POTENTIAL NUTRIENT EMISSIONS FROM PRESCRIBED FIRE IN THE LAKE TAHOE BASIN

Transcription

1 POTENTIAL NUTRIENT EMISSIONS FROM PRESCRIBED FIRE IN THE LAKE TAHOE BASIN Project Team and Contact Information Principal Investigators: Paul S.J. Verburg, Ph.D, Division of Earth & Ecosystem Sciences Richard B.Susfalk, Ph.D, Division of Hydrologic Sciences Lung-Wen Antony Chen, Ph.D, Division of Atmospheric Sciences Nevada System of Higher Education, Desert Research Institute 2215 Raggio Parkway Reno, NV Phone: (775) , Fax: (775) Grants Contact : Linda Piehl, Business Manager, Division of Earth & Ecosystem Sciences Phone: (775) FAX: (775) Linda.Piehl@dri.edu Theme and sub-theme: Theme 3.A. Forest Management Activities: Implications for Ecosystem and Public Health

2 II Justification Statement A combination of insect and disease mortality, overstocked vegetation, and fire suppression has resulted in a large build-up of fuels within the Lake Tahoe Basin increasing the risk of catastrophic wildfires. To reduce this risk, management options should be aimed at reducing fuel loads while minimizing impacts on air and water quality in the Lake Tahoe Basin. One potential management option is prescribed fire. Although prescribed fire is frequently used it has potential adverse effects including smoke production and increased nutrient and sediment runoff which can affect both public health and water quality. The proposed study will assess the potential impacts of fuel reductions through prescribed fire on water and air quality in the Lake Tahoe basin as a function of fuel condition. This information can be used by forest managers to optimize burn practices by balancing environmental impacts with the management objectives. In addition, it will allow forest managers to assess potential long-term effects of prescribed fire on ecosystem nutrient stocks which may impact forest health. III Background/Problem Statement Lake Tahoe, a pristine sub-alpine lake located in the eastern Sierra Nevada on the border of Nevada and California, is known for its extraordinary clarity and deep blue color. Its scenic quality and the availability of year-round outdoor activities have caused Lake Tahoe to have great recreational appeal. Because of this appeal and its ecological assets, Lake Tahoe has been designated an Outstanding National Water Resource in which no long-term degradation is permitted. Despite this protection, Lake Tahoe clarity has decreased during the last four decades as a result of algal growth stimulated by nutrient input from atmospheric deposition and urban runoff (Byron and Goldman, 1986; Jassby et al. 1994). Increased suspended sediment loading is also a concern for its direct impact on lake clarity and for the particulate nutrients carried into the lake. Increases in nutrients and sediments negatively affect the many beneficial uses of Lake Tahoe, from aesthetic enjoyment by residents and tourists to the health of aquatic life. Forest management in the Lake Tahoe Basin, as in many areas of the United States, has focused on suppressing wildfires. This has caused biomass to increase dramatically which has increased the risk of a catastrophic wildfire. Therefore, it has become increasingly clear that fuel reduction is necessary to limit the risk of large wildfires. One of the more frequently used management options is prescribed fire. Prescribed burns are used to eliminate waste products, control insect populations and plant diseases, improve wildlife habitat and forage production, increase water yield, maintain natural succession of plant communities, reduce the need for pesticides and herbicides, and reduce wildfire danger. However, the beneficial results of prescribed fire carry potential undesirable side effects. Burning of wildland fuel represents one major source of primarily-emitted fine particulate matter (PM 2.5, i.e., particles with aerodynamic diameter less than 2.5 µm). Particles released from prescribed burning scatter and absorb solar radiation, thus modifying the Earth s energy budget and atmospheric chemistry. The elevated particle concentration affects air quality for instance by increasing regional haze and may cause adverse human health effects. In addition, particles released through biomass burning often carry substantial amounts of nutrients including potassium, phosphate, and nitrates. The potential for long-range transport can cause these nutrients to be lost from the source 1

