Evapotranspiration (ET) is driven by evaporative demand, available solar energy and soil moisture (SM) as well as by plant physiological activity which may be substantially affected by elevated CO2 and O3. A multi-year study was conducted in outdoor sunlit-controlled environment mesocosm containing ponderosa pine seedlings growing in a reconstructed soil-litter system. The study used a 2 x 2 factorial design with two concentrations of CO2 (ambient and elevated), two levels of O3 (low and high) and three replicates of each treatment. The objective of this study was to assess the effects of chronic exposure to elevated CO2 and O3, alone and in combination, on daily ET. This study evaluated three hypotheses: (i) because elevated CO2 stimulates stomatal closure, O3 effects on ET will be less under elevated CO2 than under ambient CO2; (ii) elevated CO2 will ameliorate the long-term effects of O3 on ET; and (iii) because conductance (g) decreases with decreasing SM, the impacts of elevated CO2 and O3, alone and in combination, on water loss via g will be greater in early summer when SM is not limiting than to other times of the year. A mixed-model covariance analysis was used to adjust the daily ET for seasonality and the effects of SM and photosynthetically active radiation when testing for the effects of CO2 and O3 on ET via the vapor pressure deficit gradient. The empirical results indicated that the interactive stresses of elevated CO2 and O3 resulted in a lesser reduction in ET via reduced canopy conductance than the sum of the individual effects of each gas. CO2-induced reductions in ET were more pronounced when trees were physiologically most active. O3-induced reductions in ET under ambient CO2 were likely transpirational changes via reduced conductance because needle area and root biomass were not affected by exposures to elevated O3 in this study.

We demonstrate an approach for evaluating the level of protection attained using a variety of forms and levels of past, current, and proposed Air Quality Standards (AQSs). The U.S. Clean Air Act requires the establishment of ambient air quality standards to protect health and public welfare. However, determination of attainment of these standards is based on ambient pollutant concentrations rather than prevention of adverse effects. To determine if a given AQS protected against adverse effects on vegetation, hourly ozone concentrations were adjusted to create exposure levels that "just attain" a given standard. These exposures were used in combination with a physiologically-based tree growth model to account for the interactions of climate and ozone. In the evaluation, we used ozone concentrations from two 6-year time periods from the San Bernardino Mountains in California. There were clear differences in the level of vegetation protection achieved with the various AQSs. Based on modeled plant growth, the most effective standards were the California 8-hr average maximum of 70 ppb and a seasonal, cumulative, concentration-weighted index (SUM06), which if attained, resulted in annual growth reductions of 1% or less. Least effective was the 1-hr maximum of 120 ppb which resulted in a 7% annual reduction. We conclude that combining climate, exposure scenarios, and a process-based plant growth simulator was a useful approach for evaluating effectiveness of current or proposed air quality standards, or evaluating the form and/or level of a standard based on preventing adverse growth effects.

Salmon runs have declined over the past two centuries in the Pacific Northwest region of North America. Reduced inputs of salmon-derived organic matter and nutrients (SDN) may limit freshwater production and thus establish a negative feedback loop affecting future generations of fish. Restoration efforts use the rationale of declining SDN to justify artificial nutrient additions, with the goal of reversing salmon decline. The forms of nutrient addition include introducing salmon carcasses, carcass analogs (processed fish cakes), or inorganic fertilizers. While evidence suggests that fish and wildlife may benefit from increases in food availability as a result of carcass additions, stream ecosystems vary in their ability to use nutrients to benefit salmon. Moreover, the practice may introduce excess nutrients, disease, and toxic substances to streams that may already exceed proposed water quality standards. Restoration efforts involving nutrient addition must balance the potential benefits of increased food resources with the possible harm caused by increased nutrient and toxin loads.

