Understory Vegetation Response to Thinning Pinyon-Juniper Woodlands

Abstract

Portions of the western United States, extending from west Texas to eastern Oregon, are thin to densely populated with juniper (Juniperus spp.) woodlands. Managing tree densities, such as through mechanical thinning, has numerous ecological implications for rangeland watersheds and wildlife habitat. The objective of this study was to determine vegetation response to juniper removal in a pinyon (Pinus edulis Engelm)− and juniper (Juniperus monosperma Engelm)− dominated landscape in north-central New Mexico. Herbaceous cover and standing crop were collected from six 1.00- to 1.35-ha catchments situated within the New Mexico State University (NMSU)−Santa Fe Ranch, Santa Fe County, New Mexico, before treatment and at 5 and 10 yr following juniper removal. About 70% of mature juniper trees were cut in 2009 in three of the catchments (treated) while the remaining catchments were left as controls (untreated). Herbaceous cover and standing crop were measured in 2009 (pretreatment), 2014, and 2019 (post treatment) to test our hypothesis that juniper reductions would significantly increase herbaceous standing crop. After 10 yr, gravel, grass, and forb cover were comparable in the treated and the untreated, whereas bare soil cover was significantly decreased in the treated (30% ± 2.5) compared with the untreated (42% ± 2.5). Litter cover in the treated was higher (18% ± 2.6) than the untreated (5% ± 2.6). Herbaceous standing crop significantly increased in the treated compared with the untreated. Standing crop of grass in the treated was two to three times greater than the untreated. Forb standing crop was not influenced by treatment but increased over time in both treatments. Study findings provide land managers with critical information regarding one-seed juniper clearing effects on herbaceous vegetation response in the warm-climate rangeland ecosystems of the southwestern United States.