3 region, thus altering biogeochemical cycles. In addition, specifically for the Lake Tahoe basin, the nutrients released from forests could be deposited directly into the lake through atmospheric deposition and/or runoff, thereby contributing to eutrophication of the lake. Biomass burning emission is known to highly depend on fuel type, moisture condition, and combustion phase. High-temperature flames tend to promote oxidation, producing nitrogen oxides (NO and NO 2 ), while ammonia (NH 3 ) is the major nitrogen product during low-temperature smoldering combustion, along with significant semi-volatile organic compounds. Typically, flaming combustion immediately follows ignition, lasts for a short period of time, and consumes a majority of the fuel. The combustion then shifts to smoldering which, despite a lower burning rate, could last for a long time. Fresh fuels with high moisture content tend to show extended smoldering combustion because energy consumption for evaporating water prevents the increase of temperature to the flaming threshold. Prescribed fire can also affect nutrient release from the soil. Caldwell et al. (2002) and Johnson et al. (1998) showed that volatile C and N losses during fires greatly exceed leaching losses in Sierra Nevadan ecosystems in terms of total amounts of nutrients. However, nutrients released from soils into streams and surface water can be directly used by aquatic biota and can therefore immediately contribute to eutrophication of surface waters. Several studies have been conducted addressing soil responses to fire but the results to date have been inconclusive. For instance, Murphy et al. (2006a) found no effects of prescribed fire on soil solution nutrient concentrations or nutrient fluxes as measured by resin lysimeters in the Eastern Sierra Nevada. Conversely, these same authors found large increases in soil solution nutrient concentrations and nutrient fluxes after the Gondola fire, a wildfire within the Tahoe Basin (Murphy et al. 2006b). There may be several reasons for this discrepancy. First, soil types were different between the two studies; the prescribed fire site was located on soils derived from andesite while the Gondola fire burned an area located on granitic soils. Andesitic soils typically have a higher adsorption capacity than soils derived from granite (Susfalk, 2001) which may have contributed to the lower leaching losses. Second, the wildfire may have been higher-temperature flames that mobilized more nutrients. Additional factors include differences in fuel type, completeness of the burn, season, prevailing weather, and duration of the burn all of which can affect nutrient release during fire (e.g. Blank et al., 1996). Whether or not changes in soil nutrients result in changes in water quality entering Lake Tahoe will depend on water flow pathways. Nutrients released after fire can infiltrate into the soil and end up in the groundwater. However, Miller et al. (2005) found very high nutrient concentrations in surface water runoff which may directly affect stream water quality. The importance of overland flow is likely to be affected by water infiltration characteristics of the soil. Fire is known to increase hydrophobicity of the soil (e.g. Huffman et al., 2001) thereby affecting water flow pathways. Despite the multitude of studies conducted within and outside the Lake Tahoe Basin regarding the effects of fire on nutrient emissions, few studies have combined both air and soil emissions. In addition, to our knowledge no rigorous assessment has been made of the impacts of fuel type and burn conditions on these emissions that can be used by forest managers to assist in the determination of optimal burn conditions that minimize the impact on nutrient emissions while achieving desired fuel reduction objectives. 2

