Response of understory vegetation and soil moisture to infrequent heavy defoliation of chemically thinned juniper woodland

Targeted grazing programs that use small ruminants to control invasion of undesired woody plants are increasing across western North America. However, information regarding the potential impact of these herbivory regimes on non-target vegetation is lacking. We conducted a study in central New Mexico to determine the effects of removing all above-ground herbaceous biomass during winter in plots located beneath (under-canopy) or close to (inter-canopy) live one-seed juniper trees (Juniperus monosperma) or dead tree snags that had been chemically treated 10 years previously. In under-canopy plots (defoliated in 2005 and 2006) understory herbaceous cover was not affected by the defoliation treatment; however, biomass production of Bouteloua gracilis and other perennial grasses decreased. In inter-canopy plots (defoliated in 2006), the defoliation treatment increased total herbaceous cover in plots between live trees, whereas biomass production was not affected. Simulated infrequent targeted grazing in a juniper woodland had slight detrimental effects on non-target plants in under-canopy plots by decreasing biomass production of native grasses under dead and live trees but had positive effects on inter-canopy plots by increasing total herbaceous basal cover. Superficial volumetric water content was not affected by the defoliation treatment, suggesting that simulated targeted grazing does not increase superficial soil moisture.