Environmental Pollution 147 (2007) 438e445
www.elsevier.com/locate/envpol

Integrated effects of air pollution and climate change on forests:
A northern hemisphere perspective
Andrzej Bytnerowicz a, Kenji Omasa b, Elena Paoletti c,*ba
USDA Forest Service, 4955 Canyon Crest Drive, Riverside, CA 92507, USA
Laboratory of Biological and Environmental Information Engineering, Department of Biological and Environmental Engineering,
Graduate School Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japanc
IPP-CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Firenze, Italy

Received 15 August 2006; accepted 20 August 2006

Simultaneous addressing air pollution and climate change effects on forests is an opportunity for capturing synergies
in future research and monitoring.
Abstract
Many air pollutants and greenhouse gases have common sources, contribute to radiative balance, interact in the atmosphere, and affectecosystems. The impacts on forest ecosystems have been traditionally treated separately for air pollution and climate change. However, thecombined effects may significantly differ from a sum of separate effects. We review the links between air pollution and climate change andtheir interactive effects on northern hemisphere forests. A simultaneous addressing of the air pollution and climate change effects on forests
may result in more effective research, management and monitoring as well as better integration of local, national and global environmental
policies.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Acidification; Climate change; Eutrophication; Forest monitoring; Tropospheric ozone

1. Introduction
Climate change and air pollution are closely linked,
although in applied scientific research and even more in political negotiations they have been largely separated. Many of the
traditional air pollutants (APs) and greenhouse gases (GHGs)
have not only common sources, but may also interact physically and chemically in the atmosphere causing a variety ofenvironmental impacts on the local, regional and global scales.
The impacts on forest ecosystems have been traditionallytreated separately for air pollution and climate change. However, the combined effects of numerous climate change andair pollution factors may significantly differ from a sum of* Corresponding author. Fax: þ39 055 5225 666.
E-mail address: e.paoletti@ipp.cnr.it (E. Paoletti).
0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.envpol.2006.08.028

separate effects due to an array of various synergistic or antagonistic interactions. The net effect varies for different ecosystem types and geographic regions, and depends on magnitudeof climate or AP drivers, and types of interactions between
them (Bazzaz and Sombroek, 1996).
In this review, we will discuss links between air pollution
and climate change and their interactive effects on northernhemisphere forests from a perspective of the key ecological,
economic and societal values of forests. Our aim is to stimulate a simultaneous addressing of the air pollution and climatechange effects on forests. This is an opportunity for capturingsynergies and avoiding overlaps between two lines of traditional research (Swart et al., 2004). This could result in
more effective research, monitoring and management as well
as better integration of local, national and global environmental policies.

A. Bytnerowicz et al. / Environmental Pollution 147 (2007) 438e445

2. Links between air pollution and climate change
in the atmosphere
Many traditional APs and GHGs have common sources andinteract in the atmosphere. The most important GHG, carbon
dioxide (CO2), is mainly produced by burning fossil fuels,
which is also an important source of several APs. Elevated
CO2 itself can be considered a pollutant, depending on theterminology.
Some APs contribute to radiative forcing (Fig. 1). CO2 isa main driver in that regard, and is followed by methane(CH4), halocarbons, and nitrous oxide (N2O). Aerosols andparticulate matter (PM) affect climate depending on their composition. Soot enhances warming, while other aerosol constituents (like the APs S and N species) generally have a cooling
effect (Houghton et al., 2001). Climate change, especially highradiation and temperature, promote increases in troposphericozone (O3), the secondary pollutant generated from the
GHGs non-methane volatile organic compounds (VOCs), carbon monoxide (CO) and nitrogen oxides (NOx). O3 is a potent
GHG itself and indirectly influences lifetimes of other GHGs
such as CH4 (Fiore et al., 2002). Methane has not been considered an O3 precursor, because of its long lifetime, until intercontinental transport was recognized to contribute to O3
pollution (Derwent et al., 2004).
Acidic deposition and eutrophication affect natural emissions of the GHGs CH4 and N2O from soils (Brink et al.,2001). For the oxidized N species, climate change increases
the amount of nitric acid (HNO3) (Sanderson et al., 2006).
For reduced N species, both climate change and AP increases
act to convert more ammonia (NH3) to ammonium sulphate.
This change occurs via increased aqueous phase oxidation of
SO2, indicating that the S and reduced N cycles are closely
linked (Sanderson et al., 2006).
Climate change may affect distribution patterns and mixture of APs. Such changes are caused by changing wind439

