Reconciling Assumptions in Bottom-Up and Top-Down Approaches for Estimating Aerosol Emission Rates From Wildland Fires Using Observations From FIREX-AQ.

Accurate fire emissions inventories are crucial to predict the impacts of wildland fires on air quality and atmospheric composition. Two traditional approaches are widely used to calculate fire emissions: a satellite-based top-down approach and a fuels-based bottom-up approach. However, these methods often considerably disagree on the amount of particulate mass emitted from fires. Previously available observational datasets tended to be sparse, and lacked the statistics needed to resolve these methodological discrepancies. Here, we leverage the extensive and comprehensive airborne in situ and remote sensing measurements of smoke plumes from the recent Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign to statistically assess the skill of the two traditional approaches. We use detailed campaign observations to calculate and compare emission rates at an exceptionally high-resolution using three separate approaches: top-down, bottom-up, and a novel approach based entirely on integrated airborne in situ measurements. We then compute the daily average of these high-resolution estimates and compare with estimates from lower resolution, global top-down and bottom-up inventories. We uncover strong, linear relationships between all of the high-resolution emission rate estimates in aggregate, however no single approach is capable of capturing the emission characteristics of every fire. Global inventory emission rate estimates exhibited weaker correlations with the high-resolution approaches and displayed evidence of systematic bias. The disparity between the low-resolution global inventories and the high-resolution approaches is likely caused by high levels of uncertainty in essential variables used in bottom-up inventories and imperfect assumptions in top-down inventories.

Toward the improvement of total nitrogen deposition budgets in the United States.

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

Calibration of a high spectral resolution lidar using a Michelson interferometer, with data examples from ORACLES.

The NASA Langley airborne second-generation High Spectral Resolution Lidar (HSRL-2) uses a density-tuned field-widened Michelson interferometer to implement the HSRL technique at 355 nm. The Michelson interferometer optically separates the received backscattered light between two channels, one of which is dominated by molecular backscattering, while the other contains most of the light backscattered by particles. This interferometer achieves high and stable contrast ratio, defined as the ratio of particulate backscatter signal received by the two channels. We show that a high and stable contrast ratio is critical for precise and accurate backscatter and extinction retrievals. Here, we present retrieval equations that take into account the incomplete separation of particulate and molecular backscatter in the measurement channels. We also show how the accuracy of the contrast ratio assessment propagates to error in the optical properties. For both backscattering and extinction, larger errors are produced by underestimates of the contrast ratio (compared to overestimates), more extreme aerosol loading, and-most critically-smaller true contrast ratios. We show example results from HSRL-2 aboard the NASA ER-2 aircraft from the 2016 ORACLES field campaign in the southeast Atlantic, off the coast of Africa, during the biomass burning season. We include a case study where smoke aerosol in two adjacent altitude layers showed opposite differences in extinction- and backscatter-related Ångström exponents and a reversal of the lidar ratio spectral dependence, signatures which are shown to be consistent with a relatively modest difference in smoke particle size.

An empirical inferential method of estimating nitrogen deposition to Mediterranean-type ecosystems: the San Bernardino Mountains case study.

The empirical inferential method (EIM) allows for spatially and temporally-dense estimates of atmospheric nitrogen (N) deposition to Mediterranean ecosystems. This method, set within a GIS platform, is based on ambient concentrations of NH3, NO, NO2 and HNO3; surface conductance of NH4(+) and NO3(-); stomatal conductance of NH3, NO, NO2 and HNO3; and satellite-derived LAI. Estimated deposition is based on data collected during 2002-2006 in the San Bernardino Mountains (SBM) of southern California. Approximately 2/3 of dry N deposition was to plant surfaces and 1/3 as stomatal uptake. Summer-season N deposition ranged from <3 kg ha(-1) in the eastern SBM to ∼ 60 kg ha(-1) in the western SBM near the Los Angeles Basin and compared well with the throughfall and big-leaf micrometeorological inferential methods. Extrapolating summertime N deposition estimates to annual values showed large areas of the SBM exceeding critical loads for nutrient N in chaparral and mixed conifer forests.

Atmospheric deposition of nitrogen, sulfur and base cations in jack pine stands in the Athabasca Oil Sands Region, Alberta, Canada.

