Forest Growth Responses to the Pollution Climate of the 21st Century

The tall, aerodynamically rough surfaces of forests provide for the efficient exchange of heat and momentum between terrestrial surfaces and the atmosphere. The same properties of forests also provide for large potential rates of deposition of pollutant gases, aerosols and cloud droplets. For some reactive pollutant gases, including SO₂, HNO₃ and NH₃, rates of deposition may be large and substantially larger than onto shorter vegetation and is the cause of the so called “filtering effect” of forest canopies. Pollutant inputs to moorland and forest have been compared using measured ambient concentrations from an unpolluted site in southern Scotland and a more polluted site in south eastern Germany. The inputs of S and N to forest at the Scottish site exceed moorland by 16% and 31% respectively with inputs of 7.3 kg S ha−1 y and 10.6 kg N ha−1 y−1. At the continental site inputs to the forest were 43% and 48% larger than over moorland for S and N deposition with totals of 53.6 kg S ha−1 y−1 and 69.5 kg N ha−1 y−1 respectively.The inputs of acidity to global forests show that in 1985 most of the areas receiving > 1 kg H+ ha−1 y−1 as S are in the temperate latitudes, with 8% of total global forest exceeding this threshold. By 2050, 17% of global forest will be receiving > 1 kg H−1 ha−1 as S and most of the increase is in tropical and sub-tropical countries.Forests throughout the world are also exposed to elevated concentrations of ozone. Taking 60 ppb O₃ as a concentration likely to be phytotoxic to sensitive forest species, a global model has been used to simulate the global exposure of forests to potentially phytotoxic O₃ concentrations for the years 1860, 1950, 1970, 1990 and 2100. The model shows no exposure to concentrations in excess of 60 ppb in 1860, and of the 6% of global forest exposed to concentrations > 60 ppb in 1950, 75% were in temperate latitudes and 25% in the tropics. By 1990 24% of global forest is exposed to O₃ concentrates > 60 ppb, and this increases to almost 50% of global forest by 2100. While the uncertainty in the future pollution climate of global forest is considerable, the likely impact of O₃ and acid deposition is even more difficult to assess because of interactions between these pollutants and substantial changes in ambient CO₂ concentration, N deposition and climate over the same period, but the effects are unlikely to be beneficial overall.

Natural environmental changes and human activities have altered forest growth for centuries. Recent long-term growth investigations indicate an increasing growth trend in European forests. The investigations are based on forest inventory, permanent plot and tree analysis data. The observed trends are species specific, locally varying and modified by remarkably large periodic growth variations. On a European scale, species and site specific quantitative information about the extent and spatial as well as temporal variation in growth acceleration is lacking. Future growth development may differ from past observations. A better understanding of changes in site conditions, their causes and consequences is needed to guide sustainable management of European forests.

Wet and dry deposition of atmospheric nitrogen (N) compounds into forest ecosystems and their effect on physical, chemical and microbial processes in the soil has attracted considerable attention for many years. Still the consequences of atmospheric N deposition on N metabolism of trees and its interaction with soil microbial processes has only recently been studied. Atmospheric N deposited to the leaves is thought to enter the general N metabolism of the leaves, but the processes involved, the interaction with different metabolic pathways, and the connection between injury by atmospheric N and its metabolic conversion are largely unknown.Laboratory and field experiments have shown that N of atmospheric NO₂ and NH₃, desposited to the leaves of trees, is subject to long-distance transport in the phloem to the roots. This allocation can result in considerable decline of N uptake by the roots. Apparently, the flux of N from the soil into the roots can be down-regulated to an extent that equals N influx into the leaves. This down-regulation is not mediated by generally enhanced amino-N contents, but by elevated levels of particular amino acids. Field experiments confirm these results from laboratory studies: Nitrate (NO₃) uptake by the roots of trees at a field sites exposed to high loads of atmospheric N is negligible, provided concentrations of Gln in the roots are high.At the ecosystem level, consequences of reduced N uptake by the roots of trees exposed to high loads of atmospheric N are (1) an increased availability of N for soil microbial processes, (2) enhanced emission of gaseous N-oxides from the soil, and (3) elevated leaching of NO₃ into the ground water. How recent forest management practices aimed at transforming uniform monocultures to more structured species-rich forests will interact with these processes remains to be seen. Possible implications of these forest management practices on N metabolism in trees and N conversion in the soil are discussed particularly in relation to atmospheric N deposition.

The effects of pollutant nitrogen on forest soils and the potential feedbacks on tree growth are discussed using data from recent plot and catchment manipulation studies. Results indicate that N applied to the soil affects both soil N transformations and base cation status in some forest soils. Whilst reductions in tree growth are infrequently reported, a greater understanding of the effect of increased N deposition on soil N transformations, and associated changes in soil acidification and nutritional balance, is essential if sensitive stands are to identified. In particular, the factors controlling soil N accumulation rates and thus the onset of nitrate leaching are not clearly understood with increased nitrogen availability potentially increasing nitrogen accumulation due to abiotic fixation and lignolytic enzyme suppression, whilst the onset of nitrification as the C/N ratio of forest floor material declines, may reduce N retention efficiency. The switch from increased to decreased tree growth rate in response to N additions in some experiments highlights the need for long-term studies as a necessary component of future research.

Atmospheric deposition of strong acid and sulphur compounds has been measured at Danish forest sites since 1985. Results from 12 years of measurements are reported from Ulborg experimental forest site situated on the sandy soils of West Jutland. Measurements and model calculated deposition estimates indicate a cumulated acid input from the atmosphere of 40 kmoL during the last 30 years; far more than the forest soils could neutralise by weathering or by base cation exchange. During the period 1985–1997 an increase in soil water concentration of protons (acid) and aluminium were seen and the molar ratio between calcium and aluminium dropped to a minimum of 0.1. A decrease in forest growth and an increase in needle litter fall were seen in the same period.

