Philip J. Riggan

Deposition and processing of airborne nitrogen pollutants in Mediterranean-type ecosystems of southern California.

Atmospheric nitrogen deposition, associated with chronic urban air pollution, has produced stream water nitrate concentrations as high as 7.0 mg of N L-l in chaparral watersheds in the San Gabriel Mountains of Los Angeles County, CA. Stream water [NO3-] and discharge were greatest at high flow and may contribute significantly to existing groundwater NO3- pollution. Annual NO3- discharge ranged from 0.04 to 10.0 kg of N ha-1 over 4 years. Canopy throughfall and precipitation inputs of 23.3 and 8.2 kg of N ha-1 year-1 were high relative to other undisturbed ecosystems nationwide. Dry deposition was apparently a major source of the throughfall nitrogen. NO3- concentrations from nearby, relatively unpolluted watersheds were lower by 1–3 orders of magnitude. NO3- yield was elevated on watersheds where chaparral was converted to grassland in 1960 and may be greatly accelerated after wildfire because of high postfire NH4+ concentrations and rapid nitrification in terrestrial and aquatic ecosystems.

The magnitude and persistence of soil NO, N2O, CH4, and CO2 fluxes from burned tropical savanna in Brazil

Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2O, CH4, and CO2 from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N-trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2O and a sink for atmospheric CH4. Burning also elevated CO2 fluxes, which remained detectably elevated 1 year later.

Interaction of Fire and Community Development in Chaparral of Southern California

Fire is an ecosystem property rather than an exogenous force in southern California chaparral, and it interacts with processes of drought-mediated canopy devel- opment, production, and mortality to affect stability of community composition. Where species that must reproduce from seed, such as Ceanothus crassifolius or Ceanothus olig- anthus, are predominant, composition can be altered by a single fire with little or no recruitment after initial postfire establishment. Water balance apparently regulates subsequent leaf area development; after 15-22 yr of postfire growth, foliage biomass in monospecific C. crassifolius stands in this study had approached a maximum that was unrelated to incident solar radiation and insensitive to initial population density over a 10-fold range. Thus, establishment success, above that required for canopy closure, should have little effect on the foliage biomass that sustains combustion. After canopy closure, total biomass accumulated at an accelerating rate through at least two decades with aboveground net primary production as great as 12-13 Mg ha- I yr- 1. C. crassifolius mortality was substantially less than predicted from growth rates and the -3/2 power model of Yoda et al. (1963), and there was no approach to a common asymptotic density by stands of disparate initial density. With low deadwood biomass and absence of ground fuels, C. crassifolius cannot sustain burning in the absence of wind, steep slopes, or exceptionally low live-fuel moisture. Increased Ceanothus abundance in multispecies communities with Adenostoma fascicu- latum or Salvia mellifera alters biomass structure and could modify subsequent fire effects even if foliage area fully redevelops in concert with site water balance. Rare, low-intensity fires can devastate Ceanothus chaparral, that reproduces only from seed. Salvia mellifera and Eriogonumfasciculatum can occupy resulting openings in the canopy, and their abun- dant deadwood and compact biomass can readily spread low-intensity fires, thereby per- petuating the degraded community. Productive stands within a chaparral association are probably subject to especially severe fires that limit nutrient accumulation and may also limit subsequent productivity. Copious nitrogen volatilization during burning is promoted by high nitrogen concentrations in foliage and fine woody biomass of Ceanothus and heavy leaf litter of Quercus dumosa and C. crassifolius. The communities most prone to severe fires also accumulate and cycle nitrogen and phosphorus rapidly.

Effects of fire severity on nitrate mobilization in watersheds subject to chronic atmospheric deposition.