Atmospheric deposition in the Athabasca Oil Sands Region decreased exponentially with distance from the industrial center. Throughfall deposition (kg ha(-1) yr(-1)) of NH(4)-N (.8-14.7) was double that of NO(3)-N (.3-6.7), while SO(4)-S ranged from 2.5 to 23.7. Gaseous pollutants (NO(2), HNO(3), NH(3), SO(2)) are important drivers of atmospheric deposition but weak correlations between gaseous pollutants and deposition suggest that particulate deposition is also important. The deposition (eq ha(-1)) of base cations (Ca + Mg + Na) across the sampling network was highly similar to N + S deposition, suggesting that acidic deposition is neutralized by base cation deposition and that eutrophication impacts from excess N may be of greater concern than acidification. Emissions from a large forest fire in summer 2011 were most prominently reflected in increased concentrations of HNO(3) and throughfall deposition of SO4-S at some sites. Deposition of NO(3)-N also increased as did NH(4)-N deposition to a lesser degree.

Polioencephalomalacia secondary to hypernatremia in squirrel monkeys (Saimiri sciureus).

Squirrel monkeys (Saimiri spp) are one of the most consistently used New World primates in biomedical research and are increasingly being used in neuroscience research, including models of drug abuse and addiction. Spontaneous neurologic disease in the squirrel monkey is uncommonly reported but includes various infectious diseases as well as cerebral amyloidosis. Hypernatremia is an extremely serious condition of hyperosmolarity that occurs as a result of water loss, adipsia, or excess sodium intake. Neurologic effects of hypernatremia reflect the cellular dehydration produced by the shift of water from the intracellular fluid space into the hypertonic extracellular fluid space. Severe hypernatremia may result in cerebrocortical laminar necrosis (polioencephalomalacia) in human patients as well as in a number of domestic species, including pigs, poultry, and ruminants. We report the clinical, histopathologic, and immunohistochemical findings of polioencephalomalacia in 13 squirrel monkeys. Polioencephalomalacia in these animals was associated with hypernatremia that was confirmed by serum levels of sodium greater than 180 mmol/L (reference range, 134.0-154.0 mmol/L [mEq/L]). All animals had concurrent diseases or experimental manipulation that predisposed to adipsia. Immunohistochemical investigation using antibodies to neuronal nuclei (NeuN), CNPase, Iba-1, and CD31 revealed necrosis of predominantly cerebral cortical layers 3, 4, and 5 characterized by neuronal degeneration and loss, oligodendrocytic loss, microglial proliferation, and vascular reactivity. The squirrel monkey is exquisitely sensitive to hyperosmolar metabolic disruption and it is associated with laminar cortical necrosis.

Nitrogen critical loads and management alternatives for N-impacted ecosystems in California.

Empirical critical loads for N deposition effects and maps showing areas projected to be in exceedance of the critical load (CL) are given for seven major vegetation types in California. Thirty-five percent of the land area for these vegetation types (99,639 km(2)) is estimated to be in excess of the N CL. Low CL values (3-8 kg N ha(-1) yr(-1)) were determined for mixed conifer forests, chaparral and oak woodlands due to highly N-sensitive biota (lichens) and N-poor or low biomass vegetation in the case of coastal sage scrub (CSS), annual grassland, and desert scrub vegetation. At these N deposition critical loads the latter three ecosystem types are at risk of major vegetation type change because N enrichment favors invasion by exotic annual grasses. Fifty-four and forty-four percent of the area for CSS and grasslands are in exceedance of the CL for invasive grasses, while 53 and 41% of the chaparral and oak woodland areas are in exceedance of the CL for impacts on epiphytic lichen communities. Approximately 30% of the desert (based on invasive grasses and increased fire risk) and mixed conifer forest (based on lichen community changes) areas are in exceedance of the CL. These ecosystems are generally located further from emissions sources than many grasslands or CSS areas. By comparison, only 3-15% of the forested and chaparral land areas are estimated to be in exceedance of the NO(3)(-) leaching CL. The CL for incipient N saturation in mixed conifer forest catchments was 17 kg N ha(-1) yr(-1). In 10% of the CL exceedance areas for all seven vegetation types combined, the CL is exceeded by at least 10 kg N ha(-1) yr(-1), and in 27% of the exceedance areas the CL is exceeded by at least 5 kg N ha(-1) yr(-1). Management strategies for mitigating the effects of excess N are based on reducing N emissions and reducing site N capital through approaches such as biomass removal and prescribed fire or control of invasive grasses by mowing, selective herbicides, weeding or domestic animal grazing. Ultimately, decreases in N deposition are needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect epiphytic lichen communities.

Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis.

Atmospheric nitrogen (N) deposition is a recognized threat to plant diversity in temperate and northern parts of Europe and North America. This paper assesses evidence from field experiments for N deposition effects and thresholds for terrestrial plant diversity protection across a latitudinal range of main categories of ecosystems, from arctic and boreal systems to tropical forests. Current thinking on the mechanisms of N deposition effects on plant diversity, the global distribution of G200 ecoregions, and current and future (2030) estimates of atmospheric N-deposition rates are then used to identify the risks to plant diversity in all major ecosystem types now and in the future. This synthesis paper clearly shows that N accumulation is the main driver of changes to species composition across the whole range of different ecosystem types by driving the competitive interactions that lead to composition change and/or making conditions unfavorable for some species. Other effects such as direct toxicity of nitrogen gases and aerosols, long-term negative effects of increased ammonium and ammonia availability, soil-mediated effects of acidification, and secondary stress and disturbance are more ecosystem- and site-specific and often play a supporting role. N deposition effects in mediterranean ecosystems have now been identified, leading to a first estimate of an effect threshold. Importantly, ecosystems thought of as not N limited, such as tropical and subtropical systems, may be more vulnerable in the regeneration phase, in situations where heterogeneity in N availability is reduced by atmospheric N deposition, on sandy soils, or in montane areas. Critical loads are effect thresholds for N deposition, and the critical load concept has helped European governments make progress toward reducing N loads on sensitive ecosystems. More needs to be done in Europe and North America, especially for the more sensitive ecosystem types, including several ecosystems of high conservation importance. The results of this assessment show that the vulnerable regions outside Europe and North America which have not received enough attention are ecoregions in eastern and southern Asia (China, India), an important part of the mediterranean ecoregion (California, southern Europe), and in the coming decades several subtropical and tropical parts of Latin America and Africa. Reductions in plant diversity by increased atmospheric N deposition may be more widespread than first thought, and more targeted studies are required in low background areas, especially in the G200 ecoregions.

Empirical and simulated critical loads for nitrogen deposition in California mixed conifer forests.

Empirical critical loads (CL) for N deposition were determined from changes in epiphytic lichen communities, elevated NO(3)(-) leaching in streamwater, and reduced fine root biomass in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at sites with varying N deposition. The CL for lichen community impacts of 3.1 kg ha(-1) year(-1) is expected to protect all components of the forest ecosystem from the adverse effects of N deposition. Much of the western Sierra Nevada is above the lichen-based CL, showing significant changes in lichen indicator groups. The empirical N deposition threshold and that simulated by the DayCent model for enhanced NO(3)(-)leaching were 17 kg N ha(-1) year(-1). DayCent estimated that elevated NO(3)(-) leaching in the San Bernardino Mountains began in the late 1950s. Critical values for litter C:N (34.1), ponderosa pine foliar N (1.1%), and N concentrations (1.0%) in the lichen Letharia vulpina ((L.) Hue) are indicative of CL exceedance.

Atmospheric deposition inputs and effects on lichen chemistry and indicator species in the Columbia River Gorge, USA.

Topographic and meteorological conditions make the Columbia River Gorge (CRG) an 'exhaust pipe' for air pollutants generated by the Portland-Vancouver metropolis and Columbia Basin. We sampled fog, bulk precipitation, throughfall, airborne particulates, lichen thalli, and nitrophytic lichen distribution. Throughfall N and S deposition were high, 11.5-25.4 and 3.4-6.7 kg ha(-1) over 4.5 months at all 9 and 4/9 sites, respectively. Deposition and lichen thallus N were highest at eastern- and western-most sites, implicating both agricultural and urban sources. Fog and precipitation pH were frequently as low as 3.7-5.0. Peak NO(x), NH(3), and SO(2) concentrations in the eastern CRG were low, suggesting enhanced N and S inputs were largely from particulate deposition. Lichens indicating nitrogen-enriched environments were abundant and lichen N and S concentrations were 2x higher in the CRG than surrounding national forests. The atmospheric deposition levels detected likely threaten Gorge ecosystems and cultural resources.