Long-term and short-term N deposition effects on N₂O and NO emissions from forest soils were compared. Long-term NH₃ deposition (> 20 years) from a poultry farm to a downwind woodland (decreasing from 73 to 18 kg N ha−1 y−1, 30 to 110 m downwind of the farm) resulted in the re-emission of 6% and 14% of NH₃-N deposited as N₂O-N and NO-N, respectively. However, when in short-term (2–3 years) field experiments the atmospheric N deposition to mature conifer plantations was raised by fumigation with NH₃ tol5 kg N ha−1 y−1 or by acid mist to 48 and 96 kg N ha−1 y−1 the N deposited was immobilised. In the acid mist experiment more than 2 years of acid mist (48 and 96 kg N ha−1 y−1) were required to significantly increase N₂O emissions from −0.3 μg N₂O-N m−2 h−1 (control) to 0.5 and 5.7 μg N₂O-N m−2 h−1, respectively. This suggests, that N deposition simulation studies in soil ecosystems, which have previously not been exposed to high rates of N (by deposition or fertilisation), need to be long-term. Also, measurements of N₂O and/or NO may be a non-destructive, quick indicator of the N status of the soil.

In permanent observation plots across Switzerland, nitrogen (N) concentration in the foliage of mature beech has increased by 15% and phosphorus (P) concentrations in beech and mature Norway spruce decreased by 12 and 13% respectively between 1984 and 1995, leading to increased N:P ratios. Modelled N deposition was correlated with stem increment in both beech and spruce, with indications of P limitation in some beech plots. Experimental application of 0–160 kg N ha−1 yr−1 over four to five years caused nutrient imbalances in various afforestation plots comparable to those observed in the permanent observation plots. The changes in the trees caused by N treatment led to increased attacks by parasites such as Apiognomonia errabunda, Phomopsis sp., Phyllaphis fagi in beech and Botrytis cinerea, Sacchiphantes abietis and Cinara pilicornis in Norway spruce. The results suggest current N deposition in Switzerland induces significant changes in the forest ecosystem.

The effects of enhanced (NH₄)₂SO₄ (NS) deposition on Norway spruce (Picea abies [L.] Karst) fine root biomass, vitality and chemistry were investigated using root-free in-growth cores reproducing native organic and mineral soil horizons. The cores were covered and watered every 2 weeks with native throughfall or throughfall supplemented with NS to increase deposition by 75 kg ha−1 a−1 NH₄+-N (86 kg ha−1 a−1 SO₄−2 -S). The in-growth cores were sampled after 19 months and assessed for root biomass, necromass, length, tip number, tip vitality and fine root chemistry. Root biomass and fine root aluminium (Al) concentration were negatively correlated, but NS deposition had no effect on root growth or root tip vitality. NS deposition caused increased fine root nitrogen (N) concentrations in the organic horizon and increased Calcium (Ca) concentrations in the mineral horizon. Fine root biomass was higher in the organic horizon, where fine root Al and potassium (K) concentrations were lower and Ca concentrations higher than in the mineral horizon. Results highlighted the importance of soil stratification on fine root growth and chemical composition.

Measurements were made of NO₃-N and NH₄-N in bulk deposition, throughfall and soil solution on six permanent plots in pine and spruce stands located along a transect from the south to the north of Poland. Location differed both in the level of air pollution level and in climatic parameters. The total N load calculated from throughfall ranged from 12.5 to 34 kg−la−1. The load of NH₄-N exceeded the NO₃-N contribution. Differences in total N load were not reflected in foliar N concentration. Present forest health status of stands determined by defoliation class, and do not appear to be related to their N deposition.

Nitrogen (N) was added over two years to a spruce-dominated (Picea abler) montane forest at Alptal, central Switzerland. A solution of ammonium nitrate (NH₄NO₂) was frequently sprinkled on the forest floor (1500 m2) to simulate an additional input of 30 kg N ha−1 yr−1 over the ambient 12 kg bulk inorganic N deposition. The added nitrogen was labelled with 15NH₄15NO₃ during the first year. Results are compared to a control plot.Neither the trees nor the ground vegetation showed any increase in their N content. Only 4.1% of N in the ground vegetation came from the N addition. Current-year needles contained 11 mg N g−1 dry weight, of which only 2% was from labelled N; older needles had approximately half as much 15N. The uptake from the treatment was therefore very small. Redistribution of N also took place in the trunks: 1 to 2-year-old wood contained 0.7% labelled N, tree rings dating back 3 to 14 years contained 0.4%.Altogether, the above-ground vegetation took up 12% of the labelled N. Most 15N was recovered in the soil: 13% in litter and roots, 63% in the sieved soil. Nitrate leaching accounted for 10%. Factors thought to be influencing N uptake are discussed in relation to plant use of N and soil conditions.

