The forests of the San Bernardino Mountains have been subject to ozone and nitrogen (N) deposition for some 60 years. Much work has been done to assess the impacts of these pollutants on trees, but little is known about how the diverse understory flora has fared. Understory vegetation has declined in diversity in response to elevated N in the eastern U.S. and Europe. Six sites along an ozone and N deposition gradient that had been part of a long-term study on response of plants to air pollution beginning in 1973 were resampled in 2003. Historic ozone data and leaf injury scores confirmed the gradient. Present-day ozone levels were almost half of these, and recent atmospheric N pollution concentrations confirmed the continued air pollution gradient. Both total and extractable soil N were higher in sites on the western end of the gradient closer to the urban source of pollution, pH was lower, and soil carbon (C) and litter were higher. The gradient also had decreasing precipitation and increasing elevation from west to east. However, the dominant tree species were the same across the gradient. Tree basal area increased during the 30-year interval in five of the sites. The two westernmost sites had 30–45% cover divided equally between native and exotic understory herbaceous species, while the other sites had only 3–13% cover dominated by native species. The high production is likely related to higher precipitation at the western sites as well as elevated N. The species richness was in the range of 24 to 30 in four of the sites, but one site of intermediate N deposition had 42 species, while the easternmost, least polluted site had 57 species. These were primarily native species, as no site had more than one to three exotic species. In three of six sites, 20–40% of species were lost between 1973 and 2003, including the two westernmost sites. Two sites with intermediate pollution had little change in total species number over 30 years, and the easternmost site had more species in 2003. The easternmost site is also the driest and has the most sunlight filtering to the forest floor, possibly accounting for the higher species richness. The confounding effects of the precipitation gradient and possibly local disturbances do not show a simple correlation of air pollution with patterns of native and invasive species cover and richness. Nevertheless, the decline of native species and dominance by exotic species in the two westernmost polluted sites is cause for concern that air pollution is affecting the understory vegetation adversely.

Air pollution, principally in the form of photochemical ozone and deposition of nitrogen compounds, has significantly affected mixed conifer forests in the mountains of southern California. Foliar injury, premature needle abscission, crown thinning, and reduced growth and vigor have been well documented, particularly for ponderosa (Pinus ponderosa Laws.) and Jeffrey (P. jeffreyi Grev. and Balf.) pines on the western side of the pollution deposition gradient in the San Bernardino Mountains. Tree mortality of the more ozone-susceptible ponderosa and Jeffrey pines has led to alterations in stand composition, in favor of increased dominance by more ozone-resistant species such as incense cedar (Calocedrus decurrens (Torr.) Florin), white fir (Abies concolor (Gord. & Glend.) Lindl.), and sugar pine (P. lambertiana Doug.). Increased rates of litter deposition, alterations in C/N ratios in litter and soil, and reductions in fine root biomass of trees have also altered the dynamics of biogeochemical processing in stands impacted by ozone and excess N deposition. Research into the effects of atmospheric deposition across the mixed conifer forests of the San Bernardino Mountains continues to provide insights into the complex interactions among anthropogenic and natural stresses in a forest ecosystem.

Toxic effects of photochemical smog on ponderosa and Jeffrey pines in the San Bernardino Mountains were discovered in the 1950s. It was revealed that ozone is the main cause of foliar injury manifested as chlorotic mottle and premature needle senescence. Various morphological, physiological and biochemical alterations in the affected plants have been reported over a period of about 40 years of multidisciplinary research. Recently, the focus of research has shifted from studying the effects of ozone to multiple pollutant effects. Recent studies have indicated that the combination of ozone and nitrogen may alter biomass allocation in pines towards that of deciduous trees, accelerate litter accumulation, and increase carbon sequestration rates in heavily polluted forests. Further study of the effects of multiple pollutants, and their long-term consequences on the mixed conifer ecosystem, cannot be adequately done using the original San Bernardino Mountains Air Pollution Gradient network. To correct deficiencies in the design, the new site network is being configured for long-term studies on multiple air pollutant concentrations and deposition, physiological and biochemical changes in trees, growth and composition of over-story species, biogeochemical cycling including carbon cycling and sequestration, water quality, and biodiversity of forest ecosystems. Eleven sites have been re-established. A comparison of 1974 stand composition with data from 2000 stand composition indicate that significant changes in species composition have occurred at some sites with less change at other sites. Moist, high-pollution sites have experienced the greatest amount of forest change, while dryer low-pollution sites have experienced the least amount of stand change. In general, ponderosa pine had the lowest basal area increases and the highest mortality across the San Bernardino Mountains.