4 Finally, no estimate has been made of the potential basin-wide impacts of prescribed fire on nutrient emissions. IV Goals, Objectives and Statement of Hypotheses The main goal of the proposed research is to assess the potential impacts of prescribed fire on air and water quality as a function of antecedent conditions (e.g. fuel type and fuel moisture) by combining laboratory experiments, field fuel inventories and GIS-based spatial analysis. Given the inherent difficulty in fully capturing changes to air, soil, and water quality prior to and after prescribed burns, and the logistical challenges in obtaining pre-burn samples, we are proposing the use of laboratory-based measurements as input for the spatial analysis. Laboratory studies will assess potential nutrient releases from the soil and vegetation by coupling air quality and soil-based measurements. These results will then be scaled up to the watershed level utilizing field measurements of preand post-burn fuel conditions used to assess fuel loads, spatial patterns in fire regime and fuel consumption during prescribed fires. Combined, these data will be used to build the framework for a GIS-based spatial model to provide basin-level estimates of nutrient emissions resulting from prescribed fire within the Lake Tahoe basin. The proposed research is the first step in the creation of a model that will require further field-based data collection for better refinement and validation. Our ultimate goal is to provide land managers with a tool to evaluate the potential nutrient emissions as a function of burn conditions and fuel loads. We have formulated the following hypotheses: 1) The ratio of volatile nutrient losses vs. soil nutrient losses will increase with increasing fuel moisture. 2) Fuel reductions will decrease with increasing fuel moisture 3) Total ecosystem nutrient losses will decrease with increasing fuel moisture. 4) Dry fuels will maximize fuel load reduction but will also maximize nutrient emissions. To test our hypotheses we plan to conduct the following tasks: 1) Determine the emission factors of selected nutrients for the major fuel types as a function of moisture and partition nutrient losses into volatilization and leaching. 2) Construct an empirical model that allows for optimizing fuel reduction with associated nutrient emissions. 3) Determine spatial variability in burn conditions, fuels, and fuel consumption in up to five prescribed fires. 4) Assess the type, quantity, condition, and distribution of fuels in the Lake Tahoe basin utilizing both existing data sources and new field measurements. 5) Construct the framework for a spatial basin-wide model based on the aforementioned fuel reduction empirical model. 3

5 V Approach To accomplish our goals we will use a combination of laboratory experiments and field surveys. With the laboratory experiments we will directly measure the potential nutrient emissions as a function of fuel type and burn condition. These results will be combined with field surveys to provide watershed and basin scale estimates of potential nutrient emissions. We will collaborate with the US Forest Service s controlled burn program for field site selection in order to provide sample collection prior to and after controlled burns. In this proposal we will not conduct intensive field measurements related to nutrient transport such as stream water quality, soil and atmospheric chemistry and erosion in response to prescribed fire since this would exceed our capabilities given the budget limitations. We will however assess the hydrophobicity of the soil to obtain an initial assessment of the potential changes in water infiltration rates into the soil in response to burn conditions. We anticipate that the spatial model framework to be developed in this proposal will guide future field studies that can be used to test and refine the model. V.I Laboratory Studies (Tasks 1 and 2) V.I.I Combustion Experiments For our laboratory experiments we will collect representative vegetation and soil samples (forest floor and top of the mineral soil) for combustion studies. We will burn approximately 100 g of pre-weighed forest floor, mineral topsoil and vegetation in a combustion chamber using three different moisture levels and an estimated 3 replicates per sample/moisture combination up to a total of 50 samples. The moisture levels will be chosen to cover the variability found in the field. Subsequently, soil and vegetation samples will be leached to determine the leaching potential for various nutrients. We will not address effects of combustion temperature on nutrient losses since the burns will be conducted in a combustion chamber that will not allow for controlling burn temperature. Previously, effects of temperature have been studied using a muffle furnace (e.g. Blank et al., 1996, Saito et al., 2007). However, this may create potential artifacts due to limitations in oxygen supply. V.I.II Air Quality Measurements An in-plume system (Kuhns et al., 2004; Chen et al., 2006a) developed at DRI capable of performing real-time measurements for particulate and gaseous species will be used to sample the smoke coming out of the combustion chamber during combustion of the soil and vegetation samples (Fig. 1). The in-plume system ensures that 1) only smoke fractions prone to long-range transport are sampled and 2) multiple instruments sample the same air reflecting the chemical composition of the plume despite the spatial heterogeneity. This system also allows for differentiation between flaming and smoldering combustion emissions through the high-time resolution measurement (Chen et al., 2006b). The in-plume system contains a Fourier Transform Infrared Spectrometer that measures NH 3, EPA criteria pollutants (NO, NO 2, CO, and O 3 ), greenhouse gases (CO 2 and H 2 O), and toxics (e.g., acetaldehyde and formaldehyde) concentration in the smoke at ~10 s resolution. It uses an Electric Low Pressure Impactor and DustTraks to obtain 4