In the last three decades high industrial nitrogen (N) emissions have led to eutrophication of a Scots pine stand (Pinus sylvestris [L.]) near Schwedt characterized by a broad cover of Calamagrostis epigejos [L.] Roth (Poaceae). In comparison to the relatively unimpacted control site (low N site), this high N site showed a remarkably low mycorrhizal frequency (percentage of mycorrhizas on total amount of root tips) with seasonal lows down to 27%. At the low N site the highest number of mycorrhizal root tips was found in the organic layer. At the high N site the amount of mycorrhizas per soil volume was similar in both organic and mineral soil layers, and also significantly lower when compared to the amount at the low N site. The high N site revealed only nine mycorrhizal morphotypes instead of eighteen found at each sampling date at the control site. 80% of the coenosis at the high N site were represented by only four morphotypes resulting in a low diversity. The seasonal decrease in the mycorrhizal frequency, the small amount of mycorrhizas and the low diversity suggest that the high N deposition at this site has reduced the ability of the pine trees to withstand natural stresses such as prolonged drought or frost periods. This corresponds well with the 42% reduction in tree stocking density at the high N site compared to the low N site.

For the first time concentrations of trace nitrogenous (N) air pollutants, gaseous nitric acid (HNO₃), nitrous acid (HNO₂), ammonia (NH₃), and fine particulate nitrate (NO₃) and ammonium (NH₄), were measured in the montane forests of southern Poland. Determinations were performed in two forest locations of the Silesian Beskid Mountains in the western range of the Carpathian Mountains, and in an industrial/urban location in Katowice, Poland. The measurements performed in summer 1997 with honeycomb denuder/filter pack systems showed elevated concentrations of the studied pollutants. These findings agree with the low carbon/nitrogen (C/N) ratios and the results of 15N analyses of soil and moss samples. High concentrations of N air pollutants help to explain previously determined high levels of NO₃ and NH₄ deposition to Norway spruce (Picea abies Karst.) canopies in these mountains. Ambient concentrations of sulfur dioxide (SO₂) and ozone (O₃) were elevated and potentially phytotoxic. Deficiencies of phosphorus (P) and magnesium (Mg) in Norway spruce foliage were found while concentrations of other nutrients were normal.

The perceived health of forest ecosystems over large temporal and spatial scales can be strongly influenced by the frames of reference chosen to evaluate both forest condition and the functional integrity of sustaining forest processes. North American forests are diverse in range, species composition, past disturbance history, and current management practices. Thereforethe implications of changes in environmental stress fromatinospheric pollution and/or global climate change on health of these forests will vary widely across the landscape. Forest health surveys that focus on the average forest condition may do a credible job of representing the near -term trends in economic value while failing to detect fundamental changes in the processes by which these values are sustained over the longer term. Indications of increased levels of environmental stress on forest growth and nutrient cycles are currently apparent in several forest types in North America. Measurements of forest ecophysiological responses to air pollutants in integrated case studies with four forest types (southern pine, western pine, high elevation red spruce, and northeastern hardwoods) indicate that ambient levels of ozone and/or acidic deposition can alter basic processes of water, carbon, and nutrient allocation by forest trees. These changes then provide a mechanistic basis for pollutant stress to enhance a wider range of natural stresses that also affect and are affected by these resources. Future climatic changes may ameliorate (+ CO₂) or exacerbate (+ temperature, + UV-B) these effects. Current projections of forest responses to global climate change do not consider important physiological changes induced by air pollutants that may amplify climatic stresses. These include reduced rooting mass, depth, and function, increased respiration, and reduced water use efficiency. Monitoring and understanding the relative roles of natural and anthropogenic stress in influencing future forest health will require programs that are structured to evaluate responses at appropriate frequencies across gradients in both forest resources and the stresses that influence them. Such programs must also be accompanied by supplemental process -oriented and pattern -oriented investigations that more thoroughly test cause and effect relationships among stresses and responses of both forests and the biogeochemical cycles that sustain them.

Tropospheric ozone (O₃) may adversely affect tree growth, with critical levels for O₃, being exceeded in many parts of Europe. However, unequivocal evidence for O₃-induced foliar injury on woody species under field conditions has only been found in a few places. Visible O₃ injury appears to occur mainly in the Mediterranean Basin, which is also the area where the least amount of information is available on O₃ exposure as well as the sensitivity of individual species. Overall, the quantitative risk assessment of O₃ impacts on mature trees and forests is vague at the European scale, as most knowledge is derived from controlled O₃ fumigations of young trees, grown in isolation in exposure chambers. Research suggests that risks exist, but these need to be validated for stand conditions. O₃-induced changes in resource allocation rather than productivity appear to be crucial as they affect competitiveness and predisposition to parasite attack and may eventually lead to the loss of genetic diversity. ‘Free-air’ O₃ fumigations in forest canopies may reveal processes that are susceptible to O₃ stress under field conditions and provide a scientific basis towards quantitative risk assessment and realistic definitions of critical levels for O₃ in forest ecosystems.

Tropospheric ozone is considered as the major pollutant of concern to the health and productivity of forests in the eastern United States and has more recently become of increasing concern within the forests of southern Europe. Recent observations have clearly demonstrated foliar injury symptoms to be occurring on many tree and native plant species within remote forested areas. Several plant genera (and a few species within genera) found in both the forests of Switzerland and the southern coastal region of Spain exhibit field symptoms typical of ambient ozone exposures. Ozone exposures for many species have been conducted under controlled CSTR conditions and within open-top chambers within the study areas. Results have confirmed that the O₃-like foliar symptoms as observed under natural forest and open grown conditions for many native tree, shrub, and herbaceous species in Spain and Switzerland are caused by exposures to ambient O₃.