Over the next ten to twenty years, California's population is projected to increase, particularly in air basins upwind of the Sierra Nevada mountain range (i.e., San Francisco Bay Area, Sacramento and San Joaquin Valleys). Related trends in ozone (O 3 ) concentrations are a matter of special concern, due to a long history of O 3 injury to sensitive pines in national forests of the Sierra Nevada. While the USDA Forest Service has limited authority over polluted air masses crossing into forest boundaries, monitoring results can be used in a collaborative effort with state and federal environmental protection agencies to protect resources at risk. One such effort was initiated under the Sierra Nevada Framework for Conservation and Collaboration. An ambient O 3 concentrations and O 3 effects monitoring system is under development that will integrate previous and current monitoring efforts across agencies, and will track pollutant concentrations and effects over the entire bioregion.

A of biologists, recruited from the AIBS membership, was formed under the auspices of People-to-People Ambassador Programs. Our delegation, led by Judith Weis, traveled to South Africa in January 2002. Members of the delegation had interests in ecology, conservation biology, environmental protection, park planning and management, and environmental education. We came from academia, government agencies, and the private sector. Our goals were to participate in formal and informal exchanges with scientists and resource managers, to spend time in the field, and to learn directly about ecology and environmental problems and solutions from a South African perspective. Unlike the Constitution of the United States, the Constitution of the Republic of South Africa recognizes that a healthy environment is a basic human right. It specifies that the government must prevent pollution and damage to the environment, promote conservation, and balance economic, social, and environmental development. Since the adoption of the new constitution in 1994, several environmental laws have been passed, including the new National Water Act, the National Forest Act, the Marine Living Resources Act, and the National Environmental Management Act (DEAT 1999). South Africa is a dry country: More than half of the country (60 percent) is arid or semiarid, it has only one lake, just 10 percent of the area receives more than 750 millimeters of rain, and few rivers are permanent (Van Riet et al. 1997). Before widespread human settlement, the vegetation ranged from desert and stem-succulent shrublands in the west to savannas and grasslands on the high central plateau; a few remnant native forests in the east; and the unique fynbos vegetation in the Cape region (Cowling et al. 1997). The limited supply of fresh water is overexploited by agriculture, industry, forest plantations, and invasive plant species. In the past 100 years, agriculture and forestry have replaced large natural areas, and mining has altered landscapes. Urbanization is occurring in many areas, and squatter settlements are common. The previous apartheid policies exacerbated environmental problems by imposing artificial barriers to the natural movement of goods and services and by concentrating people in unsustainable “homelands.”Over half of the wetland areas have been claimed for agricultural uses. South Africa is experiencing the same environmental pressures that are typical throughout the world: increased

Cette etude a ete realisee en 1989 et 1990 a Shirley Meadow dans le sud de la sierra Nevada en Californie. L'objectif consistait a evaluer les effets de l'ozone (O 3 ) sur la reponse physiologique et la croissance de semis de Pinus ponderosa Dougl. ex P. & C. Laws. (pin ponderosa). Les semis on ete exposes a trois niveaux de O 3 dans des chambres a ciel ouvert (air filtree au charbon de bois (CF), niveau ambiant de O 3 (1xO 3 ) ou deux fois le niveau ambiant de O 3 (2xO 3 ));les concentrations moyennes pour 24 h etaient respectivement de ∼20, ∼60 et ∼120 ppb. Au cours de la premiere saison de croissance, les aiguilles de l'annee chez les plants exposes a 2xO 3 ont subi des dommages plus prononces ainsi que des diminutions periodiques de la fluorescence chlorophyllienne et de la photosynthese nette, mais O 3 n'a eu aucun effet significatif sur la croissance. Au cours de la seconde saison de croissance, les aiguilles de 1 an exposees a 2xO 3 ont subi une reduction significative de leurs reponses physiologique et biochimique ainsi qu'une diminution de croissance comparativement aux plants exposes a CF et a 1xO 3 . Ces resultats laissent entrevoir la possibilite que O 3 cause une diminution insidieuse de la croissance chez les semis de pin ponderosa apres plusieurs saisons de croissance successives.

