Robert M. Fecko

Thinning and Prescribed Fire Effects on Forest Floor Fuels in the East Side Sierra Nevada Pine Type

Abstract Forest thinning accomplished with cut-to-length and whole-tree harvesting systems combined with prescribed underburning were assessed for their impacts on downed and dead fuel loading of all timelag categories in eastern Sierra Nevada Jeffrey pine (Pinus jeffreyi Grev. & Balf.). Cut-to-length harvesting resulted in an approximate doubling of total fuel loads with the greatest increases occurring in the 100-hr and 1000-hr categories, while changes associated with whole tree harvesting were marginal to the extent that overall posttreatment loading differed little between whole-tree and unfhinned treatments. Following the thinning operations, 1 + 10-hr and total fuel accumulations in the cut-to-lengfh treatment and 1000-hr fuels in the whole-tree treatment were positively correlated with harvested basal area and harvested foliage, branch, bole, and total tree biomass in simple regression models. Subsequent consumption during underburning eliminated 1 + 10-hr and 100-hr fuel additions from cut-to-lengfh harvesting along with a portion of the natural loading in these categories but was much less effective in reducing the 1000-hr fuels generated by this harvesting. Consumption of 1 + 10-hr, 100-hr, and total fuels in all thinning treatments was positively correlated with the amounts present within each category before underburning. Results reported here provide insight into fuel load modifications resulting from field practices that are being increasingly integrated into comprehensive management efforts to improve forest health and fire resilience in the western United States.

FOREST HEALTH IMPACTS OF BARK BEETLES, DWARF MISTLETOE, AND BLISTER RUST IN A LAKE TAHOE BASIN MIXED CONIFER STAND

Abstract Interactions between forest health variables and mensurational characteristics in an uneven-aged eastern Sierra Nevada mixed conifer stand were examined. The stand was located in the Lake Tahoe Basin on a site featuring a coarsely textured granitic soil and numerous rock outcrops. Its composition was dominated by California white fir (Abies concolor var. lowiana [Gord.] Lemm.), with Jeffrey pine (Pinus jeffreyi Grev. & Balf.) and sugar pine (Pinus lambertiana Dougl.) less prominent and incense-cedar (Libocedrus decurrens Torr.) and mountain alder (Alnus tenuifolia Nutt.) the most minor constituents. The majority of saplings and seedlings were white fir. The stand exhibited no evidence that its development had been influenced by fire and, overall, it consisted of numerous small trees accruing little radial growth. Nearly one-quarter of all standing stems pole size or larger were dead, with mortality concentrated in white fir. Forest-floor fuel accumulations were excessive, and coarse debris was especially prominent. A fir engraver beetle (Scolytus ventralis LeConte) epidemic in white fir contrasted against apparent endemic population levels of the Jeffrey pine (Dendroctonus jeffreyi Hopkins) and red turpentine (Dendroctonus valens LeConte) beetles in Jeffrey pine and of the mountain pine beetle (Dendroctonus ponderosae Hopkins) in sugar pine. The severity of fir engraver attack on white fir was weakly related to overall tree size and to the proportion of composition consisting of this host species, while in Jeffrey pine and sugar pine, bark beetle attacks were strongly correlated with the individual proportions of these 2 hosts. Across all species, basal area explained a substantial proportion of the variation in overall attack severity. We found light infestations of true fir dwarf mistletoe (Arceuthobium abietinum Engelm. ex Munz f. sp. concoloris) in white fir and western dwarf mistletoe (Arceuthobium campylopodum Engelm.) in Jeffrey pine, plus an early stage of infection by the white pine blister rust (Cronartium ribicola J.C. Fischer) in sugar pine. Collectively, this case study characterized and quantified many of the conditions, symptoms, and causative agents inherent in a decadent mixed conifer stand in the eastern Sierra Nevada.