Ponderosa pines (Pinus ponderosa Dougl. ex. Laws) 21 to 60 yr old were used to assess the relative importance of environmental stressors (O_3, drought) versus an enhancer (N deposition) on foliar retention, components of aboveground growth, and whole tree biomass allocation. Sites were chosen across a well-described gradient in ozone exposure (40 to 80 ppb per h, 24 h basis, 6 month growing season) and nitrogen deposition (5 to 40 kg ha−1 yr−1) in the San Bernardino Mountains east of Los Angeles, California. A high level of chlorotic mottle indicated high 0, injury at sites closest to the pollution source, despite potential for the mitigating effects of N deposition. At the least polluted site, foliar biomass was evenly distributed across three of the five needle-age classes retained. At the most polluted site, 95% of the foliar biomass was found in the current year’s growth. High N deposition and O₃ exposure combined to shift biomass allocation in pine to that of a deciduous tree with one overwintering needle age class. Based on whole tree harvests, root biomass was lowest at sites with the highest pollution exposure, confirming previous chamber exposure and field studies. Aboveground growth responses in the high-pollution sites were opposite to those expected for O₃ injury. Needle and lateral branch elongation growth, and measures of wood production increased with increasing proximity to the pollution source. An enhancement of these growth attributes suggested that N deposition dominated the ponderosa pine response despite high O₃ exposure.

Damage of Pinus ponderosa Dougl. Ex P. and C. Laws. in the San Bernardino Mountains in Southern California is a well-known example for ozone-phytotoxic effects in forests. In October 1997, needles were collected at the severely impacted Dogwood plot, where pines showing severe crown thinning (‘symptomatic’) and asymptomatic ones grow together. Visible chlorosis was not or only mildly developed on the sampled current year’s (c) and previous year‘s (c+1)needles at the time of collection. In sun exposed needles of symptomatic trees, but not in needles which were dark adapted overnight, significantly lower glutathione contents were found compared to asymptomatic trees. In asymptomatic trees sun-exposed and dark adapted needles contained 85 to 90% of the glutathione pool in the reduced state, whereas in sun exposed needles of symptomatic trees a significantly higher proportion of oxidised glutathione was observed (>20%). The redox state of the ascorbate pool remained unaffected, but needles of asymptomatic trees showed a significant decrease of total ascorbate upon darkening. Tocopherol contents were not significantly different between symptomatic and asymptomatic individuals. The results indicate oxidative alterations in needles of symptomatic trees. Since these changes are detectable before severe visual symptoms appear on the needles, they represent initial physiological damage and might be useful for an early risk assessment of ozone phytotoxic potential on pine trees in the field.

During the summer of 1991 ozone injury trend plots in Great Smoky Mountains National Park, USA, consisting of mature black cherry, sassafras and yellow-poplar were established near three ozone monitors, ranging in elevation from 597–1265 m. Beginning in mid-August 1991–1993, three exposed branches each from the upper- and mid- to lower-crown of each tree were collected and evaluated for ozone injury. Of the trees examined, 63%, 52% and 36% exhibited some amount of foliar injury in 1991, 1992 and 1993, respectively. Ozone injury across species was the greatest at Cove Mountain in all three years of the study. Overall, across sites and years, 11, 12 and 11% of all leaves examined exhibited visible injury for black cherry, sassafras and yellow-poplar, respectively. The percentage of injured leaves per branch was greater in the mid- to lower-canopy for black cherry, across all sites. Trees for each species that exhibited the greatest or least amounts of visible injury did so in all three years of the study, indicating a differential sensitivity within each species population. No significant ozone exposure-tree response relationships were observed with any variable tested. These data indicate that ozone concentrations are high enough to cause visible symptoms to selected trees within Great Smoky Mountains National Park, USA.

Incidence and severity of foliar symptoms due to ambient ozone exposures were documented on mature black cherry (Prunus serotina) in two National Parks [Great Smoky Mountains National Park (GRSM) and Shenandoah National Park (SHEN)] in the Appalachian Mountains of the eastern USA during the summer of 1991–1993. Three plots in each park containing 30 trees each (Big Meadows in SHEN had 60 trees) with 90 and 120 trees total trees were evaluated in GRSM and SHEN, respectively. Plots were established at different elevations adjacent to ozone monitoring stations. Samples of foliage were collected and three exposed branches from the upper-crown and three branches from the mid-to-lower crown were examined for symptoms of foliar ozone injury. Incidence was greatest in 1991 at both locations; 60% and 45% for GRSM and SHEN, respectively. In 1992 and 1993, incidence was very similar in both parks, with approximately 33% of the trees affected. Black cherry at the highest elevations exhibited the greatest amount of symptoms in both parks all three years of the study. These sites also exhibited the highest ozone concentrations. In addition, the percent of trees injured by ozone was positively correlated with SUM06 and W126. These results along with forest surveys and open-top chamber studies indicate that black cherry may be a reliable bioindicator of foliar injury due to ambient ozone.

Since 1985, severe defoliation and foliar symptoms have been observed on Ailanthus altissima Desf. ratmets located at a site where high ozone concentration (up to 128 n1·1−1), strongly acidic drizzle and dew (to pH 1.4) were recorded. The behaviour of plants growing in an open field (directly exposed to the pollutants), and below the canopy of an old stone pine stand was compared. Compared to the below-canopy trees, the open field trees show a variety of foliar symptoms (diffuse yellowing, apical, marginal and spot-like necrosis and interveinal bronzing and stippling), defoliation and much less vegetative vigour. Experimental treatments with acidic mist caused the same necrosis observed in the field, but failed to reproduce bronzing and stippling. Since then, stipple and bronzing have been observed again at a variety of locations in the same area as well as throughout Italy, while necrosis has only seldom been recorded. Recently, surveys on symptoms have been supported by anatomical analysis by mean of light microscope and by experiments in fumigation chambers. On the whole, the results led us to suspect ozone sensitivity. These findings suggest that the symptoms observed since 1985 in the surveyed area were probably caused by the combined effects of both strongly acidic episodes and exposure to high ozone levels.