patterns, and amount and intensity of precipitation. The intensity of precipitation determines the atmospheric concentrationand deposition of acidifying compounds. This may alsochange frequency and extent of pollution episodes (e.g., O3).
However, AP emission is more important than the effect of climate change on the dispersion and chemical transformation ofAPs (Mayerhofer et al., 2002), while regional air pollution (inthe form of sulfate aerosols) was not found to have a large effect on climate change in Europe (Alcamo et al., 2002).
3. Integrated effects of air pollution and climate change
on forests
Climate change and air pollution interact in affecting forests by changes in soil processes, tree growth, species composition and distribution, increased plant susceptibility tostressors, increased fuel built-up and fire danger, water resources, recreation value, etc. Climate change can alter the effects of APs on ecosystems, and vice versa, APs can modify
responses of ecosystems to specific climatic change impacts.
3.1. Soil processes
Climate change (especially temperature) alters many soil
processes, having consequences for the entire ecosystem.
Higher temperatures, changed precipitation patterns and modified net primary production (NPP) increase the weathering
rate, resulting in higher critical loads (i.e., lower sensitivityof ecosystems to APs) (Posch, 2002). The increased mineralization increases N availability and leaching (Mol-Dijkstra
and Kros, 2001). Deposition of various APs also increases N
availability. Climate change acts to worsen the problem ofacidification by increasing the productiondand deposition tosoilsdof HNO3 from NO, and the proportion of NH3 converted into ammonium sulfate, which in turn may result in further acidification of soils (Sanderson et al., 2006). Higher CO2
levels can increase soil moisture due to changes in evaporation(Eguchi et al., 2005). Natural ability of forest soils to take up
CH4 decreases due to N deposition (http://www.bae.
uky.edu/IFAFS/FAQS.htm).
3.2. Tree healthFig. 1. The global mean radiative forcing of the climate system for the year
2000, relative to 1750 (Houghton et al., 2001). The rectangular bars represent
estimates of the contributions of these forcings, some of which yield warming,and some cooling.

APs may affect forests (Table 1). SO2 is probably the best
known example (Legge et al., 1999). NOx and NH3 as well as
HNO3 vapor may have direct phytotoxic effects but only athigh concentrations (Bytnerowicz et al., 1999). Gaseous N
pollutants and the dissolved in water N compounds contributeto N deposition that can have various effects on forests (Fenn
et al., 1998). O3 has the highest phytotoxic potential and ispredicted that by 2100 half of the World’s forest will be exposed to phytotoxic O3 levels (Fowler et al., 1999). However,high levels of APs (mainly O3) do not necessarily translate
into substantial negative effects on forests (e.g., Paoletti,2006). In contrast, an increase in forest growth has been demonstrated for several European countries (Spiecker et al.,1996). The presumed reasons are increased N deposition,

A. Bytnerowicz et al. / Environmental Pollution 147 (2007) 438e445

440

Table 1
Effects of primary air pollutants on forests can be seen as a multi-pollutant,multi-effect problem extended towards radiative forcing (EEA, 2004b).
SO2 NOx NH3 VOC CO PM CH4 CO2 þ GHGs
Ecosystems
Acidification
Eutrophication
Surface ozone