Effects of Mechanized Thinning and Prescription Fire on Stand Structure, Live Crown, and Mortality in Jeffrey Pine

ABSTRACT Forest thinning, using cut-to-length and whole-tree harvesting systems with subsequent underburning were assessed for their influence on stand structure, health, and fire resilience in uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf). Stand attributes, derived from measuring trees ≥ 10.2 cm diameter at breast height (DBH), were collected from permanent plots. Trees were divided into three size classes that generally corresponded to dominant/codominant, intermediate, and suppressed crown classes. Comparisons of post- to pre-burning mortality revealed significant thinning and fire main treatment effects as well as significant interaction between these two treatments in the two larger size classes. Mortality increased by 250% in the intermediate crown class within the burned stand portion of the whole-tree treatment, whereas among dominant/codominant trees mortality rose by 160% in the burned cut-to-length treatment combination. Pre- to post-burning shifts in live crown, expressed as a percentage of total tree height, were significantly influenced by both thinning and fire main treatments in the two larger size classes, while the interaction of these treatments was also significant among the largest trees. Within both of these size classes, decreases in live crown percentage were greatest in the burned portion of the unthinned treatment, where intermediate crown class trees lost over 20% of their crowns, while reductions in dominant/codominant trees averaged nearly 25%. The second highest losses for both size classes occurred within the burned cut-to-length treatment. In the smallest trees, mortality rose sharply and live crown decreased substantially after burning in both thinning treatments and in the unthinned control. Within the two larger size classes, preburn live crown size was negatively correlated with changes in crown size subsequent to underburning while DBH was negatively correlated with postburning changes in mortality, but only in intermediate crown class trees. These results present land managers with outcomes of differing management practices presently being evaluated for their potential to enhance forest health and reduce wildfire risk in the Sierra Nevada and similar dry forest regions.

Influences of Thinning and Prescribed Fire on Water Relations of Jeffrey Pine I. Xylem and Soil Water Potentials

Abstract Forest thinning accomplished with cut-to-length and whole-tree harvesting systems, and prescribed underburning were assessed for their impacts on water relations in eastern Sierra Nevada Jeffrey pine (Pinus Jeffreyi Grev. & Balf.) during a period of extended drought. Predawn and midday measurements of xylem water potential in dominant and codominant crown class trees more than a century old were made on six days spread over three growing seasons, accompanied by measurements of soil water potential completed between the predawn and midday sessions of each day. With the exception of a single predawn session, the only one of a total of 12 in which xylem water potentials did not differ among treatments, the potentials in trees of thinned stand subunits were 0.67 MPa higher on average during predawn sessions and 0.71 MPa higher during midday sessions than those in trees of the unthinned treatment. Differences between the cut-to-length and whole-tree treatments were marginal and uncommon, but when they occurred, potentials were higher in the former. Prescribed fire effects on xylem water potential were also uncommon, but when occurring generally indicated lower stress levels in the burned than in the unburned treatment. Soil water potentials largely coincided with those of xylem water, with higher potentials in either the cut-to-length or whole-tree treatments, and usually both, than in the unthinned treatment on each of the six days of measurement. Underburning influences on soil water were rare, but when evident, potentials were higher in the burned than in the unburned treatment by substantial margins. For a majority of the measurement sessions, xylem water potential was found to be negatively correlated with residual basal area but positively correlated with soil water potential. In turn, coarse fragments and organic matter in the soil profile intermittently influenced soil water potential, with the former a negative factor while the latter was positive. Overall, results of this study suggest that substantial ecophysiological advantages can be derived from density management in older, dry site forests, which at minimum are not compromised by subsequent implementation of controlled underburning.

Fire Injury Severity in an Eastern Sierra Nevada Mixed Conifer Stand: Variability and Influencing Factors

Variability of postfire injury and stand and individual tree factors that affected survival responses of eastern Sierra Nevada conifers to wildfire were examined. Prefire measurements served as a basis of comparison for postfire conditions in a mixed conifer stand located in the eastern portion of the Lake Tahoe Basin and provided insight into predisposing influences on survival. Species composition consisted primarily of Jeffrey pine (Pinus jeffreyi Grev. & Balf.) and California white fir (Abies concolor var. lowiana [Gord.] Lemm.) along with a minor component of sugar pine (Pinus lambertiana Dougl.). Postfire survival was higher in Jeffrey pine than white fir but was highest overall in sugar pine. Catastrophic crown loss occurred less frequently in Jeffrey pine than in the fir but was least common in sugar pine. Survival generally increased with tree size, but this relationship did not extend to the largest trees in the stand. Among an array of regression models used to evaluate selected variables for their predictive capacity regarding postfire survival, prefire stand density was found to negatively influence that of Jeffrey and sugar pine, and survival of Jeffrey pine and white fir was negatively correlated with bole char. These results provide natural resource managers guidance in the selection of viable trees for retention during the salvage harvesting operations that often follow wildfire events.