Severe fires in chaparral watersheds subjectto air pollution from metropolitan Los Angeles mobilized accumulated nitrogen and caused streamwater to be polluted with nitrate at concentrations exceeding the Federal Water Quality Standard. Streamwater NO 3 - concentrations were elevated during peak flows, the largest of which was a debris flow that transported NO 3 - at concentrations as high as 1.12 mequiv/L. Annual NO 3 - loss from severely burned watersheds, averaging 1.2 kequiv/ha, was 40 times greater than that from areas that remained unburned. Fires of moderate intensity produced a more subdued response in stream discharge and soil nitrification and less than one-seventh the NO 3 - loss observed after severe burning

WRF-Fire: coupled weather-wildland fire modeling with the weather research and forecasting model

AbstractA wildland fire-behavior module, named WRF-Fire, was integrated into the Weather Research and Forecasting (WRF) public domain numerical weather prediction model. The fire module is a surface fire-behavior model that is two-way coupled with the atmospheric model. Near-surface winds from the atmospheric model are interpolated to a finer fire grid and are used, with fuel properties and local terrain gradients, to determine the fire’s spread rate and direction. Fuel consumption releases sensible and latent heat fluxes into the atmospheric model’s lowest layers, driving boundary layer circulations. The atmospheric model, configured in turbulence-resolving large-eddy-simulation mode, was used to explore the sensitivity of simulated fire characteristics such as perimeter shape, fire intensity, and spread rate to external factors known to influence fires, such as fuel characteristics and wind speed, and to explain how these external parameters affect the overall fire properties. Through the use of theoret...

REMOTE MEASUREMENT OF ENERGY AND CARBON FLUX FROM WILDFIRES IN BRAZIL

Temperature, intensity, spread, and dimensions of fires burning in tropical savanna and slashed tropical forest in central Brazil were measured for the first time by remote sensing with an infrared imaging spectrometer that was designed to accommodate the high radiances of wildland fires. Furthermore, the first in situ airborne measurements of sensible heat and carbon fluxes in fire plumes were combined with remote measurements of flame properties to provide consistent remote-sensing-based estimators of these fluxes. These estimators provide a means to determine rates of fuel consumption and carbon emission to the atmosphere by wildland fires as required for assessments of fire impacts on regional air pollution or global emissions of greenhouse gases. Observed fires developed complex fire-line geometry and thermal structure, even as average whole-fire temperatures varied little. Flame temperatures sometimes exceeded 1600 K along the leading edge of actively spreading fire lines, yet >90% of the radiant energy from observed fires was associated with temperatures of 830–1440 K. Fire in a partially slashed forest encompassed a high-intensity flaming front and a trailing reach of residual combustion extending 400 m. Fire fronts in tropical savanna typically formed with little depth and a high proportion of their radiant flux density associated with high temperatures due to low levels of residual combustion. Measured fires had such low and variable radiance compared with that of a blackbody of comparable temperature as to preclude the use of fire radiance at a single wavelength as a measure of fire intensity or temperature. One-half of the radiant flux density from a measured savanna fire was associated with values of a combined emissivity–fractional-area parameter <0.091 m2/m2; for a slash fire this fraction was associated with values <0.37 m2/m2. Observations reported here show wildland fires to be so complex and dynamic as to require frequent high-resolution measurements over their course and duration in order to specify their effects in the environment; an understanding of global fire impacts may require such measurements over a large sample of individual fires.

Chapter 17 Air Pollution Increases Forest Susceptibility to Wildfires: A Case Study in the San Bernardino Mountains in Southern California