6 particle mass and size distribution at 5 to 10 s resolution allowing for determination of the amount of pollutant emitted per unit C burned (note CO 2 and CO comprise >95% of the carbon emission). The amount of pollutant emitted per fuel mass can be calculated by knowing the C content of the fuel (Moosmüller et al., 2003). By multiplying the emission factors to the fuel mass, total gas and particle emissions can be determined. An air flow of ~5 L/min will be passed through the combustion chamber providing oxygen for combustion and carrying out the smoke. Four channels in the in-plume system will be used to collect particles on filters (Fig. 1). The filter samples will then be submitted to DRI laboratories for analysis of particle mass, nutrient elements, and C fraction. Water soluble K, NO 3, and PO 4 will be analyzed using ion chromatography. Organic N will be determined by a thermal evolution method described in Li and Yu (2004). Providing that the particle mass emission is known, emissions of nutrient elements can be estimated. INLET FROM SOURCE Q = 30 lpm Bendix Cyclone Mixing Plenum Q = 113 lpm Q = 113 lpm Q = 30 lpm Real Time PM Module TSI DustTrak TSI DustTrak Nephelometer Photoacoustic Spectrometer Q = 1.7 lpm 8 port RS232 to Ethernet Comtrol Q = 1.7 lpm Q = 2.5 lpm Q = 2.5 lpm TSI ELPI Q = 10 lpm 8 port RS232 to Ethernet Comtrol Ethernet Hub Q = 100 lpm Gas and DAQ Module MIDAC FTIR TSI Mass Flow Meter Field Computer DAQ Quartz Filter Holder Filter Module 8 port RS232 to Ethernet Comtrol Quartz Filter Holder Teflon Filter Holder Teflon Filter Holder TSI Mass Flow Meter Vacuum Pump Gast Pump Pump Gast Gast Pump Gast Pump Gast Pump Gast Pump Gast Pump Figure 1. Schematic of the in-plume system to measure volatile nutrient emissions from combusted samples. V.I.III Soil Nutrient Measurements After the simulated burnings the crucibles will be cooled and weighed to determine weight loss. Following weighing, 50 g of each soil and vegetation sample will be transferred to centrifuge tubes and 150 ml of deionized water will be added. The tubes will be shaken for 30 minutes and filtered through 0.45 µm nylon filters. The water samples will be analyzed for NH 4, NO 3, ortho-p, total P and total N in the Water Analysis Laboratory at DRI. The remaining 50 g of the soils samples will be tested for 5