N saturation symptoms in chaparral catchments are not reversed by prescribed fire.

Fire is a critical ecosystem process in many landscapes and is particularly dominant in the chaparral shrublands of southern California which are also exposed to high levels of atmospheric N deposition. Few studies have addressed the combined effects of these two disturbance factors. In this study we evaluate the hydrologic and biogeochemical response of a control and a prescribed burn catchment over a 15-year period. Streamwater nitrate concentrations and export in the burned catchment were higher than those in the unburned catchment for 7-10 years after the burn and concentrations remained high in both catchments during the entire study. Therefore, fire is not an effective mitigation tool for N deposition in these semi-arid systems. Additionally, the extended N export in this system indicates that chaparral ecosystems do not recovertheir N retention capabilities as rapidly as humid systems do when subjected to disturbance.

Nutrient status and plant growth effects of forest soils in the Basin of Mexico.

The nutrient status of forest soils in the Mexico City Air Basin was evaluated by observing plant growth responses to fertilization with N, P or both nutrients combined. P deficiency was the most frequent condition for soil from two high pollution sites and N deficiency was greatest at a low N deposition site. Concentrations of Pb and Ni, and to a lesser extent Zn and Co, were higher at the high pollution sites. However, positive plant growth responses to P and sometimes to N, and results of wheat root elongation bioassays, suggest that heavy metal concentrations were not directly phytotoxic. Further studies are needed to determine if heavy metal toxicity to mycorrhizal symbionts of eucalyptus (Eucalyptus camaldulensis Dehnh.) from high pollution sites may explain the P deficiency and stunted growth. P deficiency is expected to limit the capacity for biotic N retention in N saturated forested watersheds in the Basin of Mexico dominated by Andisols.

California black oak response to nitrogen amendment at a high O3, nitrogen-saturated site.

In a nitrogen (N) saturated forest downwind from Los Angeles, California, the cumulative response to long-term background-N and N-amendment on black oak (Quercus kelloggii) was described in a below-average and average precipitation year. Monthly measurements of leaf and branch growth, gas exchange, and canopy health attributes were conducted. The effects of both pollutant exposure and drought stress were complex due to whole tree and leaf level responses, and shade versus full sun leaf responses. N-amended trees had lower late summer carbon (C) gain and greater foliar chlorosis in the drought year. Leaf water use efficiency was lower in N-amended trees in midsummer of the average precipitation year, and there was evidence of poor stomatal control in full sun. In shade, N-amendment enhanced stomatal control. Small differences in instantaneous C uptake in full sun, lower foliar respiration, and greater C gain in low light contributed to the greater aboveground growth observed.

Characterisation of atopic and non-atopic wheeze in 10 year old children.

BACKGROUND: Wheezing occurs in both atopic and non-atopic children. The characteristics of atopic and non-atopic wheeze in children at 10 years of age were assessed and attempts made to identify whether different mechanisms underlie these states. METHODS: Children were seen at birth and at 1, 2, 4 and 10 years of age in a whole population birth cohort study (n = 1456; 1373 seen at 10 years). Information was collected prospectively on inherited and early life environmental risk factors for wheezing. Skin prick testing, spirometry, and methacholine bronchial challenge were conducted at 10 years. Wheezing at 10 years of age was considered atopic or non-atopic depending on the results of the skin prick test. Independent significant risk factors for atopic and non-atopic wheeze were determined by logistic regression. RESULTS: Atopic (10.9%) and non-atopic (9.7%) wheeze were equally common at 10 years of age. Greater bronchial hyperresponsiveness (p<0.001) and airways obstruction (p = 0.011) occurred in children with atopic wheeze than in those with non-atopic wheeze at 10 years. Children with atopic wheeze more often received treatment (p<0.001) or an asthma diagnosis for their disorder, although current morbidity at 10 years differed little for these states. Maternal asthma and recurrent chest infections at 2 years were independently significant factors for developing non-atopic wheeze. For atopic wheeze, sibling asthma, eczema at 1 year, rhinitis at 4 years, and male sex were independently significant. CONCLUSIONS: Non-atopic wheeze is as common as atopic wheeze in children aged 10 years, but treatment is more frequent in those with atopic wheeze. Different risk factor profiles appear relevant to the presence of atopic and non-atopic wheeze at 10 years of age.

Characterization of wheezing phenotypes in the first 10 years of life.