Radiative forcing
DirectVia aerosols
Via OH


























temperature, and availability of CO2 as well as changes in silvicultural practices. The inherent aspecificity of tree response
indicators of AP effects is another likely reason of the observed discrepancies between high pollution and low effects
on forests (e.g., Ferretti et al., 2007).
Climate change parameters that trigger stomata opening(e.g., increasing temperature) increase the sensitivity of plants
to APs like SO2 and O3. Parameters that lead to stomata closure (e.g., water stress, increased CO2) help to protect theplant from APs. A complication is that O3 slows the stomatal
response to reduced water availability (Paoletti, 2005). Climate change parameters that lead to a longer growing season(e.g., warming) increase the exposure of plants to APs likeSO2 and O3, whereas parameters that shorten the growing
season (e.g., water stress) reduce the exposure and damage(Guardans, 2002). A simulation for Norway spruce and European beech showed that climate change sensitivity increases inboreal areas and decreases in temperate areas due to temperature and water stress (Guardans, 2002). The AP impacts wereless severe in 2000 than in 1990, but the cumulative stress was
still significant.
3.3. Growth of trees and carbon sequestration
Climate warming, increased mineralization in soils, andhigher N availability increase plant growth and thus the C
sink especially if N is the limiting nutrient (Bazzaz and Sombroek, 1996). Increased NPP has been hypothesized to be due
to increases in N deposition (Nadelhoffer et al., 1999), atmospheric CO2 concentrations (Friedlingstein et al., 1995), temperature, and longer growing season (Myneni et al., 1997).
The contribution of N deposition to the increase in C in standing biomass is approximately 10 to 20 Mton y1, i.e., 3.5e7%of the estimated forest growth (de Vries et al., 2003). By far
the largest amount of C stored in northern hemisphere forests
is in the soil (de Vries et al., 2003). Increased N deposition
causes an increased rate of soil organic matter accumulation
due to an increased leaf/needle biomass and litter production,
and a reduced decomposition of organic matter (Schulze,2000). An increase in CO2 concentrations favors both growth
and water use efficiency of plants. However, trees may adapt
and such an effect may diminish soon (Tognetti et al.,2000). Using a modeling approach, temperature has been

claimed to be relatively unimportant, whereas the combinationof elevated CO2 and N may account for a 15e20% increase inforest NPP (Rehfuess et al., 1999). In this context, N deposition is claimed to be the most important factor of the increased
forest growth. The remaining explanation would be the impactof forest management.
The AP effects on forests may provide an important control
on the C cycle that has not yet been widely considered. Prolonged exposure to O3 may suppress gains in C sequestrationof trembling aspen as it has been shown in a free-air CO2 þ O3
experiment (Karnosky et al., 2003). Including O3 in a forest
ecosystem model applied to the north-eastern United States,
offsets a substantial portion of the growth increases caused
by CO2 and N deposition (Ollinger et al., 2002). Felzer
et al. (2004) have incorporated empirical equations derived
for trees (hardwoods and pines) and crops into the Terrestrial
Ecosystem Model to explore the effects of O3 on NPP and C
sequestration across the conterminous US. The results showthat C sequestration since the 1950s has been reduced by
18e38 Tg C y1 with the presence of O3.
More frequent extreme drought events may counteract theeffects of the anticipated mean warming and lengthening of
the growing season, and erode the health and productivity of
ecosystems, reversing sinks to sources (see Section 3.5).
3.4. Biodiversity changes
Species composition is mainly driven by climate, soil andforest type, but N and S deposition has also a significant
impact (de Vries et al., 2003). Some crops and trees need
low temperatures in winter to trigger bud bursting in spring.
These species can no longer grow in areas where winter temperatures are becoming too high. An overload with N leads toeutrophication and changed ecosystem composition in Europe(Bakkenes et al., 2002). Since critical loads of AP are speciesspecific, the critical loads in a region may change if ecosystemcomposition changes, meaning that the sensitivity to AP isalso altered. The changes in composition may also change
the sensitivity of an ecosystem to climate change. Changes
in biodiversity have been observed under historic climatechange (Prentice et al., 1998) and are also projected for the future (Bakkenes et al., 2002). Assessment of climate change
impacts on terrestrial ecosystems in Japan suggests that the
average NPP increase could change the vegetation type
(Shimizu et al., 2005). In North America, modeling effortssuggest substantial changes in potential habitats of several
species and communities (Bachelet et al., 2001; Iverson andPrasad, 2001). The forest area in the USA is projected to decrease by an average of 11%, with the lost forest replacedby savanna and arid woodland biome. Community types predicted to increase include oak/hickory and oak/pine in the
East and ponderosa pine and arid hardwoods in the West.
Seven of the 80 eastern species are predicted to be reducedin suitable habitat by at least 90%. Potential habitats for several subalpine conifers are simulated to contract substantially.
The potential habitat for big sagebrush (Artemisia tridentata)will shift from the US into Canada (Hansen and Dale, 2001).

