Understory cover responses to pinon-juniper treatments across tree dominance gradients in the Great Basin

Authors: ROUNDY, BRUCE - Brigham Young University; MILLER, RICHARD - Oregon State University; TAUSCH, ROBIN - Us Forest Service (FS); YOUNG, KERT - Brigham Young University; Hulet, April; Rau, Benjamin; JESSOP, BRAD - Bureau Of Land Management; CHAMBERS, JEANNE - Us Forest Service (FS); EGGET, DENNIS - Brigham Young University

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2014
Publication Date: 9/1/2014
Publication URL: http://handle.nal.usda.gov/10113/60847
Citation: Roundy, B.A., Miller, R.F., Tausch, R.J., Young, K., Hulet, A., Rau, B.M., Jessop, B., Chambers, J.C., Egget, D. 2014. Understory cover responses to pinon-juniper treatments across tree dominance gradients in the Great Basin. Rangeland Ecology and Management. 67;482-494. DOI: 10.2111/REM-D-13-00018.1.

Interpretive Summary: Piñon and juniper trees are often removed to restore native vegetation and avoid high severity fires where they have invaded sagebrush communities. We evaluated three common fuel reduction methods for removing trees including prescribed fire, tree felling, and tree shredding across a tree infilling gradient, and examined at what phase of tree infilling relative to understory cover should prescribed fire and mechanical treatments target. Prescribed fire reduced shrub cover at all but the highest range of tree cover where shrub cover was minimal before treatment. Prescribed fire increased cheatgrass cover when tree cover was = 10%. Tree cutting and shredding produced similar responses and increased tall grass, perennial herbaceous, and cheatgrass cover at mid to upper ranges of tree cover. Responses were relatively consistent among sites and indicate beneficial effects of fuel treatments from mid to high tree cover where residual tall grass and perennial herbaceous cover are at least 8-10%.

Technical Abstract: Piñon (Pinus spp.) and juniper (Juniperus spp.) trees are reduced to restore native vegetation and avoid high severity fires where they have invaded sagebrush (Artemisia tridentata Nutt.) communities. To recommend treatment implementation which avoids threshold-crossing to invasive plant dominance we asked: 1) what phase of tree infilling relative to understory cover should prescribed fire and mechanical treatments target, and 2) are patterns of response consistent enough to support region-wide recommendations? Prescribed fire and tree felling were applied to 8-20 ha treatment plots at 11 sites across the Great Basin with a tree shredding treatment also applied to four Utah sites. Treatments were applied across a tree infilling gradient as quantified by two covariates, tree cover (TC) and tree dominance index (TDI= tree cover/tree + shrub + tall perennial grass cover). Mixed model analysis of covariance indicated that treatment x covariate interactions were significant (P<0.05) for most functional groups 3 years after treatment. Fire reduced shrub cover by an average of 6.3% at all but the highest ranges of TDI (<0.8) and TC (<40%) where shrub cover was minimal before treatment. Fire increased cheatgrass (Bromus tectorum L.) cover by an average of 4.4% at all ranges of TDI and at =10% TC. Cutting produced similar responses as shredding and increased tall grass, perennial herbaceous, and cheatgrass cover by an average of 8.1, 10.3, and 3.6%, at mid to upper ranges of TDI (=0.1, = 0.2, = 0.45) and TC (= 10, = 20, = 35%). Cheatgrass cover was <11% and patchy at all but two Utah sites, where high cover (>30%) indicates risk of weed dominance with recurring fire. Responses were relatively consistent among sites and indicate beneficial effects of fuel treatments from mid to high tree cover where residual tall grass and perennial herbaceous cover are at least 8-10%.