BACKGROUND: Childhood wheezing illnesses are characterized into different phenotypes. However, severity of the disease associated with these phenotypes has not been extensively studied. OBJECTIVES: To determine characteristics of childhood wheezing phenotypes in the first decade of life using health outcomes plus measurements of atopy, lung function and bronchial hyper-responsiveness. METHODS: A whole population birth cohort (n = 1456) was prospectively studied to examine the natural history of childhood wheezing. Children were seen at 1, 2, 4 and 10 years for questionnaire completion and prospectively collected data used to define wheezing phenotypes. Assessment was made of adverse health outcomes plus spirometry, bronchial hyper-responsiveness, serum IgE measurement at 10 years and skin test sensitization at both 4 and 10 years for wheezing phenotypes. RESULTS: Phenotypic analysis identified that 37% early life wheezers (symptom onset by age 4 years) still wheezed at 10 years. These persistent wheezers showed significantly more physician-diagnosed asthma in early life (P < 0.005 at 2 years) than early transient wheezers (wheezing transiently with onset by age 4 years). Overall they experienced greater multiple hospital admissions (P = 0.024), specialist referral (P = 0.009) and use of inhaled (P < 0.001) and oral steroids (P < 0.001) than early transient wheezers. They also demonstrated enhanced bronchial hyper-responsiveness compared with early transient wheezers (P < 0.001). However, both groups of early life wheezers showed impairment of baseline lung function at 10 years in comparison with non-wheezers: FEV1 (P < 0.029) and FEV1/FVC ratio (P < 0.001) with persistent wheeze and PEF (P = 0.036) with early transient wheeze. Late-onset wheezers (onset from 5 years onwards) had similar BHR to persistent wheezers but maintained normal lung function at age 10 and had lower cumulative prevalence of adverse health outcomes than persistent wheezers. CONCLUSIONS: Persistent wheezing with early childhood onset is associated with substantial morbidity in the first decade of life in association with high levels of atopy, bronchial hyper-responsiveness and impaired lung function at 10 years of age. Late-onset wheezing in the first decade of life could harbour potential for similarly significant disease subsequently.

The prevalence of asthma and wheezing illnesses amongst 10-year-old schoolchildren.

Asthma and wheezing illnesses carry a significant burden of disease during childhood. Prevalence studies have the capacity to provide invaluable insights into the nature of these common conditions. As part of the Isle of Wight Whole Population Birth Cohort Study (n=1456) we have examined wheezing and asthma development amongst 10-year-old children. At this age 1373 children completed ISAAC written questionnaires whilst 1043 children performed further testing including skin-prick testing, serum inhalant IgE antibody screening, spirometry and bronchial challenge. At 10-years, prevalence of current wheeze was 18.9%, current asthma (symptomatic bronchial hyper-responsiveness--BHR) 14.4% and currently diagnosed asthma (current wheeze and asthma ever--CDA) 13.0%. Both wheezing and asthma at 10 years were associated with average symptom onset at 3 years of age indicating an early life origin for such conditions. Current wheeze (P=0.011) and CDA (P=0.008) showed significant male predominance. Considerable disease morbidity was identified for these states that tended to be greatest amongst children defined asthmatic rather than simply current wheezers. Wheezing and asthma were significantly associated with both atopy (P<0.001) and allergic co-morbidity Children with these states, particularly current asthma, also demonstrated impaired lung function (FEV1, P<0.001 and FEV1/FVC, P=0.010) and increased BHR (inverse slope, P<0.001). In conclusion, Asthma and wheezing showed substantial prevalence at 10 years of age. Strong associations to male gender, atopy, impaired lung function and BHR were seen for both wheeze and asthma. In regard to prevalence and morbidity characteristics, a questionnaire-based definition of currently diagnosed asthma gave similar results to the use of symptomatic BHR in defining current asthma.

Rhinitis in 10-year-old children and early life risk factors for its development.