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

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.

Influences of Mechanized Thinning and Prescribed Underburning on Radial Growth of Jeffrey Pine

Abstract Forest thinning utilizing cut-to-length and whole-tree harvesting systems with subsequent underburning were assessed for their impacts on several components of radial increment in uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) on the east slope of the Sierra Nevada during a period of extended drought. At the conclusion of three growing seasons following thinning and two seasons after burning, increment cores were extracted from dominant and codominant crown class trees over a century old as well as from intermediate crown class trees that were approximately 20 years younger. Separate measurements of early wood and late wood ring width during each of the 10 growing seasons preceding treatment and three seasons subsequent to thinning were summed for each year to obtain total ring width and permitted the calculation of proportions of total increment of the respective components. Comparisons of posttreatment values or averages of each component as well as of the magnitude of change, in both absolute and relative terms, in their values following treatment from 10-year pretreatment means were made among treatments for three posttreatment phases. Thinning treatment differences in direct measurements of the several components were marginal and uncommon. However, disparities in the magnitude of shifts in radial increment resulting from thinning were highly significant. Total ring width in dominant and codominant trees averaged 14% below to 13% above the pretreatment level in thinned portions of the stand over the three postthinning phases examined, while in the unthinned control total increment was 30-53% lower than the pretreatment mean, with the lowest values in all treatments occurring during the driest period of the study and highest observed after annual precipitation increased nearer to normal. In intermediate crown class trees, changes in ring width among the thinning treatments were similar to those in the dominant and codominant trees. Differences between the cut-to-length and whole-tree treatments were more pronounced in the dominant/codominant trees and comparisons generally revealed higher relative growth in the latter than in the former, while in the intermediate crown class, trees in the cut-to-length treatment performed marginally better in this regard than those in the whole-tree treatment. Influences of prescribed fire were limited to only a few components of radial growth in both sets of trees considered. The magnitude of change in several components of radial growth following thinning was found to be negatively correlated with residual basal area, but only in dominant and codominant trees. Live crown percentage influenced radial growth preceding thinning in both the dominant/ codominant and intermediate crown classes, in which it was a positive factor, as well as changes in increment subsequent to thinning in the former alone, when it was negative. Overall, results of this study demonstrate that considerable improvement in stand productivity can be realized with density management in an older, dry site forest type, which at minimum is not diminished by subsequent underburning.

Stem dimensional fluctuation in Jeffrey pine from variation in water storage as influenced by thinning and prescribed fire