An experimental study was performed in open-top chambers to assess the relative O₃-sensitivity of the following Mediterranean woody plants: Quercus ilex ssp. ilex, Quercus ilex ssp. ballota, Olea europaea ssp. sylvestris, Ceratonia siliqua and Arbutus unedo. Assessment of O₃ ‘sensitivity’ was based on the development of O₃-induced visible injury and the extent of the reduction in relative growth rate induced by O₃. Two-year-old seedlings were exposed in open-top chambers to charcoal filtered air (CFA), Non-filtered air (NFA) or Non-filtered air supplied with 40 ppb O₃ from 09:00 to 18:00 hours (NFA+40). No clear relationships were found between the development of visible injury and effects on plant growth or accumulated biomass. Olea europea ssp. sylvestris and Quercus ilex ssp. ballota appeared to be the most O₃-sensitive tree species screened; a ten month exposure to ambient O₃ levels (AOT40 = 12.4 ppm.h) caused reductions in height and/or stem growth. Further research is needed to characterise the physiological, biochemical and anatomical characteristics that may underlie the ‘sensitivity’ of certain Mediterranean species to O₃

Urban air basins produce many pollutants, such as oxides of nitrogen (NOx) and ozone, which move into rural areas. NOx itself has been shown not to be especially phytotoxic except at high levels. More problematical is the conversion of NOx into inorganic nitrogen that the plant can utilize. In ecosystems which are nitrogen-limited, excess nitrogen can radically increase plant growth by stimulating the nitrogen-handling metabolic pathways which then alters “normal” carbon (C) metabolism. On the other hand, ozone alters plant productivity by inducing a decline in growth, speeding developmental events to senescence, lowering reproductive rates, and impairing resistance to other stresses. Three major metabolic events induced by ozone exposure have been well studied: 1) increased turnover of antioxidant systems, 2) production of symptoms similar to tissue wounding, especially ethylene production; and 3) decline in photosynthesis. In order to understand the effect upon productivity of any pollutant, one must understand the mechanisms by which that effect is generated. Those mechanisms must not be merely descriptive statements, but must be firmly modeled with predictions. Any model will be highly complex; however here, the focus will be upon C metabolism. Both a closure of the stomata and a decline in the ability to fix CO₂ within the chloroplast can alter the photosynthetic rate of leaves. Yet the movement of C throughout the plant (translocation) seems also to be altered by pollutants. The loss of carbohydrate to roots and growing shoot tips has a profound effect upon the plant’s ability to respond normally to the integrated effects of the total environment; loss may cause the most pronounced and long lasting dilemma for plants.

Elevated carbon dioxide concentrations and limited water supply have been shown to reduce the impact of ozone pollution on the growth and physiology of Quercus petraea in a long-term factorial experiment. These responses can be explained by observed reductions in stomatal conductance, and thus potential ozone exposure of 28% and 40% for CO₂ and drought treatments respectively. However, parameterisation of a stomatal conductance model for Quercus robur and Fagus sylvatica grown under ambient and elevated CO₂ concentrations in a separate experiment has demonstrated that elevated CO₂ also reduces the responsiveness of stomata to both saturation deficit (LAVPD) and soil moisture deficit (ψ) in beech, and to a lesser extent, in oak. Season-long model simulations of ozone fluxes suggest that LAVPD and ψ conductance parameters derived at ambient CO₂ concentrations will lead to these fluxes being underestimated by 24% and 2% for beech and oak respectively at 615 ppm CO₂.

Over the years, a series of trembling aspen (Populus tremuloides Michx.) clones differing in O₃ sensitivity have been identified from OTC studies. Three clones (216 and 271[(O₃ tolerant] and 259 [O₃ sensitive]) have been characterized for O₃ sensitivity by growth and biomass responses, foliar symptoms, gas exchange, chlorophyll content, epicuticular wax characteristics, and antioxidant production. In this study we compared the responses of these same clones exposed to O₃ under field conditions along a natural O₃ gradient and in a Free-Air CO₂ and O₃ Enrichment (FACE) facility. In addition, we examined how elevated CO₂ affected O₃ symptom development. Visible O₃ symptoms were consistently seen (5 out of 6 years) at two of the three sites along the O₃ gradient and where daily one-hour maximum concentrations were in the range of 96 to 125 ppb. Clonal differences in O₃ sensitivity were consistent with our OTC rankings Elevated CO₂ (200 ppm over ambient and applied during daylight hours during the growing season) reduced visible foliar symptoms for all three clones from 31 to 96% as determined by symptom development in elevated O₃ versus elevated O₃ + CO₂ treatments. Degradation of the epicuticular wax surface of all three clones was found at the two elevated O₃ gradient sites. This degradation was quantified by a coefficient of occlusion which was a measure of stomatal occlusion by epicuticular waxes. Statistically significant increases in stomatal occlusion compared to controls were found for all three clones and for all treatments including elevated CO₂, elevated O₃, and elevated CO₂ + O₃. Our results provide additional evidence that current ambient O₃ levels in the Great Lakes region are causing adverse effects on trembling aspen. Whether or not elevated CO₂ in the future will alleviate some of these adverse effects, as occurred with visible symptoms but not with epicuticular wax degradation, is unknown.

