Over the last 50 years, humans have changed the Earth’s ecosystems more rapidly and extensively than in any comparable period of time in human history ([13]). The global annual average nitrogen deposition over land is expected to increase by a factor of about 2.5, leading to an increased eutrophication and acidification. For present-day conditions, Lamarque et al. ([9]) suggest that the deposition over land ranges between 25 and 40 Tg(N)/year. Over forests, the deposition is expected to increase globally from 10 Tg(N)/year to 20 Tg(N)/year by 2100. Tropospheric ozone (O₃) is a global air pollution problem and an important greenhouse gas. Although not a new issue, ground level O₃ remains one of the most pervasive of the global air pollutants with impacts on human health, food production and the environment. Between the late 19^th century and 1980, concentrations of background O₃ in the Northern Hemisphere mid-latitudes doubled to about 30-35 ppb and have since increased by a further 5 ppb to 35-40 ppb. Future O₃ concentrations strongly depend on global emissions and global warming. However, even if emissions are reduced and background O₃ declines, it is expected that there will be an increase in the frequency of high pollution days due to the changes in weather expected under future climate change. In Europe there is likely to be an increase in the frequency of summer droughts, heat-wave events, and associated high O₃ episodes ([16]). Global greenhouse gas emissions have grown since pre-industrial times with an increase of 70% between 1970 and 2004. Depending on the model and scenarios used, projected global average surface warming is expected to increase by 0.6 to 4.0°C at 2090-2099 relative to 1980-1999. However, northern Hemisphere temperatures during the second half of the 20^th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years ([6]).

These factors, indicating a rapidly changing environment, are threatening and damaging forest ecosystems. Increasing concentrations of tropospheric O₃ may reduce the photosynthetic capacity of trees resulting in a reduced growth rate and carbon allocation ([14]). The increase in deposition of nitrogen to forest soils is threatening the integrity and functioning of forest ecosystems. Predicted changes in average temperature and precipitation will strongly affect ecological conditions of forests and their plant communities. In addition, extreme weather events like storms, high temperatures, and long lasting droughts are predicted to occur more often in the future, putting forests at risk.

The issue of air pollution, climate change and forest health problems is characterized by multi-causality with different strengths of association. The linkages between air pollutant exposures and plant ecosystem responses have complex inter relationships as their impacts depend on the environmental pollutants and conditions being considered while influenced by factors such as genetic constitution, age, nutrition and adaptation ([18]). The impacts on forest ecosystems have been traditionally treated separately for air pollution and climate change. However, the combined effects may significantly differ from a sum of their parts. Simultaneous addressing of the air pollution and climate change effects on forests is an opportunity for capturing synergies in future monitoring and research ([3]).

In order to identify future monitoring and research needs, a COST Strategic workshop on the role of “Forest ecosystems in a changing environment” assembled nearly 180 scientists from 30 countries in Istanbul from 11-13 March 2008. In plenary and parallel sessions, the workshop specifically tackled the fields of climate change and forests, ozone, atmospheric deposition and critical loads, biodiversity, as well as quality assurance in forest monitoring.

A part of the presentations during these sessions has been included in this special issue of iForest - Biogeosciences and Foresty and summarizes key studies on potential and actual impacts of air pollution and climate change on forests. The themes are centered around the following topics of global importance: linking critical thresholds and loads ([1], [22]) to forest condition, tree reactions to air pollution and climate change ([2], [19]), assessment of biodiversity ([4], [5]), long-term monitoring of deposition and integrated effects ([11], [12]), modelling carbon budgets, deposition loads and ozone fluxes ([7], [20], [21]), monitoring policy and instruments ([8], [15], [10]), and quality assurance and control activities for ecological monitoring ([17]).

There is evidence that forest ecosystems will be exposed to even more rapid changes of environmental conditions in the future. The combined effects of global warming and air pollution are complex and may differ from a sum of separate effects. In order to measure these, and to assess the environmental risk and its consequences on the ecosystem functioning such as carbon sink and protection of soil and water, a well-established and continuous forest long-term monitoring is crucial.

In the early 1980’s, large scale forest decline and deterioration led to the Convention on Long-range Transboundary Air Pollution (CLRTAP, Geneva 1979) which was crucial for launching forest monitoring within the frame work of the International Co-operative Programme on the Assessment and Monitoring of Air Pollution on Forests. Today, increasing awareness of the rapidly changing environment should lead to more integrated and intensified monitoring and research efforts enabling reliable quantification of risk assessment of global change and air pollution.

  References