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The effects of fuels treatments on soil carbon respiration in a Sierra Nevada pine plantation

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The effects of fuels treatments on soil carbon respiration in a Sierra Nevada pine plantationIntroductionMethodsSite descriptionMastication and burning treatmentsSoil respiration, temperature, and moisture measurementsData analysisResultsAnalysis of treatment effectsStages 0-1: mastication effects on SRR, Ts, and MsStages 1-2: burning effects on SRR, Ts, and MsStages 0-2: mastication+burning effects on SRR, Ts, and MsRelationship between soil temperature, water, and respirationSpatial variability between treatment units and treatmentsDiscussionTreatment effectsRoles of soil temperature and moisture in dictating SRR trendsSpatial variabilityConclusionAcknowledgementsReferencesThe effects of fuels treatments on soil carbon respirationin a Sierra Nevada pine plantationLeda N. Kobziara,*, Scott L. StephensbaSchool of Forest Resources and Conservation/School of Natural Resources and Environment,Institute of Food and Agricultural Sciences, University of Florida, P.O. Box 110410, Gainesville, FL 32611-0410, USAbDepartment of Environmental Science, Policy, and Managements, University of California, Berkeley,137 Mulford Hall #3114, Berkeley, CA 94702-3114, USAReceived 23 June 2006; received in revised form 4 September 2006; accepted 22 September 2006AbstractFire-prone forests in the American west are presently slated for extensive fuels reduction treatments, yet the effect on soil CO2efflux rates, or soil respiration, has received little attention. This study utilizes the homogeneity of a Sierra Nevada ponderosa (Pinusponderosa Dougl. ex P. & C. Laws)–Jeffrey pine (Pinus jeffreyi, Grev. & Balf.) plantation to investigate changes in soil respirationfollowing mechanical shredding of understory vegetation, or mastication, in 2004; mastication coupled with prescribed burning in2005; and burning alone also in 2005 as measured over the growing seasons from 2003 to 2005. Soil respiration, soil temperatureand soil moisture were measured in two masticated stands which were burned the following year, and in one burned stand; the threeof which were compared with two controls stands. Soil respiration response to treatments was detectable even though spatialvariability within sites was high (coefficients of variation of 39–66%). Mastication produced short-term reductions in respirationrates, reduced soil moisture by 20%, and mitigated a year-to-year reduction in soil temperature evidenced by controls. Prescribedfire in masticated stands lowered soil respiration from 3.42 to 2.68 mmol m2s1while fire in the untreated stand raised rates from3.41 to 3.83 mmol m2s1, although seasonal increases in control sites were greater than those in the untreated stand. Masticatedthen burned site soil moisture increased by 52% while soil temperature decreased over the span of the growing season. Microclimatevariables were not consistently effective in explaining spatial trends. Exponential models using soil temperature and/or moisture topredict temporal trends in respiration were only significant in treated stands, suggesting that treatment implementation increasedsensitivity to environmental factors. These results imply that fuels reduction practices in water-stressed forests may have importantconsequences for ecosystem carbon dynamics.# 2006 Elsevier B.V. All rights reserved.Keywords: Fire; Soil temperature; Soil moisture; Gas exchange; CO21. IntroductionForest soils contain more than 70% of the terrestrialworld’s soil carbon pool (Post et al., 1982), and therebyplay a major role in global carbon cycles and theirinfluence on cli mate. Yet little is known about howmanagement practices, especially prescribed fire,affect forest soil carbon emissions. Soil surface CO2efflux rates, which include respiration from bothautotrophic (root) and heterotrophic (soil macro- andmicroorganisms) sources, have been shown to accountfor up to 67% of total mean ecosystem respiration in ayoung ponderosa pine plantation in California (Xuet al., 2001). As the importance of sequestering carbonwww.elsevier.com/locate/agrformetAgricultural and Forest Meteorology 141 (2006) 161–178* Corresponding author. Tel.: +1 352 846 0901.E-mail address: [email protected] (L.N. Kobziar).0168-1923/$ – see front matter # 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.agrformet.2006.09.008to potentially offset global climate warming i ncr ease s,forest management goals may soon undergo asignificant shift. Traditional, production-based para-digms may be replace d by a new ethic of ecosyst emcarbon budgeting. A paral lel shift emphasizingrestoration of fire processes in fire-prone ecosystemsis already underway (Stephens and Moghaddas, 2005a;Stephens and Ruth, 2005). As wildfire suppression andmitigation costs throughout the western US continue torise, land ma nagers are chall enged t o ac t expediently toreduce hazardous fuels. Over 2 million hectares offorested lands are slated for extensive fuels reductiontreatments in the Sierra Nevada alone over the nextdecade, including the implementation of prescribedfire, thinning, and shredding of understory vegetation,or mastication (USDA, 2002). There is a compellingneed for understanding the relationship between fuelsreduction prescript ions and the increasingly importantissue of carbon flux in forested ecosystems.Assuring the health and fire resilience in denseplantation forests is a relatively new challenge faced bynatural resource managers. Although tree density inmost northern California plantations is currently at an‘‘acceptable’’ level, mortality risk from inter-treecompetition, disease, insect infestation and fire willincrease significantly over the next 10–20 years(Landram, 1996). High fire hazards are already presentin and around many of these plantations (Stephens andMoghaddas, 2005b), linked to high success rates inreplanting and dense post-fire understory growth, lowsummer fuel moisture, steep, mountainous terrain,frequent ignitions from lightning, and increased publicrecreation in National Forests. Such plantations, whichcover 155,000 ha in the Sierra Nevada, have also beenheralded for their capacity to sequester carbon. Yet thehigh fire hazards associated with their structuralfeatures may ultimately lead to a greater emiss ion ofCO2.How fuels reduction treatments affect soil respira-tion is closely linked to the role soil physical andchemical factors play in s oil respiration. Fire is knownto impact the organic matter content of the soil, a longwith nitrogen


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