This investigation is intended to add to the quantitative information about the sensitivity of European deciduous tree species to environmentally realistic ozone (O₃) exposure in respect to the established critical level of an AOT40 of 10 ppm.h (6-month growing season, daylight hours). Cuttings (without leaves) of Fagus sylvatica L., Sorbus aucuparia, L., Carpinus betulus L., Fraxinus excelsior L., and cuttings and seedlings of Prunus serotina Ehrh. were exposed during one growing season either to filtered air, to which 50% of ambient O₃, concentration was added, or to 50% + 30 ppb O₃, resulting in a final AOT40 of 0.3 and 20.7 ppm.h, respectively. The foliage formed per tree varied between and within the species, but was not significantly modified by O₃, whereas the number of symptomatic leaves per tree significantly increased in the 50% + 30 ppb O₃, regime. By mid July light-green spots appeared in the leaves (except C. betulus), which developed into stippling (F. excelsior), red (P. serotina) or necrotic spots by September. The CO₂ assimilation rate decreased more with increasing visual symptoms (earliest in 20- day-old leaves) than with age-dependent leaf discoloration in F. sylvatica, F. excelsior, and P. serotina (not in C. betulus). The dark-adapted photosystem II quantum yield (Fv/Fm) slightly declined with leaf age, but a considerable reduction became apparent in 107-day-old leaves with O₃,-symptoms only. By late morning yield reductions in light-adapted leaves were exaggerated in such leaves from the 50% + 30 ppb O₃ regime. The above mentioned responses were not reflected in significant changes of the net biomass production during the experiment. The critical level, therefore, is based on visual and functional symptoms rather than on production.

Two-year-old Aleppo pine (Pinus halepensis Mill.) seedlings were exposed to ambient air+50 ppb O₃ in open-top chambers (24 hours/day, 7 days/week) during May-October 1997 and to ambient air+70 ppb O₃ from May 1998 onwards. One growing season fumigation with ozone did not affect the pigment concentrations of the current-year (c) needles, nor were there any differences in photosynthesis or stomatal conductance. In May 1998, however, a marked carry-over effect was seen in the chlorophyll a and b and total carotenoid concentrations of the O₃-fumigated one-year-old (c+l) needles. The chlorophyll a and b and total carotenoid concentrations of newly flushed needles of the O₃-fumigated seedlings also seemed to be slightly decreased, as was their net photosynthesis when compared to the values of the filtered-air control needles. The chlorotic mottle and the changes in chloroplast pigments and photosynthesis of the c and/or c+l needles of the NFA+O₃ seedlings in May 1998 indicate that frequent episodes of ozone concentrations of ≥ 100 ppb, especially when they also occur during the evening and night hours, as in some areas in southern Europe, may result in visible needle damage on Aleppo pine.

Critical levels of tropospheric ozone, established for the protection of crops and other plants, are now reported as being exceeded over large forested areas, giving rise to the need for an extensive monitoring program to confirm ambient levels within the forest and to detect related forest health effects. The requirement for an inexpensive monitor that can be used in remote locations prompted the development of the Can Oxy PlateTM passive ozone monitor and a monitoring protocol by the air pollution research group of the Canadian Forest Service, Forest Health Network. The monitors underwent initial trials in 1996 and operational trials during 1997 that involved two 2–3 week mid summer exposures in the canopy at selected forest health monitoring plots across Canada, and at adjacent forest openings. In both trials monitors were also co-located with the nearest instrumental ozone monitor. This allowed for the production of a field calibration for quality assurance assessment under field conditions. Results from 1996 indicate highly significant correlations with accumulated ambient ozone concentrations from the instrumental monitors at the co-located sites (r=0.88, p=0.0002). However, no such relationship was found between these sites and the forest plots which were up to 200 kilometres away. This may indicate spatial heterogeneity in ozone exposure between the continuous air quality monitoring sites and the forest plots. This information, together with our knowledge that strong gradients of ozone exposure are found within the canopy, underlines the importance of in situ monitoring of ozone exposure of forest health plots at risk to ozone effects.

Plants represent one of the major sinks for tropospheric ozone that, at high concentrations, can affect plants’ physiological activity with consequent serious damage. A research project has been promoted by the Lombardy Foundation for the Environment to investigate the effects of air pollution on forest ecosystems. The areas of study are located on the southern slopes of the Italian Alps in two valleys, only 10 km apart, selected because of their different plant injury: Val Gerola and Val Masino. Air quality (O₃, NO_x SO_x, VOC) and meteorological parameters were monitored during four summer seasons (1994–97) using automatic sampling devices providing hourly mean values for each variable. Data analysis showed very different ambient ozone concentrations at the two sites, with average concentration values observed in the more damaged valley (Val Gerola) twice those measured at the other site. Multivariate data analyses have been used to interpret the observed differences in long-term O₃ exposure between the two sites and to identify possible underlying processes,.

The status of different response indicators of forest condition were measured and assessed between 1995 and 1997 at 6 Permanent Monitoring Plots (PMPs) in Tuscany (central Italy), where beech, holm oak and Turkey oak are the most frequent tree species. Foliage transparency, leaf damage and crown dieback have changed significantly over the monitoring period. Leaf area, length of the current year shoots, and total leaf area changed as well, but only for Turkey oak. Changes were consistent between and within the plots. Different indices of plant diversity showed marked changes, apparently linked to natural dynamics within individual ecosystems. These rapid and contrasting fluctuations in the various indicators of forest ecosystem make it difficult to derive a synthesis about the general condition of ecosystems and — especially Æ about the effects of air pollution.