7 hydrophobicity and analyzed for total C, N and P. We will use the Water Drop Penetration Time (WDPT; Letey, 1969) test to measure for hydrophobicity of the soils. For each fuel type, background levels of extractable nutrients will be measured prior to burning. Total C, N and P will be measured in the soils laboratory at DRI. V.I.IV Empirical Model Development The data collected with the laboratory experiment will be used to construct empirical relationships between fuel type, fuel moisture and nutrient emission. We will develop simple regression equations for each nutrient under consideration both for soil and vegetation. V.II Field Studies (Tasks 3, 4 and 5) V.II.I Field Inventory of Fuels, Spatial Variability in Burn Conditions, and Fuel Consumption. Spatial variability in burn conditions will be assessed in up to five prescribed fires. We will specifically assess fuel moisture conditions prior to the burn, fuel load, burn temperatures, and remaining fuels after the burn. Intensive sampling will be carried out in areas that are scheduled to be burned. Within each area, a 100 x 100 m grid will be established with 10 m grid points. Twenty five samples of forest floor and mineral topsoil will be taken by randomly selecting coordinates within the grid. The amount of forest floor will be estimated by measuring forest floor thickness at each grid point. Soil hydrophobicity using the WDPT test will be measured at 25 randomly selected points. Aboveground biomass will be estimated by measuring diameter-at-breast height for large trees and applying appropriate allometric equations for the various tree species. Many allometric equations have already been developed for the dominant species during previous studies in the Incline Creek watershed (Susfalk, pers. comm.). Understory biomass will be estimated by measuring mass density of the vegetation and use a line intersect method to assess coverage. Moisture contents of soil and vegetation will be measured by drying samples at 105ºC until they reach constant weight. Burn temperature will be assessed by placing metal strips covered with heat sensitive paint at at least 25 randomly selected points on the grid. Fuel losses and soil hydrophobicity after the burn will be measured using the same approach as the before-burn assessment. The degree of scorching of large trees will be assessed by measuring the amount of scorched bark on the individual trees. The data generated from the before and after surveys will allow us to establish a burnable fuel index defined as the fraction of total fuels consumed by the fire at the moisture levels found in he field. V.II.II Basin Wide Fuel Load Assessment and Spatially Explicit Emissions Model We will make a first assessment of the basin-wide fuel loads combining our intensive surveys with existing data sources. Whenever possible we will use available land cover data to estimate total standing biomass. The intensive surveys will be used to verify the existing land cover data. However, we anticipate the need for additional field-based data collection of antecedent pre-burn conditions from additional sites throughout the basin to address data gaps in existing data. Currently, UC Davis is conducting biomass inventories in selected watersheds and we have contacted them about potential 6

8 collaboration. Using the burnable fuel index determined from our intensive surveys, we can assess the potential amount of biomass that may be consumed at the basin level. The emission factors from the laboratory studies will subsequently allow us to conduct a basin-wide assessment of the potential nutrient emissions depending on moisture conditions and amount of area burned. The spatial assessment and model framework will be constructed using ESRI s ArcGIS suite. VI Deliverables/Products The main products of the proposed study include an assessment of the potential nutrient emissions from prescribed fire both into the soil and the air as a function of fuel type and moisture. The second product will be a first basin-wide inventory of potential emissions based on the laboratory studies, field surveys, and GIS modeling. The GIS based modeling will be the first phase in developing a tool that ultimately can be used by basin managers to estimate potential nutrient emissions. This will allow managers to refine the timing and burning locations to minimize nutrient inputs into the lake from atmospheric and aquatic sources while maximizing fuel reductions. Future areas of studies should include transport processes including particle transport in smoke plumes and soil nutrient transport through erosion, surface runoff and leaching. Currently, an ongoing study by Miller, Johnson and Weisberg at the University of Nevada, Reno funded by the US Forest Service is addressing nutrient transport from soil to streams and groundwater. PI s Verburg and Susfalk are already collaborating with some of these investigators on other projects and we will discuss the potential for integration of our combined efforts. VII Schedule of Events/Reporting and Deliverables Spring 2007: Start recruiting graduate students and student workers. Finalize agreements with US Forest Service to coordinate pre- and postburn sampling. Finalize detailed work plan. July 2007: Start of project. Purchase field and laboratory materials. Fall 2007: Conduct initial fuel load surveys. Collect fuel materials for laboratory studies. Winter 2007: Conduct laboratory studies. Spring 2008: Collect existing data sources for spatial fuel load estimates to be used in the GIS model. Fall 2008: Construct GIS fuel load inventory and conduct spatial modeling. June 2009: Finish GIS model and final report. The pre- and post-fire surveys will be conducted between the fall of 2007 and winter of 2008/2009. Exact dates will depend on management decisions made by the US Forest Service. We have discussed our plans with the US Forest Service and they have indicated that access prior to and after a burn would not be a problem and we would be informed of the burn schedule. 7