summary Nitrogen (N) resorbed from foliage before leaf abscission is a significant source of N for new leaves. The widely distributed photochemical air pollutant ozone (O 3 ) accelerates foliar senescence and leaf abscission and therefore could interfere with the process of N resorption. The objective of this study was to determine the effects of O. and drought stress on the N economy of ponderosa pine ( Pinus ponderosa Laws.), a species highly susceptible to O Potted ponderosa pine trees (2‐yr‐old) were exposed for 3 yr to three concentrations of O 3 and two soil water regimes in open‐top chambers in the Sierra Nevada mountains of California. One‐third of the trees was harvested at the end of each exposure season and each age class of needles, branches, stems, coarse roots, fine roots, and leaf litter was analysed for N. Exposure to a seasonal O 3 mean of 88 ppb increased N concentrations in pine foliage, particularly in current‐year needles. Current‐year foliage from well‐watered (WW) trees exposed to elevated O averaged 34% higher N than that from trees in charcoal‐filtered (CF) chambers. Drought‐stressed (DS) trees from all O 3 treatments averaged 10% higher N in current‐year needles than WW seedlings. Resorbed N from older age classes of needles from WW trees increased from 32% of current‐year needle N in CF seedlings to 51% of current‐year N in the high O 3 treatment. Nitrogen resorbed from older needles of DS trees increased from 15% in CF to 38% of current‐year N in the high O 3 treatment. Thus O 3 ‐injured ponderosa pine seedlings increased resorption of N from older needles and increased partitioning of N to current‐year foliage. The increased N in current‐year needles facilitated increased rates of photosynthesis in these needles, thus partly compensating for the O 3 ‐induced loss of older leaves.

Foliar injury symptoms and stem diameter growth were measured on well-watered and drought-stressed ponderosa pine (Pinusponderosa Dougl. ex Laws.) seedlings at the end of each of 3 years of exposure to three levels of ozone: charcoal-filtered air, nonfiltered air, and nonfiltered air plus 1.5 times ambient ozone. Ozone-injury indices were constructed by adding percent chlorotic mottle and percent necrosis for each needle age-class. Percent necrosis was weighted from 1 to 5 times to construct different indices. Seedlings grown in nonfiltered air plus 1.5 times ambient ozone developed severe foliar injury after 2 years of exposure and were the only seedlings with significant reductions in radial growth after three seasons of exposure to a mean seasonal ozone concentration of 88 ppb. Foliage that had developed >30% chlorotic mottle by September of the 2nd year had abscised by September of the following year. Reduction in radial growth was significantly correlated with amount of foliar injury in well-watered trees, and the best-fit regression equation occurred when percent necrosis was weighted by a factor of 4. Regressions between foliar-injury indices and radial growth in drought-stressed trees were not significant, nor were regressions between radial growth and foliar injury among well-watered trees with only 1 year of premature needle abscission. The low R2 (0.30) between foliar injury and radial growth in well-watered trees and the lack of a significant regression in drought-stressed trees suggest that detection of reductions in stem diameter growth of ponderosa pine in the field in response to ozone injury could be difficult, except for severely injured trees with fewer than 2 years of foliar retention.

A 3-year field study of the cumulative effects of ozone (O3), wet and dry acidic deposition, and soil water availability was conducted on ponderosa pine (Pinusponderosa Laws.) in the Sierra Nevada of California from 1988 to 1990. Thirty-six 2-year-old potted seedlings were placed in each of 30 chambers and exposed from May through October to three levels of O3 (charcoal-filtered (CF), nonfiltered (NF), and NF plus 1.5 times ambient O3 (NF150)); three levels of acidity in simulated rain (pH 3.5, 4.4, 5.3); two levels of dry deposition (60 or 90% filtration), and two levels of soil water availability (well watered (WW) or drought stressed (DS)). An additional six plots served as ambient air (AA) controls. One-third (432) of the trees were harvested at the end of each exposure season. Low soil water availability was the only stress factor to significantly affect growth following the first exposure season. After the second season, O3 significantly reduced foliar biomass in WW–NF150 trees, but DS seedlings did not respond to O3. After 3 years of exposure, WW–NF150 trees averaged 70% loss of 1988 needles and 48% loss of 1989 foliage. Ozone-injured seedlings compensated for these losses by increased growth of current-year needles and stems and also increased growth of fine feeder roots. Radial stem growth and coarse-root growth were significantly reduced in O3-injured trees. DS trees in NF150 chambers averaged half the needle loss of WW trees and showed no reduction in radial growth in response to O3. Rain pH and dry deposition had no direct effects on growth of ponderosa pine. These cumulative responses to interacting stresses indicate the importance of multifactorial, long-term studies to evaluate forest tree responses to atmospheric deposition.