Stem increment of mature Fagus sylvatica L. from 57 plots in Switzerland was analysed with respect to environmental factors using multilinear regression. Nitrogen (N) deposition was positively correlated with stem growth, with an increase by 41.8% (confidence interval 38.9–44.2) per 10 kg N ha−1 a−1. There was a negative relationship between ozone (03) dose and diameter increment. The fit was better for maximum rather than average exposure values. Coefficients indicated growth changes of −22.5% (14.3–28.6 confidence interval) and −35.1% (17.8–47.3) respectively per 10 ppm hours. These changes exceed the −6.1% found in experiments with beech seedlings. This difference is mainly attributed to the fact that the seedling data are calculated for a single season’s exposure whereas the stem increment data extend over four years, representing the product of four annual reductions. Increased sensitivity of mature trees compared to seedlings, however, is also suggested.

Needle fluctuating asymmetry (FA) in Scots pine (Pinus sylvestris) was studied south of the Severonikel nickel-copper smelter in the Kola Peninsula, Russia. We measured the difference in length of two needles of the same pair, and calculated FA as the ratio between this difference and the average length of the two needles of this pair. Needle FA did not depend on tree age, distance from the nearest neighbour or branch position within the crown, but increased with an increase in branching order. Needles originating in different years (1993–1997) demonstrated among-year variation in FA, which, however, was not related to annual fluctuations in SO₂ emission by the smelter. FA increased with decreasing distance from the smelter, and in the sites proximate to the smelter FA was double that found at the most distant (background) sites. The increase in FA was primarily due to an increased difference in the length of the two needles, and this difference may serve as a practicable indicator of pollution-induced stress in Scots pine.

In 1996 the assessment of tree condition in Italy was subjected to a Quality Assurance (QA) program. The QA program consisted in (i) the adoption of Standard Operating Procedures (SOPs), the set of Measurement Quality Objectives (MQOs) expressed by Data Quality Limits (DQLs), (iii) a national training and intercalibration course, and (iv) field checks, performed on a number of areas spread over the country. In general, DQLs were achieved for a number of tree condition indices. However, problems were identified with crown transparency, the most used index in the international reports. Additional problems arose when considering the data quality at plot level as a result of the combined achievement of DQLs for the different tree condition indices.

Determination of the stresses operating on a forest ecosystem demands the use of several bioindication methods. Air quality indicators were assessed from an inventory of forest decline based on the assessment of tree crowns and epiphytic lichens. Photosynthetic pigments, ascorbic acid and major macronutrients were studied in Norway spruce needles as indicators of physiological and biochemical stress. Analyses were carried out on selected forest plots in polluted areas (Zasavje district,vicinity of thermal power plants) and predominantly unpolluted areas (Triglav National Park, Julian Alps, Slovenia). For some bioindication methods, there was good agreement with measured air quality and climatological parameters. The best agreement was found between total foliar sulphur in needles and epiphytic lichens, especially in more polluted areas. Agreement with forest decline inventories and analyses of some needle stress physiological/biochemical parameters was less convincing. The strength of agreement was further decreased by soil characteristics and climatic parameters, influenced also by biotic parameters and forest stand history and management. It was concluded that there is no simple bioindication method available to evaluate the vitality of a forest.

Initial analysis of needle nutrient status and visible symptoms suggested that a declining Norway spruce (Picea abies L. Karst.) stand in south Sweden was suffering from B and Cu deficiency. Effects of addition of 1000 kg ha−1 of a commercial N-free fertilizer containing macro nutrients plus B, Cu, and Zn were investigated. B concentrations and B/N ratios were increased significantly in needles from fertilized trees compared to the control, to levels well above critical levels of deficiency and optimum nutrient ratios. In contrast, the treatment did not change the low needle Cu status and there was no effect on growth measured as needle mass after one growing season. Graphical vector analysis together with the absence of change in needle mass suggested a luxury uptake of B. Application of the concepts for critical levels of deficiency and optimum nutrient ratios in relation to N suggested B and Cu deficiency were contributing factors to the observed forest damage in the area. Graphical vector analysis requires a rapid response in needle mass, in combination with improved needle nutrient status in order to identify nutrient deficiencies. The applicability of the technique appears to be limited to primarily growth-promoting nutrients.

A two-step regression procedure was used to predict the impact of throughfall chemistry on the defoliation of Fagus sylvatica L. over a 10-year-monitoring period at three plots in northern Belgium. The impact of throughfall chemistry on crown condition was examined after accounting for influences of site, stand, climate and diseases. In a first step, defoliation was regressed on site, stand, climate and disease parameters. The residual defoliation of this standard set was correlated with 27 throughfall variables.Climatic variables of the year preceding the year of crown assessment accounted for 79% of the variation in current defoliation. Site, stand and disease factors were not included and were still part of the residual defoliation. The study of the residual defoliation revealed that high throughfall depositions of sulphate and ammonium and high throughfall ratios of SO₄/Mg enhanced defoliation.

The effect of heavy metal and gaseous pollution on genetic make up due to selection and mortality was studied in eight European Scots pine (Pinus sylvestris L.) populations. Genetic structure of these populations was evaluated after 13 years of growth at two sites, one located in the vicinity of a copper smelter in Glogow (southern Poland) and another in a control area (Komik, central Poland). Based on data from 10 enzymatic loci, studied by starch gel electrophoresis, the following genetic parameters were characterized: mean number of alleles and genotypes per locus, heterozygosity and genotype polymorphism index, genetic diversity among sites and genetic similarity according to Nei and Roychoudhury. Most of the parameters differed both between sites as well as between pairs of populations. Mean multilocus observed heterozygosity and genotype polymorphism indices were 8% and 6% lower respectively at the control site than in the polluted one. Populations from the site polluted by heavy metals demonstrate higher genetic diversity then those from the control. Genetic similarities showed differentiation of particular populations from both locations in response to industrial pollution. These results provide evidence that the general pattern of adaptation strategy of populations to pollution stress is connected with increasing genetic variation. The best isoenzymatic markers for monitoring changes in the genetic structure of Scots pine populations under the influence of industrial pollution are fluorescent esterase (FEST), glutamicoxaloacetatic transaminase (GOT-A and GOT-B), alcohol dehydrogenase (ADH), malate dehydrogenase (MDH-A) and shikimate dehydrogenase (ShDH-A) loci.