9 References Blank, R.R., Allen, F.L., and Young, J.A. (1996) Influence of simulated burning of soillitter from low sagebrush, squirreltail, cheatgrass, and medusahead on watersoluble anions and cations. International Journal of Wildland Fire. 6: Byron, E.R., and Goldman, C.R. (1986) A technical summary of changing water quality in Lake Tahoe: The first five years of the Lake Tahoe Interagency Monitoring Program. Tahoe Research Group Institute of Ecology. University of California, Davis. 62pp. Caldwell, T.G., Johnson, D.W., and Miller, W.W. (2002) Forest floor carbon and nitrogen losses due to prescription fire. Soil Science Society of America Journal. 66: Chen, L.W.A., Moosmüller, H., Arnott, W.P., Watson, J.G., and Mousset-Jones, P. (2006a) Novel approaches to measure diesel emissions. Journal of the Mine Ventilation Society of South Africa. (April/June): Chen, L.-W.A., Moosmüller, H., Arnott, W.P., Chow, J.C., Watson, J.G., Susott, R.A., Babbitt, R.E., Wold, C., Lincoln, E., and Hao, W.M. (2006b) Particle emissions from laboratory combustion of wildland fuels: In situ optical and mass measurements. Geophysical Research Letters. 33: 1-4. Huffman, E.L., MacDonald, L.H., and Stednick, J.D. (2001) Strength and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range. Hydrological Processes. 15: Jassby, A.D., Reuter, J.E., Axler, R.P., Goldman, C.R., and Hackley, S.H. (1994) Atmospheric deposition of nitrogen and phosphorus in the annual nutrient load of Lake Tahoe (California-Nevada). Water Resources Research. 30: Johnson, D.W., Susfalk, R.B., Dahlgren, R.A., and Klopatek, J.M. (1998) Fire is more important than water for nitrogen fluxes in semi-arid forests. Environmental Science and Policy. 1: Kuhns, H.D., Chang, M.C., Chow, J.C., Etyemezian, V., Chen, L.-W.A., Nussbaum, N.J., Nathagoundenpalayam, S.K., Trimble, T.C., Kohl, S.D., MacLaren, M., Abu- Allaban, M., Gillies, J.A., and Gertler, A.W. (2004) DRI Lake Tahoe Source Characterization Study. Prepared for California Air Resources Board, Sacramento, CA, by Desert Research Institute, Reno, NV. Letey, J. (1969) Measurements of contact angle, water drop penetration time and critical surface tensions. In Proceedings of Symposium of Water Repellent Soils, 6 10 May University of California, Riverside. p Li, Q., and Yu, J.Z. (2004) Determination of total aerosol nitrogen by thermal evolution. Aerosol Science and Technology. 38: 1-9. Miller, W.W., Johnson, D.W., Denton, C., Verburg, P.S.J., Dana, G.L., and Walker, R.F. (2005) Inconspicuous nutrient laden surface runoff from mature forest Sierran watersheds. Water, Air and Soil Pollution. 163:

10 Moosmüller, H., Mazzoleni, C., Barber, P.W., Kuhns, H.D., Keislar, R.E., and Watson, J.G. (2003) On-road measurement of automotive particle emissions by ultraviolet lidar and transmissometer: Instrument. Environmental Science and Technology. 37: Murphy, J.D., Johnson, D.W., Miller, W.W., Walker, R.F., Blank, R.R. (2006a) Prescribed fire effect on forest floor and soil nutrients in a Sierra Nevada forest. Soil Science. 171: Murphy, J.D., Johnson, D.W., Miller, W.W., Walker, R.F., Carroll, E.F., and R.R. Blank. (2006b) Wildfire effects on soil nutrients and leaching in a Tahoe Basin watershed. Journal of Environmental Quality. 35: Saito, L., Miller, W.W., Johnson, D.W., Qualls, R.G., Provencher, L., Carroll, E, and Szameitat, P. (2007) Fire effects on stable isotopes in a Sierran forested watershed. Journal of Environmental Quality. 36: Susfalk, R.B. (2001) Relationships of soil-extractable and plant-available phosphorus in forest soils of the eastern Sierra Nevada. Graduate Program of Hydrologic Sciences, University of Nevada, Reno. 228pp. (Doctoral Dissertation). 9