Influences of Mechanized Thinning and Prescribed Fire on Understory Vegetation in an Uneven-Aged Jeffrey Pine Stand

Abstract

Thinnings using cut-to-length and whole-tree harvesting systems followed by prescribed underburning were assessed for their effects on shrub, forb, and grass understory species in a second-growth Jeffrey pine (Pinus jeffreyi Grev. & Balf.) stand. Prior to treatment installation, a mixed shrub-dominated understory featuring antelope bitterbrush (Purshia tridentata [Pursh] DC.) with mules ears (Wyethia mollis A. Gray) as the only forb and Sandberg bluegrass (Poa secunda J. Presl) the most prominent among sparse grasses was inventoried such that both percent cover and dry weight by species were revealed. Five growing seasons after thinning and four after underburning, this inventory was repeated. By either abundance measure, bitterbrush was reduced by approximately two-thirds in the cut-to-length treatment and by one-half in the whole-tree treatment in comparison to that in the unthinned control at the final inventory. For the cut-to-length treatment, a similar reduction in mules ears was noted, but that in the whole-tree treatment was somewhat less for this species. Bluegrass was reduced by approximately one-half in the former treatment and by threefourths in the latter. Prescription fire reduced bitterbrush to less than one-tenth and mules ears to approximately one-half of that in the unburned treatment, but bluegrass prevalence was more than 10× greater in the burned than in the unburned treatment. Among an array of regression models used to evaluate selected variables for their predictive capacity regarding understory plants, abundance of each species noted above among others was negatively correlated with overstory density across treatments.