Genetic variability can have profound effects on the interpretation of results from elevated CO₂ studies, and future forest management decisions. Information on which varieties are best suited to future atmospheric conditions is needed to develop future forest management practices. A large-scale screening study of the effects of elevated CO₂ on 15 half-sibling sources of genetically superior ponderosa pine (Pinus ponderosa Dougl ex P. Laws.) is presented. These sources represent multiple elevations and latitudes throughout California. Among-provenance variability in the effects of elevated CO₂ on gas exchange and growth, and their correlation with geographic origin were investigated in ponderosa pine seedlings subjected to ambient or elevated CO₂ concentrations (525 μmol mol−1 CO₂, and 700 μmol mol−1 CO₂) for more than two years in open-top chambers. Substantial among-provenance variability in growth response to elevated CO₂ was evident, with 8 sources demonstrating no significant growth response to elevated CO₂ while 7 sources responded positively. For all sources, elevated CO₂ increased photosynthesis (ranging from 19% increase at 525 μmol mol−1 CO₂ to 49% increase at 700 μmol mol−1 CO₂). A modest correlation existed between geographic origin and above ground growth response to elevated CO₂ .

Biochemical and gene expression changes in response to elevated atmospheric CO₂ were investigated in five maternal half-sibling breeding families of Ponderosa pine. Seedlings were grown in a common garden located at Lawrence Livermore National Laboratory, in open-topped chambers (OTC) for two years. Chamber atmospheres were maintained at ambient, ambient + 175 μL L−1 CO₂, or ambient + 350 μL L−1CO₂. Growth measurements showed significant increases in stem volumes and volume enhancement ratios in three of the five families studied when grown under elevated CO₂. Biochemical and gene expression studies were undertaken to gain a mechanistic understanding of these phenotypic responses. Biochemical studies focused on sucrose phosphate synthase (SPS) specific activities at increased CO₂ levels. Kinetic evaluations of SPS showed an increase in V_Max. Specific SPS probes revealed increases in the transcriptional levels of one SPS gene with exposure to increasing CO₂. RT-PCR differential gene displays showed that overall only a small fraction of visualized gene transcripts responded to elevated CO₂ (8–10%). There were also significant differences between the gene expression patterns of the different families, some of which correlated with alterations in growth at elevated CO₂ levels.

Three year old spruce trees (Picea omorika) were exposed to 100 and 225 nl 1−1 SO₂ and H₂S for three weeks. The number of chromosomal aberrations and the mitotic index in the root tip meristems, and glutathione and cysteine contents in fine roots were determined twice weekly. An increase in glutathione content in fine roots of H₂S exposed plants was only detectable after 13 days of fumigation. The number of chromosomal aberrations increased significptly after 9 days of exposure to 225 nl 1−1 H₂S and after 13 days of exposure to 225 nl 1−1 SO₂ or 100 nl 1−1 H₂S. This increase in chromosomal damage persisted up to the end of the 3 week treatment. Neither SO₂ nor H₂S exposure affected the cysteine content or the redox state of glutathione in fine roots. These results suggest that the development of chromosomal aberrations during SO₂ and H₂S exposures does not directly reflect changes in thiol/glutathione content or redox state in the fine roots.

Blue spruce [Picea pungens Engelmann.] seedlings were reared from seed for 8 weeks under one of seven UV-B doses ranging from 0.0 to 9.2 kJ m −2 d−1. Emerging primary needles were chopped and incubated 48 h (22°C; 750 μmol m−2 s−1 PAR) with [1−14C] CH₃COONa. Radioactivity incorporated into epicuticular waxes was measured using radio thin-layer chromatography. Biosynthesis of nonacosan-10-ol, the dominant constituent, was affected by the UV-B dose. The results suggest that, if no other factors are limiting, the optimum UV-B dose for wax biosynthesis in emerging primary needles of blue spruce is 6−7 kJ m−2 d−1. This dose is below that routinely measured in some northern temperate forests and well within the range of predicted values under stratospheric ozone depletion scenarios. UV-B dose levels above this threshold may cause changes to the wax composition that may predispose the tree to damage from other environmental stresses.

In order to address the problem of pollutant effects on trees against the background of environmental fluctuations a simulation model using radial increment as an indicator of tree productivity is proposed. Ring times series of large trees provide one of the most acceptable biological indicators for the assessment of pollutant impact. This research focused on the tallest severely damaged and healthy trees growing close to each other in the surroundings of Jonava Mineral Fertilizer plant, which primarily emits nitrogen (N) and sulphur (S) compunds. The annual increment was found to be strongly correlated with mean September air temperature of the previous year and S load. The correlation coefficient (k=0.961) between modelled and observed tree ring times series of severely damaged trees proves the impact of S load on growth and vitality of a tree and enables fulfillment of tree condition prognosis.

Concurrent IUFRO Working Party (WP) sessions were convened and structured to address the state of science within each subject area. Oral summary presentations were made by rapporteurs during the closing plenary. The written output from the five sessions convened is included here in order of IUFRO WP designation.