J. D. Murphy

PRESCRIBED FIRE EFFECTS ON FOREST FLOOR AND SOIL NUTRIENTS IN A SIERRA NEVADA FOREST

The objectives of this study were to quantify the effects of prescribed fire on forest floor C and nutrient content, soil chemical properties, and soil leaching in a Jeffrey pine (Pinus jeffreyi [Grev. and Balf.]) forest in the eastern Sierra Nevada Mountains of California. The study included a prescribed fire and three timber harvest treatments: whole-tree (WT) thinning, cut-to-length (CTL) thinning, and no harvest (CONT). Prescribed fire resulted in significant decreases in forest floor C (−8 to −23 mg ha−1, or 39% to 61% decrease), N (−114 to −252 kg ha−1, or −31% to 51% decrease), S (0 to −15 kg ha−1, or 0% to 48% decrease), and K (−3 to −45 kg ha−1, or 12% to 51% decrease) contents but no significant change in Ca or Mg contents. In each case, the decreases were greatest in the CTL treatment, where slash accumulation before burning was greatest. Burning caused statistically significant effects on soil total nitrogen, C:N ratio, pH, water-extractable ortho-P, and water-extractable SO42− in some cases, but these effects were generally small, inconsistent among harvest treatments and horizons, and in the case of ortho-P much less than the temporal variation in both burned and unburned plots. There were no statistically significant effects of burning on total C, Bray-extractable P, bicarbonate-extractable P, and exchangeable Ca2+, K+, or Mg2+. Burning had no significant effect on soil solution pH, ortho-P, SO42−, NO3−, or NH4+ as measured by ceramic cup lysimeters and no effect on the cumulative leaching of ortho-P, NO3−, or NH4+ as measured by resin lysimeters. Burning had no effect on needle weight or nutrient contents as measured by the vector analysis. We conclude that prescribed fire had minimal effects on soils or water quality at this site, and that the most ecologically significant effect was the loss of N from the forest floor.

Fire Effects on Carbon and Nitrogen Budgets in Forests

Estimates of C and N loss by gasification during a wildfire in a Jeffrey pine (Pinus Jeffreyii [Grev. and Balf.]) forest in Little Valley, Nevada are compared to potential losses in more mesic forests in the Integrated Forest Study (IFS). In Little Valley, the fire consumed the forest floor, foliage, and an unknown amount of soil organic matter, but little standing large woody material. on an ecosystem level, the fire consumed approximately equal percentages of C and N (12 and 9%, respectively), but a considerably greater proportion of aboveground N (71%) than C (21%). Salvage logging was the major factor in loss, and C lost from the site will not be replenished until forest vegetation is established and succeeds the current shrub vegetation. N2 fixation by Ceanothus velutinus [Dougl.l in the post-fire shrub vegetation appears to have more than made up for N lost by gasification in the fire over the first 16 yr, and may result in long-term increases in C stocks once forest vegetation takes over the site. N loss from the fire equaled > 1,000 years of atmospheric N deposition and > 10,000 years of N leaching at current rates. Calculations of C and N losses from theoretical wildfires in the IFS sites show similar patterns to those in Little Valley. Calculated losses of N in most of the IFS sites would equal many centuries of leaching. Conceptual models of biogeochemical cycling in forests need to include episodic events such as fire.

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.

The combined effects of thinning and prescribed fire on carbon and nutrient budgets in a Jeffrey pine forest

Abstract• Both burning and harvesting cause carbon and nutrient removals from forest ecosystems, but few studies have addressed the combination of these effects. For a Pinus jeffreyii forest in the Sierra Nevada Mountains of California, we posed the question: what are the relative impacts of thinning and subsequent burning on carbon and nutrient removals?• The thinning methods included whole-tree thinning (WT, where all aboveground biomass was removed) cut to length (CTL, where branches and foliage were left on site in a slash mat on top of skid trails) and no harvest (CONT). Total C and nutrient exports with thinning and burning were greater in the WT and CTL than in the CONT treatments. Total C and N removals were approximately equal for the WT and CTL treatments, although harvesting dominated exports in the WT treatment and burning dominated exports in the CTL treatment. Total removals of P, K, Ca, Mg and S were greatest in the WT treatments, where harvesting dominated removals.• Comparisons of nutrient removals with ecosystem capital and calculations of potential replenishment by atmospheric deposition suggested that N is the nutrient likely to be most depleted by harvesting and burning treatments.Résumé• L’un et l’autre, le brûlage et les récoltes, causent un enlèvement important de carbone et de nutriments dans les écosystèmes forestiers, mais peu de travaux se sont attelés à l’étude de la combinaison de ces effets. Pour une forêt de Pinus jeffreyii nous posons la question suivante : quels sont les impacts relatifs d’une éclaircie et d’un brûlage ultérieur sur l’enlèvement du carbone et des nutriments, dans les Montagnes de la Sierra Nevada en Californie?• Les méthodes d’éclaircie comprennent : éclaircie totale des arbres (WT, toute la biomasse au-dessus du sol est enlevée), coupe à la dimension désirée (CTL, branches et feuillage ont été laissés sur site en andains) et pas de récolte (CONT). L’exportation totale de carbone et de nutriments avec éclaircie et brûlage dans les traitements WT et CTL a été plus importante que dans le traitement CONT. Les sommes totales de carbone et d’azote enlevées ont été approximativement les mêmes pour les traitements WT et CTL, bien que la récolte est été prédominante dans les exportations pour le traitement WT et que le brûlage soit prédominant dans les exportations pour le traitement CTL. Le total des enlèvements de P, K, Ca, Mg et S dans le traitement WT a été le plus grand dans les traitements WT où la récolte était plus importante que l’enlèvement.• La comparaison des nutriments enlevés par rapport au capital de l’écosystème et le calculs de réapprovisionnement par des apports atmosphériques suggère que l’azote (N) est le nutriment probablement le plus épuisé par les traitements de récolte et de brûlage.

Effects of mechanical harvest plus chipping and prescribed fire on Sierran runoff water quality.

Fire suppression in Sierran ecosystems creates a substantial wildfire hazard and may exacerbate nutrient inputs into Lake Tahoe by allowing the buildup of O horizon material, which serves as a source for high N and P concentrations in runoff water. The purpose of this study was to evaluate the effects of biomass reduction using cut-to-length mechanical harvest followed by chipping and controlled burning on surface runoff volume and water quality. Based on previous findings regarding N and P leaching flux and soil solution concentrations, we hypothesized that controlled burning and/or mechanical harvest with residue chipping does not increase inorganic N, P, and S concentrations in overland flow. Runoff, snowmelt, and rainfall were collected, volume measurements were taken, and samples were analyzed for NO(3)-N, NH(4)-N, PO(4)-P, and SO(4). Runoff volume, season, and year were identified as important parameters influencing overland flow nutrient concentrations and loads. Higher nutrient concentrations were commonly associated with summer rather than winter runoff, but the opposite was true for nutrient loads due to the higher runoff volumes. Treatment (unharvested, harvested, unburned, burned) effect was a strong predictor for discharge loads of NO(3)-N and SO(4) but was a weak predictor for PO(4)-P. Discharge loads of NO(3)-N and SO(4) were greater for the unburned harvested and the burned unharvested treatments than for the unburned, unharvested control sites or the burned and harvested combined treatment. Although mechanical harvest and/or controlled burning had a small initial impact on increased nutrient loading, the effects were minimal compared with background levels. Hence, these management practices may have the potential to improve forest health without the danger of large-magnitude nutrient mobilization and degradation of runoff water quality found with wildfire.

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.