AIM: To study the prevalence, characteristics of and risk factors for childhood rhinitis. METHODS: In a whole population birth cohort study (n = 1,456) the prevalence and characteristics of rhinitis among 10-y-old children were examined. At this age 1373 children (94%) completed standardized questionnaires, 1,043 (72%) skin-prick testing, 953 (65%) serum inhalant immunoglobulin E antibody screening and 784 (54%) methacholine bronchial challenges. RESULTS: At the age of 10 y the prevalence of hayfever ever was 18.6% and current nasal symptoms (rhinitis) 22.6%. Rhinitis at 10 y was largely seasonal and associated with low morbidity, although 62.7% of cases required pharmacological treatment. Atopy (positive skin test) and other allergic states were associated with rhinitis (p < 0.001). Wheeze or diagnosed asthma was higher with coexistent rhinitis. Among wheezing children physician-diagnosed asthma (p < 0.024) and inhaled corticosteroid use (p < 0.001) were greater with the presence of rhinitis. Significant bronchial hyperresponsiveness (methacholine concentration giving a 20% fall in forced expiratory volume in I s <4.0 mg ml(-1)) was greater if rhinitis was present even when the child did not wheeze (p < 0.001). Risk factor analysis for rhinitis identified the independent significance for atopy (p < 0.001) and eczema (p = 0.009) at the age of 4 y plus paternal rhinitis (p < 0.001), maternal rhinitis (p = 0.033) and maternal food allergy (p = 0.016). CONCLUSION: Rhinitis is common at the age of 10 y, with strong associations with atopy, wheezing, asthma and bronchial hyperresponsiveness. An inherited predisposition towards atopy appears to predominate over environment in the aetiology of this state.

Concentrations, deposition, and effects of nitrogenous pollutants in selected California ecosystems.

Atmospheric deposition of nitrogen (N) in California ecosystems is ecologically significant and highly variable, ranging from about 1 to 45 kg/ha/year. The lowest ambient concentrations and deposition values are found in the eastern and northern parts of the Sierra Nevada Mountains and the highest in parts of the San Bernardino and San Gabriel Mountains that are most exposed to the Los Angeles air pollution plume. In the Sierra Nevada Mountains, N is deposited mostly in precipitation, although dry deposition may also provide substantial amounts of N. On the western slopes of the Sierra Nevada, the majority of airborne N is in reduced forms as ammonia (NH3) and particulate ammonium (NH4+) from agricultural activities in the California Central Valley. In southern California, most of the N air pollution is in oxidized forms as nitrogen oxides (NOx), nitric acid (HNO3), and particulate nitrate (NO3-) resulting from fossil fuel combustion and subsequent complex photochemical reactions. In southern California, dry deposition of gases and particles provides most (up to 95%) of the atmospheric N to forests and other ecosystems. In the mixed-conifer forest zone, elevated deposition of N may initially benefit growth of vegetation, but chronic effects may be expressed as deterioration of forest health and sustainability. HNO3 vapor alone has a potential for toxic effects causing damage of foliar surfaces of pines and oaks. In addition, dry deposition of predominantly HNO3 has lead to changes in vegetation composition and contamination of ground- and stream water where terrestrial N loading is high. Long-term, complex interactions between N deposition and other environmental stresses such as elevated ozone (O3), drought, insect infestations, fire suppression, or intensive land management practices may affect water quality and sustainability of California forests and other ecosystems.

A case study of nitrogen saturation in western U.S. forests.

Virtually complete nitrification of the available ammonium in soil and nitrification activity in the forest floor are important factors predisposing forests in the San Bernardino Mountains of southern California to nitrogen (N) saturation. As a result, inorganic N in the soil solution is dominated by nitrate. High nitrification rates also generate elevated nitric oxide (NO) emissions from soil. High-base cation saturation of these soils means that soil calcium depletion or effects associated with soil acidification are not an immediate risk for forest health as has been postulated for mesic forests in the eastern U.S. Physiological disturbance (e.g., altered carbon [C] cycling, reduced fine root biomass, premature needle abscission) of ozone-sensitive ponderosa pine trees exposed to high N deposition and high ozone levels appear to be the greater threat to forest sustainability. However, N deposition appears to offset the aboveground growth depression effects of ozone exposure. High nitrification activity reported for many western ecosystems suggests that with chronic N inputs these systems are prone to N saturation and hydrologic and gaseous losses of N. High runoff during the winter wet season in California forests under a Mediterranean climate may further predispose these watersheds to high nitrate leachate losses. After 4 years of N fertilization at a severely N saturated site in the San Bernardino Mountains, bole growth unexpectedly increased. Reduced C allocation below- ground at this site, presumably in response to ozone or N or both pollutants, may enhance the bole growth response to added N.