Influences of mechanized thinning and prescribed fire on natural regeneration and understory vegetation in an uneven-aged Jeffrey pine stand

The application of thinning and prescribed fire treatments to meet forest management objectives necessitates a comprehensive understanding of the implications for a host of forest attributes. Thinnings using cut-to-length and whole-tree harvesting systems followed by prescribed underburning were assessed for their effects on seedling and sapling demography as well as shrub, forb, and grass understory species in a second growth, uneven-aged Jeffrey pine stand containing a minor component of California white fir. Initial seedling and sapling populations were quantified early in the first growing season immediately of lowing implementation of the thinning treatments and again four years later approximately three years after prescribed fire implementation. Depression of seedling counts due to forest floor disturbance associated with the thinning operations was followed by a recovery largely confined to Jeffrey pine regeneration in the whole-tree treatment where final seedling counts exceeded those found initially. The postburn substrate was more favorable for the establishment of Jeffrey pine than white fir seedlings, and the largest increase overall in seedling counts between the initial and final inventories occurred in the burned portion of the whole-tree treatment. Live sapling losses from thinning were greatest in the cut-to-length treatment, while underburning induced complete mortality within this size class. For understory vegetation, a mixed shrub-dominated composition featuring antelope bitterbrush with mules ears as the only forb and Sandberg bluegrass 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. 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 three-fourths 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 10X greater in the burned than in the unburned treatment. Results presented here provide land managers insight into the impacts of six combinations of thinning and burning treatment on natural regeneration and understory community development in eastern Sierra Nevada Jeffrey pine and similar dry site forest types.