Long-term vegetation responses to pinyon-juniper woodland reduction treatments in Nevada
Ernst-Brock, C.; L.T. Turner; R.J. Tausch, E.A. Leger. 2019. Long-term vegetation responses to pinyon-juniper woodland reduction treatments in Nevada, USA. Journal of Environmental Management 242:315-326.
Wellington Hills, 65 km (40 mi) southeast of Carson City, Nevada.
Description of Site
Elevation – 2,200 m (7,200 ft); Ppt – 500 mm (20 in); Soils -very gravelly coarse sand, Toejom Series (frigid-xeric); Plant community – singleleaf pinyon (Pinus monophylla)/mountain big sagebrush (Artemisia tridentata spp. vaseyana)-bitterbrush (Purshia tridentata)-mountain mahogany (Cercocarpus ledifolius)/desert needlegrass (Acnatherum speciosum).
Treatments and Responses Measured
Treatments – Control, three types of thinning, clearcut, and cut-leave trunk cross sections; all treated plots seeded to mix of native and non-native shrubs, perennial grasses, and forbs; Time period – 32-years post-treatment; Measurements – foliar cover and tree density by size class.
Singleleaf pinyon cover ranged 2-8% in treated plots compared to 32% in control plots; treated plots contained 86-160 singleleaf pinyon trees/ha and control 111 trees/ha. Bitterbrush and mountain big sagebrush cover in tree canopy reduction plots ranged from 15-20% and 6-13% cover, repectively compared to 2 to 2.5% in control. Perennial grass cover ranged from 30- >40% in treated plots compared to <5% in the control. Cheatgrass was generally higher in treated plots and accounted for 24-27% cover, although there was high variability within and across all treatment plots.
Both shrub and perennial grass cover increased and remained significantly greater than the control 32 years following treatment. The majority literature supports this inverse relationship between trees and understory vegetation. However, the magnitude of change is closely linked to key characteristics of the site, which drive plant succession (Miller, Chambers, et al. 2014, pg 3). Seeded introduced species accounted for a significant portion of the increase in perennial grasses. Pretreatment native perennial grass cover was low, based on the control plots, so seeding should have been a consideration. The two to three-fold increase in cheatgrass across the treated plots is of concern, especially if a stand replacement fire occurs. The current density of young trees on the site will eventually result in PJ dominance, likely within the next 20-40 years.