A. Bytnerowicz et al. / Environmental Pollution 147 (2007) 438e445

3.5. Susceptibility to natural disturbances
Natural disturbances having the greatest effects on forests
include insects, disease, drought, fires, or wind storms.
Chronic excess input of N to forest ecosystems causes nutrient
imbalances (i.e., deficiencies of the macro-nutrients K, P, Mg
and Ca) which, in turn, increase the sensitivity of plants to climatic factors, such as frost or drought, and susceptibility toparasite attacks (UNECE, 2005). Ozone also decreases winterhardiness increasing the risk of frost damage (Bazzaz andSombroek, 1996).
The seasonality of fire hazards in the US is projected to increase about 10% over the next century, or even about 30% in
Alaska and south-eastern US (NSST, 2000). Examples of catastrophic fires are those in southern California in fall 2003. Yearsof fire suppression have lead to severe overstocking of themixed-conifer mountain forests. Several years of severe
drought as well as chronic exposure to elevated O3 concentrations and N deposition have contributed to fuel buildup at theforest floor. Additionally, drought and AP weakened the trees
promoting bark beetle infestation (Miller and McBride, 1999)
and massive tree dieback. Hot weather in the 2003 summer
and fall, strong winds, and fires started by arsonists resulted
in catastrophic fires (Keeley et al., 2004). Potential for such
events is high in semi-arid Mediterranean type ecosystems.
The catastrophic fires in Spain and Portugal in summer 2005
are additional examples of that increasingly important problem.
Due to the decreased proportion between roots and above
ground portions caused by N deposition, susceptibility of trees
to windthrow can increase (Bytnerowicz, 2002). Also, elevatedO3 levels reduce the supply of carbohydrates to roots reducing
their biomass (Bytnerowicz and Grulke, 1992). In Switzerland,
after the 1999 windstorm, 1600 trees were analyzed (UNECE,
2005). Broken trees showed wider annual growth ringsdand
thus decreased mechanical resistancedin the last decade compared to the unbroken trees. The presumed reasons are increased N deposition, temperature, and availability of CO2.
Higher N concentrations were measured in the wood of broken
trees that most likely reflects higher N supply. Over the past
decades storm events and damage severity to forest have increased over Europe (UNECE, 2005). In Slovakia, a storm
in November 2004 destroyed 24,000 ha of forest stands(1.2% of the total forest area) mostly in the Tatra National
Park (UNECE, 2005). The southern part of the Park has
been characterized by elevated levels of O3, S and N deposition (Bytnerowicz et al., 2004). The destroyed stands arealso mostly monocultures of mature Norway spruce trees.
Lower resistance of monocultures to windbreaks results inlarge areas of Norway spruce stands felled by wind storms
in Central Europe (Spiecker, 2000).
In the mountains of the western US and Canada, unprecedented outbreaks of bark beetles killing pine forests at a large
scale have occurred. Recent increase of temperatures, especially at high elevations, allow for shortening the life cycleof bark beetles and shifting from typical one-year cycle totwo-year cycle. In New Mexico and Arizona, bark beetles
killed about 70 million pines in 2002 and 2003. These forests,