In 1989 and 1990, the effects of multi-year ozone exposures on growth, foliar injury and physiological responses in ponderosa pine were examined. Two-year old seedlings were exposed to four ozone treatments in open-top chambers: clean air (subambient levels of oxidants and particles); ambient ozone; twice-ambient ozone; or ambient air. The study was performed at Shirley Meadow in the southern Sierra Nevada. In both years, ambient ozone levels were representative of other forests in the region. While ozone is the most phytotoxic air pollutant, seedlings also experienced elevated concentrations of nitric acid and ammonia. In 1990, ambient ozone significantly increased injury to previous year needles. Premature senescence and alterations in physiological responses were also noted. Exposure to twice-ambient ozone reduced seedling biomass, increased injury and caused decreases in a variety of physiological responses.

Seedlings of ponderosa pine (Pinus ponderosa Laws.) were grown for three years under three atmospheric ozone concentrations - clean air (CF), ambient ozone (NF), and 1.5 times ambient ozone (NF1 50) - at a moderately-polluted site in the Sierra Nevada, under either well-watered or drought-stressed conditions. When the trees were 5 years old, photosynthetic capacities of 2-year-old, 1-year-old, and current-year needles were measured during August and September of the 3rd season of exposure. Current-year needles of NF150 trees had higher photosynthetic capacity than NF and CF trees during late summer, an effect due to greatly enhanced photosynthesis in well-watered plants that had lost older needles as a result of ozone damage. This photosynthetic compensation in well-watered NF150 seedlings was related to higher tissue nitrogen concentration in the current-year foliage and possibly to increased inorganic phosphate cycling, both responses to the loss of older needles. Drought-stressed NF150 seedlings were partially protected from ozone damage by decreased stomatal conductance and did not exhibit the same degree of photosynthetic compensation. No differences in photosynthetic rate were found between CF and NF seedlings or between well-watered and drought-stressed seedlings (across ozone treatments) in any needle age class.

A 3-year field study of the potential interactions of ozone (O3), wet and dry acidic deposition, and soil water availability on foliar injury responses of 18 ponderosa pine (Pinus ponderosa Laws.) families was conducted in the Sierra Nevadas of California. Thirty-six 2-year-old seedlings in each of 30 open-top chambers (plus six ambient air plots) were exposed to combinations of three levels of O3 [charcoal-filtered (CF), non-charcoal filtered (NF), and NF plus 1.5 times ambient O3 (NF150)]; three simulated rain pH treatments (pH 3.5, 4.4, 5.3); two levels of dry deposition (60% filtration of ambient dry deposition and 90% filtration), and two irrigation regimes (approximately weekly watering vs irrigation every other week) for three growing seasons. One-third of the trees were harvested at the end of each year. O3, irrigation level, amount of dry deposition, and family (genotype) significantly affected degree of foliar injury responses to O3. The interactions of O3 with irrigation amount and O3 with dry deposition were also statistically significant. Drought-stressed seedlings had significantly less O3 injury than well-watered trees, but seedlings exposed to 60% filtration of dry deposition had significantly greater O3 injury than those in the 90% filtered treatments. Ponderosa pine families differed greatly in susceptibility to O3, ranging from two with an average of > 20% O3 injury to several with nearly no O3 injury. These results reflect the complex patterns ofponderosa pine responses to natural and pollutant stresses and emphasize the importance of long-term, multifactorial experiments to elucidate those patterns.

Four cultivars field-grown Phaseolus vulgaris L., differing in their morphological characteristics, were exposed to charcoal-filtered air (CF), non-filtered air (NF), or 1.5 times greater than ambient ozone O3 concentrations from July to September in Riverside, California (USA). The objective of the study was to determine the relative susceptibility of the four cultivars to O3 and to relate susceptibility to O3 to the physiological characteristics of the cultivars, particularly to rates of stomatal conductance. Compared with bean yields in CF chambers. ambient O3 reduced yields by 65% for ‘Sutter Pink’, 73% for ‘Yolano Pink’ and 44% for ‘Sal Small White’. One cultivar, ‘Linden Red Kidney’, had increased yields in NF relative to CF treatments. The sequence of susceptibility to O3 for both visible foliar injury and reductions in yield in the four cultivars was the same as that for the rates of stomatal conductance. The rate of maximum conductance was significantly positively correlated with both yields in CF chambers (r = 0.77; P < 0.05) and with O3-induced reductions in conductance (r=0.93; P < 0.05). The results suggests that measurements of stomatal conductance in bean cultivars could be used to screen for cultivars potentially susceptible or resistant to O3-induced yield losses in the field.