Forest thinning utilizing cut-to-length and whole-tree harvesting systems with subsequent underburning were assessed for their impacts on water storage in the extensible tissues of dominant and codominant trees in an uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) stand on the east slope of the Sierra Nevada. Prior to the onset of the third growing season following thinning and the second season after burning, manual band dendrometers were installed at breast height on the selected trees and readings of diurnal fluctuation in stem circumference, an indication of bole water status, were taken monthly for one year. Diameter and relative diameter fluctuation were calculated from the circumference measurements. Overall, thinning had a positive influence on stem water recharge capacity, with the most pronounced effects evident in the latter part of the growing season. During this period, bole contraction in thinned stand portions was 49 to 55% greater than in the unthinned control, suggesting that both a greater volume of stored water was available for transpiration and was transpired in trees of the former treatment. There was no clear evidence that harvesting method affected stem water storage and influences of underburning were also absent entirely. Seasonal effects on diurnal changes in stem diameter were prominent, as the extent to which boles contracted generally increased over the course of the growing season, whereas fluctuations were at a minimum during the colder months. The magnitude of stem dimensional flux was found to be negatively correlated with initial tree DBH in one instance, while negative relationships between the former and live crown length as well as percentage were also revealed, albeit infrequently. Changes in bole size were positively correlated with residual basal area in some cases. These results suggest that improvement in water relations can be realized from density management in a dry site forest type with no apparent compromise of this benefit by broadcast underburning.RésuméLes éclaircies en forêt utilisant les systèmes de débit à la scie et de récolte de l’ensemble de l’arbre avec ultérieurement un brûlage par le bas ont été évaluées pour leurs impacts sur le stockage en eau dans les tissus extensibles des arbres dominants et codominants dans un peuplement inéquienne de Pinus jeffreyi Grev. & Balf sur le versant est de la Sierra Nevada. Avant le début de la troisième année de croissance suivant l’éclaircie et la deuxième saison après le feu, des dendromètres manuels à bande ont été installés à hauteur de poitrine sur des arbres sélectionnés et la lecture des fluctuations journalières de la circonférence des arbres, une indication sur le statut hydrique du tronc, étaient faite mensuellement pendant une année. Le diamètre et les fluctuations de diamètre ont été calculés à partir des mesures de circonférence. En général, l’éclaircie a une influence positive sur la capacité de recharge en eau du tronc, avec les effets évidents les plus prononcés dans la dernière partie de la saison de croissance. Pendant cette période, la contraction du tronc dans les parties éclaircies des peuplements était de 49 à 55 % plus grande que le témoin non éclairci, suggérant qu’à la fois un plus grand volume d’eau stocké était disponible pour la transpiration et était transpiré par les arbres du premier traitement. Il n’y a pas une preuve claire que la méthode de récolte affecte le stockage d’eau dans le tronc et l’influence du brûlage était aussi totalement absente. Les effets saisonniers sur les changements diurnes du diamètre du tronc étaient proéminents, alors que l’importance des troncs contractés s’est accrue généralement dans le courant de la saison de croissance tandis que les fluctuations ont été au minimum pendant les mois froids de l’année. Dans un cas, l’ampleur de la dimension du flux a été trouvée corrélée négativement avec le diamètre initial, à hauteur de poitrine, de l’arbre, tandis qu’une relation négative entre ce dernier et la longueur de la couronne vivante comme en pourcentage a été aussi révélée, bien que peu fréquemment. Dans quelques cas, les changements dans la taille du tronc ont été positivement corrélés avec la surface terrière résiduelle. Ces résultats suggèrent qu’une amélioration des relations hydriques peut être réalisée par une gestion de la densité des peuplements dans les forêts de zones sèche sans apparemment compromettre ce bénéfice par un brûlage par surface.

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

Thinnings using cut-to-length or whole-tree harvesting systems followed by underburning were evaluated for their effects on seedling and sapling demography in a pure, uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) stand containing a minor component of California white fir (Abies concolor var. lowiana [Gord.] Lemm.). Depression of seedling counts due to forest floor disturbance associated with thinning was followed by a recovery largely confined to Jeffrey pine in the whole-tree treatment where final seedling counts exceeded those found initially. The postburn substrate was more favorable for establishment of Jeffrey pine than white fir seedlings, and the largest increase 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 sapling mortality. Absent treatment, several stand and site variables influenced seedling and sapling abundance, prominent among them a propensity for mahala mat (Ceanothus prostratus Benth.) to elevate counts of white fir within both size classes. These results provide land managers insight into the impacts of six combinations of thinning and burning treatment on natural regeneration in eastern Sierra Nevada Jeffrey pine and similar dry site forest types.

Fuel Bed Alterations by Thinning, Chipping, and Prescription Fire in a Sierra Nevada Mixed Conifer Stand

Thinning using cut-to-length harvesting coupled with on-site slash chipping and redistribution and followed by prescribed underburning were assessed for their impacts on downed and dead fuels in an eastern Sierra Nevada mixed conifer stand. California white fir (Abies concolor var. lowiana [Gord.] Lemm.) dominated stand composition and was targeted in the thinning operation while Jeffrey pine (Pinus jeffreyi Grev. & Balf.), sugar pine (Pinus lambertiana Dougl.), incense-cedar (Libocedrus decurrens Torr.), and California red fir (Abies magnifica A. Murr.) were less prevalent. Thinning increased total fuel loading 6% and bed depth 14% but the chipping operation confined the additions to the 1+10-hr and 100-hr timelag categories. Subsequent underburning consumed the fuels produced by the mechanical treatments plus a substantial portion of the preexisting load, and the resulting reduction in total loading was almost twice that combusted in the absence of the thinning and chipping operations. Among an array of regression models used to evaluate assorted variables for their predictive capacity regarding downed and dead fuels, positive correlations between both 1+10-hr and total fuels prior to treatment and pretreatment basal area were prominent. Results reported here quantify fuel load modifications from management practices that are being increasingly utilized to enhance stand health and fire resilience in the Lake Tahoe Basin and surrounding forests.