441

similarly as the southern California forest described above,
had experienced severe droughtdthe year 2002 was the driest
year in the last 1400 years. Overstocking of trees caused by almost 100 years of fire suppression led to tripled or even quadrupled stand densities and severe limitations of available
water to individual trees (Nijhuis, 2004). Although O3 was
not mentioned, one should ask if elevated O3 levels have notplayed a role in that pine dieback.
The positive effects of rising temperature on plant growth
may be offset by an increased risk of water shortage. Europe
experienced a particularly extreme climate anomaly during
2003, with July temperatures even 6  C above long-termmeans, and annual precipitation deficits 50% below the average. Forest sites experienced a significant reduction of GPP(gross primary productivity) and net C uptake, with some forests even temporarily becoming net C sources in August (Ciais
et al., 2005). GPP did not entirely recover from the summer
stress during the remainder of the year. Several Mediterraneansites showed a smaller decrease in C uptake, largely dominated by less respiration. Pronounced soil water deficits compensated for the effect of warmer temperatures in affecting soil
respiration (Ciais et al., 2005). Although 2003 was not the driest year on record, the impact of drought was amplified byhigh summer temperatures and soil water deficits carriedover from the previous spring. The GPP anomalies correlated
better with rainfall than with summer air temperature, indicating the dominant role of water limitations (Ciais et al., 2005).
Despite climatic conditions that favor drought and heat also
increase O3 concentrations, the ambient O3 was not mentioned
in the above article.
4. Perspectives and recommendations
International policy-making on climate change still takes
place independently from air pollution. Opportunities for synergies and avoiding trade-offs have neither been integrated
into international air quality policies in Europe, such as thesixth environmental action program and the Convention on
Long-range Transboundary Air Pollution, nor are they addressed in current climate negotiations under the United Nations Framework Convention on Climate Change. At thetechnical assessment level, however, the opportunities are increasingly noted. For example, the Intergovernmental Panelon Climate Change addresses multiple pollutants and sourcesof GHGs (Metz et al., 2001), and the UNECE Task Force on
Integrated Assessment Modelling integrates climate change
considerations into regional air pollution policies as a logicalnext step. The complex atmospheric interactions between air
pollution and climate change, however, are presently not understood sufficiently well to allow their quantitative incorporation in integrated assessment modeling (EEA, 2004b). For
example, more precise determination of the chemical composition of aerosols is necessary in order to assess the overall net
warming or cooling effect, as this is the largest source of uncertainty in the radiative balance (Houghton et al., 2001).
Assuming a reduction of CO2 emissions to comply with the
Kyoto protocol, there will be significant ancillary benefits in442