Salmon runs have declined over the past two centuries in the Pacific Northwest region of North America. Reduced inputs of salmon-derived organic matter and nutrients (SDN) may limit freshwater production and thus establish a negative feedback loop affecting future generations of fish. Restoration efforts use the rationale of declining SDN to justify artificial nutrient additions, with the goal of reversing salmon decline. The forms of nutrient addition include introducing salmon carcasses, carcass analogs (processed fish cakes), or inorganic fertilizers. While evidence suggests that fish and wildlife may benefit from increases in food availability as a result of carcass additions, stream ecosystems vary in their ability to use nutrients to benefit salmon. Moreover, the practice may introduce excess nutrients, disease, and toxic substances to streams that may already exceed proposed water quality standards. Restoration efforts involving nutrient addition must balance the potential benefits of increased foo...

To determine whether CO2 (a key contributor to climate change), and O3 (an important air pollutant), interact to affect needle N dynamics; we grew Pinus ponderosa Dougl. ex Laws. seedlings for three seasons (1998–2000) in outdoor chambers with ambient or elevated CO2 combined with low or elevated O3. Nitrogen concentration (area basis) was measured for attached and abscised needles. Nitrogen retranslocation was determined by comparing N for attached needles in July vs. October, and N resorption by comparing N for needles which abscised from July to October vs. attached needles in October. Ozone, and to a lesser extent CO2, reduced needle N depending on age class and sampling date, but there were no CO2 × O3 interactions. There was a significant CO2 × O3 interaction for N retranslocation for 1998 needles in 1999 (O3 reduced higher retranslocation with elevated CO2), but CO2 and O3 had only marginal effects on N retranslocation or resorption for other needles. Abscission of year-old needles increased with elevated CO2, but not O3. Long-term studies are needed to determine if the responses to elevated CO2 and O3 in our seedlings persist as trees mature and have multiple age-classes of needles.

Ambient air quality standards and control strategies are implemented to protect humans and vegetation from adverse effects. We used a process-based tree-growth model (TREGRO) to show that over the past 37 years, changes in O3 exposure, with accompanying variation in climate, are reflected in changes in the growth of Pinus ponderosa Dougl. ex Laws. in the San Bernardino Mountains near Los Angeles, California, USA. Despite variation in temperature and precipitation over the study period (1963–1999), O3 exposure consistently reduced simulated tree growth. Simulated growth reductions increased concurrent with increasing O3 exposure. The maximum growth reduction occurred in 1979. As O3 exposures decreased during the 1980s and 1990s, effects on growth also decreased. This implies that emission control strategies taken to reduce exposures to attain O3 standards benefited P. ponderosa growth in the San Bernardino Mountains. This modeling approach provides a powerful tool for solving the difficult problem of evaluating regulatory effectiveness by simulating plant response using long-term climate and air pollution exposure records for a given region.

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N2-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r 2 = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha−1 y−1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r 2 = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.

Corrigendum to ‘‘Predicting the regional impact of ozone and precipitation on the growth of loblolly pine and yellow-poplar using linked TREGRO and ZELIG models’’ [Forest Ecol. Manage. 146 (2001) 247–263] J.A. Laurence, W.A. Retzlaff, J.S. Kern, E.H. Lee, W.E. Hogsett, D.A. Weinstein United States Environmental Protection Agency, National Health and Ecological Effects Research Laboratory, Western Ecology Division, 200 SW 35th Street, Corvallis, OR 97333, USA Boyce Thompson Institute for Plant Research, Tower Road, Ithaca, NY 14853-1801, USA Dynamac Corporation, 200 SW 35th Street, Corvallis, OR 97333, USA

The objective of this study was to determine if theO 3 sensitivity of Phaseolus vulgaris L. changed withplant development. Plants exposed to charcoal-filtered airor elevated O 3 throughout the study were compared tothose exposed only during the vegetative or reproductivestages. Effects of pod development on O 3 sensitivitywere also examined by removing flowers from a subset ofplants. We found that plant developmental stage significantly altered O 3 sensitivity. Exposures during vegetative stagescaused little foliar injury and had only a small impact on totalgrowth and yield. In contrast, exposures during pod development had a much greater effect on foliar injury, growth and yield. Pod development significantly increased O 3 induced foliar injury and O 3 impacts on plant growth. We suggest that thegreater O 3 sensitivity during pod filling and maturation isthe consequence of reduced energy levels available for homeostatic repair processes.

