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 89512
Phone: (775) 673-7425,
Fax: (775) 673-7485
Email: Paul.Verburg@dri.edu
Grants Contact :
Linda Piehl, Business Manager, Division of Earth & Ecosystem Sciences
Phone: (775) 673-7481
FAX: (775) 673-7485
Email: Linda.Piehl@dri.edu
Theme and sub-theme: Theme 3.A. Forest Management Activities: Implications for
Ecosystem and Public Health
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 (PM2.5, 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
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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 NO2), while ammonia (NH3) 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.
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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.
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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 NH3, EPA criteria pollutants (NO, NO2, CO, and O3), greenhouse gases (CO2
and H2O), 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
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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 CO2 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, NO3, and PO4 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
Q = 113 lpm Q = 113 lpm
Q = 30 lpm
Mixing Plenum
Q = 100 lpm
Real Time PM Module
Gas and DAQ
Module
MIDAC FTIR
Quartz Filter Holder
Quartz Filter Holder
Teflon Filter Holder
Q = 1.7lpm
TSI 40241
Mass Flow Meter
TSI DustTrak
TSI DustTrak
Q = 1.7 lpm
8 port
RS232 to Ethernet
Comtrol
Q = 2.5 lpm
Teflon Filter Holder
Filter Module
Nephelometer
Field
Computer
DAQ
Q = 10lpm
Photoacoustic
Spectrometer
Q = 2.5 lpm
8 port
Ethernet
Hub
TSI E LPI
8 port RS232 to Ethernet
Comtrol
Comtrol
Gast
Pump
Gast
Pump
Vacuum
Pump
TSI 40241
Mass Flow Meter
RS232 to Ethernet
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 NH4, NO3, 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
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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
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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:
July 2007:
Fall 2007:
Winter 2007:
Spring 2008:
Fall 2008:
June 2009:
Start recruiting graduate students and student workers. Finalize
agreements with US Forest Service to coordinate pre- and postburn sampling. Finalize detailed work plan.
Start of project. Purchase field and laboratory materials.
Conduct initial fuel load surveys. Collect fuel materials for
laboratory studies.
Conduct laboratory studies.
Collect existing data sources for spatial fuel load estimates to be
used in the GIS model.
Construct GIS fuel load inventory and conduct spatial modeling.
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.
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