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terms of additional reduced emissions of APs and reduced
costs of AP abatement (Metz et al., 2001). The indirect effects
of climate policies in Europe targeting the stabilization of
GHG concentrations in the atmosphere would cut the costsof abating SO2 emissions by 50e70% and NOx by about
50% (Van Harmelen et al., 2002). For the shorter-term implementation of the Kyoto Protocol, savings of 10% of the costsof controlling acidification and ground-level O3 were found(Syri et al., 2001). Put another way, there are strong financial
arguments for developing joint policies to reduce AP and GHG
emissions (Alcamo et al., 2002). Outside Europe, the potential
synergies between GHG mitigation and abatement of local air
pollution have received attention especially in developing
countries (Chile, Brazil, Mexico; Cifuentes et al., 2001) andin the US, where NGOs such as the World Resources Institute(MacKenzie et al., 1992) and Resources for the Future (Burtraw and Toman, 2000) have promoted an integrated approach,primarily on the basis of the advantages for human health. In
the US, harmonized options for joint abatement of APs and
GHGs are being promoted at the State level (STAPPA/
ALAPCO, 2000) and are recommended by the National Research Council recent report (NRC, 2004).
Dynamic industrial growth without an effective air pollution control and rapid increase of a number of combustion engine vehicles in Asia, especially in China, will be the main
drivers for increasing background O3 concentrations in thenorthern hemisphere in coming years (Klimont et al., 2001).
There is a need for improved understanding of O3 distribution
patterns and trends in forests. Ground-based monitoring networks, remote sensing, integration of data generated at different scales (forest stand, landscape, region, continent,
hemisphere) through various modeling efforts should be implemented. The ongoing monitoring of incidences of O3 damage to forests may provide evidence of changes in effects,
provided that the better indicators are selected. Ozone visible
injury is regarded as a result of oxidative stress, leading toa cascade of adverse effects resulting in a reduced vitality offorest species and increasing predisposition to the climatic,
edaphic and biotic factors (de Vries et al., 2003), but the ultimate link between O3 visible injury and plant functionality
and growth is still to be ascertained in a wide range of species.
Understanding responses of key forest species to ambient O3 isessential for developing secondary O3 biologically basedmodels. Until today, O3 responses of only a few tree species
in North America and Europe are known and very little is
known about the responses of Asian forests. Therefore a development of improved O3 risk assessment models is recommended (as an improvement from simple exposure indices such as
AOT40 and promotion of new indices such as those based on
stomatal O3 flux and plant defense mechanisms). The fluxbased approach provides a more biologically realistic representation of the O3 exposure of plants than indices based on
concentration only (UN, 2004). Work is needed to validate
flux-based indices for more species and to verify the applicability of the flux-approach to large-scale surveys under theroutine condition of monitoring programs (Ferretti et al.,2007). Eddy correlation may provide more robust results

and fluxes of O3 since inert gases, such as CO2, can be easily monitored. Nevertheless, ozone flux from dry soils
should be carefully evaluated (Omasa et al., 2000). In addition, a compliance with possible flux-based air quality standards might be difficult to be assessed and monitored(Ferretti et al., 2007). Increased knowledge on ecosystemspecific O3 sensitivity may help to set up different criticallevels across Europe, analogously to the EMEP effects-based
critical loads, instead of the present flat-approach criticallevels.
Climate change acts to worsen the problem of acidification
by increasing the production of HNO3 and ammonium sulfate(Sanderson et al., 2006). Implementation of existing technology to reduce emissions of precursor gases will help alleviate
the problem. Our knowledge of the long-term effects of increased N deposition on a representative range of terrestrial
ecosystems should be improved. While wet N deposition iswell characterized, contribution of dry N deposition to forest
ecosystems, especially in arid and semi-arid zones, should
be better understood. There is also a need to better understandeffects of atmospheric N deposition on tree growth as well as
the interactions between N deposition, forest stand, ecosystemstability, and biodiversity. Model development is still needed
regarding several key processes, particularly N dynamics andrelations to climate change. Understanding the N cycle insemi-natural ecosystems may be the key to understand thelong-term source or sink strength of soils for C (de Vries
et al., 2003). Since N often is the limiting nutrient in forests,
N deposition may increase wood production and accumulationof soil organic matter, thus increasing C sequestration into theforest. Earlier estimates suggested that this mechanism could
take up one third of the global CO2 emission from fossil
fuel if most of the N deposition was taken up by trees andused to form new woody biomass (Holland et al., 1997). Recent data, however, suggest that the increase in N deposition
may cause a 10-times smaller additional CO2 sequestration
in forests (Nadelhoffer et al., 1999). When the large uptake
is mainly due to elevated growth, it is likely that this is a transitory phenomenon, whereas it could be a C sink for a long period if soil accumulation is the main cause since below groundC has much lower turnover times than above ground C. The
EUROFLUX project provided measurements of C fluxes
above a range of forests across Europe (Tenhunen et al.,1998), but extrapolation of these results to a continental scaleis still prone to large uncertainties (de Vries et al., 2003). The
aboveground CO2 sequestration in trees can also be estimatedfrom yield tables and models on tree growth or can be basedon repeated forest surveys. Recent EU projects have increased
the understanding of controls in the N and C cycle in forests(e.g. Dise et al., 1998; Gundersen et al., 1998). A comprehensive evaluation of the effect of eutrophication on C sequestration in the entire ecosystem needs to trace C and N jointly
through the ecosystem and needs to include the response of
the aboveground biomass, the input of C to the soil (aboveground and belowground), the aquatic phase and the responseof soil microorganisms (emission of CO2 and NOx; formationof labile soil organic matter).