Understanding the effects of tropospheric ozone on vegetation, as called for in the U.S. Clean Air Act, has involved collection of experimental data at the species level and, in particular, at the level of the individual and populations. Frequently the studies have been regression designs involving single species and single pollutants resulting in quantitative exposure-response functions that characterize the effects on biomass or reproduction (crop yield). Recently, the 1996 EPA Oxidant Criteria Document reviewed the published research on oxidant effects on crops, forests, and ecological resources, and concluded that the current secondary National Ambient Air Quality Standard (NAAQS) for ozone is neither protective or appropriate. The subsequent discussion and decision on what is an appropriate form and level of the secondary NAAQS utilized results from single pollutant and single species studies. To estimate concentrations causing effects composite response-functions for crops and trees from a large number of species were developed, predicting crop yield loss or annual biomass loss in seedlings. This approach is useful based on available quantitative data on biological effects; however it assumes that individual plant response does Plot change in the presence of other stresses or in natural systems which are considerably more complex (e.g. species' assemblages and competition for resources). For example, the importance of biological complexity is illustrated when we examine the below-ground ecosystem, an often overlooked portion of the ecosystem. We have found that incorporating natural biological complexity into potting soils can result in carbon fluxes opposite those predicted from individual plant studies using artificial media lacking natural soil foodwebs. A future research approach to understanding O-3 effects on ecosystems is required that will develop the necessary linkages to extrapolate experimental data taken at the individual level, often in artificial conditions, to predict changes on individuals or populations in more complex native environments. We will present experimental and modelling activities from our laboratory that show how we are beginning to address the problems of scale, complexity, and multiple stresses in forested ecosystems exposed to ozone stress. Questions frequently posed in ecological risk assessments.

Understanding the effects of tropospheric ozone on vegetation, as called for in the U.S. Clean Air Act, has involved collection of experimental data at the species level and, in particular, at the level of the individual and populations. Frequently the studies have been regression designs involving single species and single pollutants resulting in quantitative exposure-response functions that characterize the effects on biomass or reproduction (crop yield). Recently, the 1996 EPA Oxidant Criteria Document reviewed the published research on oxidant effects on crops, forests, and ecological resources, and concluded that the current secondary National Ambient Air Quality Standard (NAAQS) for ozone is neither protective or appropriate. The subsequent discussion and decision on what is an appropriate form and level of the secondary NAAQS utilized results from single pollutant and single species studies. To estimate concentrations causing effects composite response-functions for crops and trees from a large number of species were developed, predicting crop yield loss or annual biomass loss in seedlings. This approach is useful based on available quantitative data on biological effects; however it assumes that individual plant response does not change in the presence of other stresses or in natural systems which are considerably more complex (e.g. species' assemblages and competition for resources). For example, the importance of biological complexity is illustrated when we examine the below-ground ecosystem, an often overlooked portion of the ecosystem. We have found that incorporating natural biological complexity into potting soils can result in carbon fluxes opposite those predicted from individual plant studies using artificial media lacking natural soil foodwebs. A future research approach to understanding O3 effects on ecosystems is required that will develop the necessary linkages to extrapolate experimental data taken at the individual level, often in artificial conditions, to predict changes on individuals or populations in more complex native environments. We will present experimental and modelling activities from our laboratory that show how we are beginning to address the problems of scale, complexity, and multiple stresses in forested ecosystems exposed to ozone stress. Questions frequently posed in ecological risk assessments.

