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 823 824 W. G. Salverson et al. 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.

Influences of Thinning and Underburning Restoration Practices on Red Turpentine Beetle Demography in Jeffrey Pine

“…Regardless, beetle colonization here as related to the change in understory vegetation paralleled that in response to most categories of downed and dead fuels insomuch as there was an array of negative correlations between the pitch tube counts and this independent variable, although the total number of such models was considerably less than that concerning fuel loading and did not include any incorporating the overall counts. Despite the substantial consumption of the shrub-dominated understory on this site by the fire (Salverson et al, 2011), it is illogical to deduce that any heat-related stress or injury was involved in these relationships also, so they may reflect an attenuated diminishment of stress in the Jeffrey pine from competition with the shrub cover that facilitated an elevated intensity of beetle attack.…”

“…Subsequently, differences among means were evaluated using the least significant difference (LSD) test with α = .05. For the statistical analysis of data concerned with mensurational attributes and tree mortality specific to the 2001 and 2011 inventories, consult Walker, Swim, Johnson, Miller, and Fecko (2014b), while such analysis of that concerning downed and dead fuels and understory vegetation is detailed in Walker et al (2006) and Salverson et al (2011), respectively.…”

“…Postburn dry weights in the burned portions of the three thinning treatments, in order as above, were 374.9, 196.7, and 352.1 kg ha −1 ; and thus the reductions therein were 1,198.6, 98.7, and 2,472.4 kg ha −1 , respectively. The understory community at this site is quantified in greater detail in Salverson et al (2011), including the impacts of the thinning and fire treatments.…”

Evaluation of Mechanized Thinning and Prescribed Fire Effects on Long-Term Fuels Accumulations in Uneven-Aged Jeffrey Pine

“…and mahala mat (Ceanothus prostratus Benth.) in the understory (Salverson et al, 2011a), potentially explaining why decomposition may have been hindered in these stand portions. The drastic reductions of 1+10-hr fuels within the unburned portions of the unthinned and cut-to-length subunits may be further explained by the added benefits of a mulching effect on their breakdown, as these two treatment combinations initially exhibited significantly greater amounts of these fuels.…”

“…Specific to mensurational and fire injury data reported here, an initial posttreatment inventory was conducted in September 2003 that relied upon 30 permanent 0.08-ha circular plots originally established in September 2000 for the collection of pretreatment mensurational and other data reported in a series of prior manuscripts (Walker et al, 2006a(Walker et al, , 2006b(Walker et al, , 2014a(Walker et al, , 2014bFecko et al, 2008aFecko et al, , 2008bFecko et al, , 2008cSalverson et al, 2011aSalverson et al, , 2011bSwim et al, 2013). These plots were situated such that each stand subunit accommodated 10 and with five each in their respective burned and unburned portions.…”