Many factors increase susceptibility of forests to wildfire. Among them are increases in human population, changes in land use, fire suppression, and frequent droughts. These and other factors have been exacerbating forest susceptibility to wildfires over the past century in southern California. We report on the significant role that air pollution has had on increasing forest susceptibility to wildfires, based on a 1999–2003 case study in the San Bernardino Mountains. Air pollution, specifically ozone (O3) and wet and dry deposition of nitrogenous (N) compounds as a by-product of fossil fuel combustion, has significantly increased since urbanization and industrialization of the region after 1945. Ozone and elevated N deposition cause specific changes in forest tree carbon (C), N, and water balance that enhance individual tree susceptibility to drought, bark beetle attack, and disease, and when combined, contribute to whole ecosystem susceptibility to wildfire. For example, elevated O3 and N deposition increase leaf turnover rates, leaf and branch litter, and decrease decomposability of litter, creating excessively deep litter layers in mixed-conifer forests affected by air pollutants. Elevated O3 and N deposition decrease the proportion of whole tree biomass in foliage and roots, thereby increasing tree susceptibility to drought and beetle attack. Because both foliar and root mass are compromised, carbohydrates are stored in the bole over winter. Elevated O3 increases drought stress by significantly reducing plant control of water loss. The resulting increase in canopy transpiration, combined with O3 and N deposition-induced decreases in root mass, significantly increases tree susceptibility to drought stress, likely contributing to successful host colonization and population increases of bark beetles. Phenomenological and experimental evidence is presented to support the role of these factors contributing to an increase in the susceptibility of forests to wildfire in southern California.

Perspectives on Fire Management in Mediterranean Ecosystems of Southern California

San Dimas Canyon seems a wild place beyond the reach of civilization. It is home to black bears, gray foxes, Anna’s hummingbirds, scrub jays, and in early summer, a multitude of biting insects. Along the steep, north-facing hillsides, the chaparral has the appearance of an ancient forest. From within the canyon it is difficult to remember that one is less than 7 km from metropolitan Los Angeles. It is also difficult to conceive of the landscape swept by flames 30- or 40-m high, or to visualize San Dimas Creek afterwards scoured by debris flows. Our difficulty in perceiving these catastrophic events makes it difficult to alter their course and consequences because to do so involves substantial cost and risks.

Estimating fire properties by remote sensing

Contemporary knowledge of the role of fire in the global environment is limited by inadequate measurements of the extent and impact of individual fires. Observations by operational polar-orbiting and geostationary satellites provide an indication of fire occurrence but are ill-suited for estimating the temperature, area, or radiant emissions of active wildland and agricultural fres. Simulations here of synthetic remote sensing pixels comprised of observed high-resolution fire data together with ash or vegetation background demonstrate that fire properties including flame temperature, fractional area, and radiant-energy flux can best be estimated from concurrent radiance measurements at wavelengths near 1.6, 3.9, and 12 pm. Successful observations at night may be made at scales to at least 1 kmn for the cluster of fire data simulated herein. During the daytime, uncertainty in the composition of the background and its reflection of solar radiation would limit successful observations to a scale of approximately 100 mn or less. Measurements at three wavelengths in the long-wave infrared would be unaffected by reflected solar radiation and could be applied to separate flame properties in a binary system of flame and background. However, likely variation in the composition of the background and its temperature limit the approach to measurements that are of high resolution in relation to the scale of the flaming front. Alternative approaches using radiances at wavelengths near 4 and 12 pm alone must fail absent a correction for the background, yet the correction is made imprecise by uncertainty in composition of the background where it comprises more than one-third of a pixel.

A prescription for controlling the air pollution resulting from the use of prescribed biomass fire: clouds

Forestry, conservation, wildfire risk reduction, and agricultural uses of planned or prescribed fires as a tool for meeting the needs of wildland managers are increasingly in collision at the air pollution control and climate change cross-roads. The inevitable conflict resulting from the disparate goals of users has long been the subject of a combination of both systems and ecologically integrated analysis attempting to minimize the environmental impact and maximize the economic and societal benefits of this land management technique. We offer here experimental evidence for the viability of implementing a pollution control option that could substantially reduce the particulate emissions from prescribed fires in biomass and explore some of the logical implications of these concepts. In nature, clouds and precipitation are the principal mechanisms by which the atmosphere is cleansed of particulate pollution, aerosols and smokes. We propose here, for consideration, using clouds as a part of the prescription for scheduling biomass fires. Since in most areas biomass fire is already carried out within a detailed prescriptive plan which includes meteorological forecasts, the addition of additional meteorological scheduling constraints should be acceptable to most users providing that the benefits are correspondingly large. Reducing particulate smoke emissions in all size classes by at least 50% seems practicable.