm•h-1 (dry run) and 102 mm•h-1 (wet run). Runoff and sediment were collected from 50 post-treatment plots: 20 control, 20 tire-tracked, and 10 mulch residue covered. Soil penetration resistance, canopy cover, ground cover, soil stability, and surface roughness were measured. Tracked soils were significantly more compacted (from 5 cm to 10 cm in soil depth) than untracked soils for interspace and shrub mound microsites. Infiltration rates of grass interspaces were significantly decreased (P < 0.05) by tire tracks but not on juniper mounds or bare interspaces. Mulch-residue-covered bare interspace plots had significantly higher (P < 0.05) infiltration rates and lower sediment yields compared to microsites without mulch residue. This study found little adverse hydrologic effect from mechanical shredding in these juniper woodlands at the patch-microsite scale. Effects of shredding at the hillslope or larger scales and on other sites should be quantified to best determine hydrologic response and guide management actions.

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