A. Bytnerowicz et al. / Environmental Pollution 147 (2007) 438e445

In addition, more research, such as that conducted by
Karnosky et al. (2003) and Marinari et al. 2007, is needed
for understanding how ambient O3 or N deposition may modify responses of trees to elevated CO2. The effects of O3 on
NPP and C sequestration should be factored into future calculations of C budget (Felzer et al., 2004).
Achieving stabilization of atmospheric GHG concentrations
would require substantial (ca. 70%) reductions in global GHG
emissions (Houghton et al., 2001), i.e., much larger global
emission reductions than agreed in the Kyoto Protocol. Even
if industrialized countries substantially reduce their GHG
emissions over the next few decades, the climate system is expected to continue changing over the coming centuries (EEA,
2004a). This is due to the long time delay before emission reduction policies have an effect on GHG concentrations and, inturn, on climate. Therefore, in addition to reducing emissions,
understanding mechanisms of adaptation of forests and otherecosystems to climate change is increasingly needed.
To quantify the long-term consequences of extreme climate
conditions on forest productivity, we need to understand betterthe consequences of xylem embolism; the effects of reduced
carbohydrate pool sizes on subsequent leaf and fine root production and turnover, and on the ability of plants to resist pathogen attacks; the impacts on soil microbial dynamics,
decomposition and nutrient-supply processes, shifting competitive abilities between plant species (Ciais et al., 2005), and therelative contribution of concurrent pollution. In this regard,a much better knowledge on the confounding factors that affect chemical and biological recovery of ecosystems is needed.
Better understanding of the effect of air pollution and climate change interactive effects on catastrophic fires is needed,
as well as improved understanding of mechanisms leading tocatastrophic windthrows. This should lead to development ofmanagement strategies to improve the resistance of forest
stands to these stressors.
The data collected at intensive APs monitoring sites may
be useful also in relation to other environmental problems
than air pollution, such as climate change and changes inbiodiversity (Manual for Integrated Monitoring, 1998). Data
collected at ICP Integrated Monitoring sites have alreadybeen used in global change research framework for calculations of, e.g., C and N pools and fluxes in Finnish forests (Ilvesniemi et al., 2002). Clearly, there is a great need for better
utilization of the existing monitoring and modeling efforts.
This could be done through the improved cooperation between various ICP activities in the EU, coordination of various monitoring programs in the US (CASTNET, NADP,
IMPROVE, FHM, FIA, LTER, etc.), and more effective cooperation between US, Canada, EU and Asia in developing
the coordinated methodologies for evaluation of critical loads
for N, S and acidity.
Experiments at different scales (branch, seedling, maturetree) are needed to better understand mechanisms of the effects of single factors or simple interactions. In this regard
long-term, large-scale experiments such as the free-air CO2
and/or O3 and N enrichment studies or gradient air pollution
studies (e.g., FACE study in Wisconsin and Italy, Kranzberg

443

Forest in Germany, San Bernardino Mountains in southern
California) should be encouraged and funded.
Collaboration of the forestry research scientists within the
IUFRO working parties, research groups, scientific divisions
and across them is needed and may greatly help in exchange
of information, technology transfer or starting new multidisciplinary and multinational cooperative projects.

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