The risk tropospheric ozone poses to forests in the United States is dependent on the variation in ozone exposure across the distribution of the forests in question and the various environmental and climate factors predominant in the region. All these factors have a spatial nature, and consequently an approach to characterization oi ozone risk is presented that places ozone exposure-response functions for species as seedlings and model-simulated tree and stand responses in a spatial context using a geographical information systems (GIS). The GIS is used to aggregate factors considered important in a risk characterization, including: (1) estimated ozone exposures over forested regions, (2) measures of ozone effects on species' and stand growth, and (3) spatially distributed environmental, genetic, and exposure influences on species' response to ozone. The GIS-based risk characterization provides an estimation oi the extent and magnitude of the potential ozone impact on forests. A preliminary risk characterization demonstrating this approach considered only the eastern United States and only the limited empirical data quantifying the effect oi ozone exposures on forest tree species as seedlings. The area-weighted response of the annual seedling biomass loss formed the basis for a sensitivity ranking: sensitive-aspen and black cherry (14%-33% biomass loss over 50% of their distribution); moderately sensitive-tulip popular, loblolly pine, eastern white pine, and sugar maple (5%-13% biomass loss); insensitive-Virginia pine and red maple (0%-1% loss). In the future, the GIS-based risk characterization will include process-based model simulations of the three- to 5-year growth response of individual species as large trees with relevant environmental interactions and model simulated response of mixed stands. The interactive nature of GIS provides a tool to explore consequences of the range of climate conditions across a species' distribution, forest management practices, changing ozone precursors, regulatory control strategies, and other factors influencing the spatial distribution of ozone over time as more information becomes available.

/ The risk tropospheric ozone poses to forests in the United States is dependent on the variation in ozone exposure across the distribution of the forests in question and the various environmental and climate factors predominant in the region. All these factors have a spatial nature, and consequently an approach to characterization of ozone risk is presented that places ozone exposure-response functions for species as seedlings and model-simulated tree and stand responses in a spatial context using a geographical information systems (GIS). The GIS is used to aggregate factors considered important in a risk characterization, including: (1) estimated ozone exposures over forested regions, (2) measures of ozone effects on species' and stand growth, and (3) spatially distributed environmental, genetic, and exposure influences on species' response to ozone. The GIS-based risk characterization provides an estimation of the extent and magnitude of the potential ozone impact on forests. A preliminary risk characterization demonstrating this approach considered only the eastern United States and only the limited empirical data quantifying the effect of ozone exposures on forest tree species as seedlings. The area-weighted response of the annual seedling biomass loss formed the basis for a sensitivity ranking: sensitive-aspen and black cherry (14%-33% biomass loss over 50% of their distribution); moderately sensitive-tulip popular, loblolly pine, eastern white pine, and sugar maple (5%-13% biomass loss); insensitive-Virginia pine and red maple (0%-1% loss). In the future, the GIS-based risk characterization will include process-based model simulations of the three- to 5-year growth response of individual species as large trees with relevant environmental interactions and model simulated response of mixed stands. The interactive nature of GIS provides a tool to explore consequences of the range of climate conditions across a species' distribution, forest management practices, changing ozone precursors, regulatory control strategies, and other factors influencing the spatial distribution of ozone over time as more information becomes available.KEY WORDS: Ecological risk assessment; GIS; Ozone; Risk characterization; Forests; Trees

To assess the impact of ozone on forests, it is necessary to quantify ozone exposure on regional scales. Since ozone monitoring stations are widely scattered and mostly concentrated in urban and suburban areas, spatial interpolation is necessary to estimate ozone exposure at non-monitored forest sites. However, ozone precursor emissions, wind transport of precursors and ozone, and weather conditions conducive to the formation of ozone exhibit considerable spatial variability. These factors are not considered in simple interpolation between monitored sites. We defined an index of ozone formation potential, which incorporated spatially distributed NO{sub x} emissions, wind directional frequencies, distance downwind from NO{sub x} sources, and a measure of solar radiation which drives the photochemical production of ozone. This index was computed for each 2Ox2O km grid cell within the range of loblolly pine (Pinus taeda) in the southeastern U.S. Monitored ozone exposure was quantified by SUM06, the monthly sum of hourly concentrations {>=}0.06 ppm. SUM06 was interpolated using cokriging with the ozone formation potential index as a covariate. For comparison, interpolations without the covariate were also done with inverse distance, inverse distance{sup 2}, and kriging methods. The incorporation of this additional information on ozone formation potential slightly increased predictive capability formore » ozone exposure, with precision and accuracy increasing in the order INVD{sup 2} < INVD < kriging < cokriging.« less