7.01.09 - Ground-level ozone
To date, ground-level ozone (O3) is considered as the most damaging air pollutant in terms of adverse effects on forest health worldwide and may become worse in the future.
This unit will examine current understanding of the processes regulating tropospheric O3 and associated impacts on forests at global to local scales from both measurements and models. The unit 7.01.09 will provide fruitful scientific discussion and assessment of the current state of knowledge aiming to identify priorities, directions and challenges for future research with an ultimate goal of improving health, sustainability and productivity of forests worldwide.
The Working Party 7.01.09 will enable: i) better understanding of ground-level O3 patterns; ii) mechanistic understanding of O3 effects on forests; iii) assessment of surface O3 exposures and quantification of potential impacts on forests; iv) quantification of the spatio-temporal trends in surface O3 concentrations and associated impacts and iv) definition of future directions, countermeasures and issues relative to O3 for forests protection.
The Working Party 7.01.09 will collaborate with the Working Parties: 7.01.01 "Detection and monitoring" to contribute towards a relevant risk assessment for forest ecosystems; 7.01.03 "Atmospheric deposition, soils and nutrient cycles", 7.01.07 "Multiple stressors on ecosystems" , the new 7.01.10 "New Nitrogen and Sulphur deposition" as ozone is a component of climate change and the effects of elevated tropospheric O3 are coupled with the nitrogen cycle and other environmental factors (e.g. drought) and with the unit 7.01.05 "Modelling and risk assessment" to develop more realistic projections of forest and their health under future climate change.
This new WG 7.01.09 is centred around the impacts of surface O3 on forests, and is planned as a forum (integrated interface) where all the competences at present distributed in different IUFRO Units can be discussed and developed.
State of Knowledge
Surface O3 is an important air pollutant, a highly reactive trace gas and the third most important greenhouse gas in terms of radiative forcing contributing to climate change.
The current annual mean background O3 concentrations are considerably higher in the Northern Hemisphere (35-50 ppb) than the Southern Hemisphere (15-25 ppb). Ozone is typically a problem for forests because the highest surface O3 concentrations are measured in rural areas, in particular at high-elevation sites, while the lowest concentrations were recorded in urban areas where freshly emitted NO (road traffic) can deplete O3 locally.
The implementation of emission control legislation, in each individual country, leads to a geographically heterogeneous impact on surface O3 levels. Ozone in some senses remains a paradox. While ground-level O3 have stabilized or has fallen in North America and Europe as a result of successful control measures of O3 precursors emissions, the background concentrations are still increasing up to 55-65 ppb and even 85 ppb at Northern Hemisphere mid-latitudes by 2100. In contrast to the situation in Europe or North America, air pollutant emissions have been increasing over the last two decades in many developing countries of Latin America, Africa and Asia (rapid economic growth, urbanization, transportation, industrialization).
The O3 profiles also change seasonally since the hemispheric transport of O3 affects the emissions of O3 precursors and hence the formation and destruction of the pollutant in the atmosphere. The consequences of these changes in O3 seasonality and associated impacts on vegetation are poorly understood. The establishment of O3 trends is important to verify the effectiveness of the European O3 control strategies for the protection of vegetation. The actions to mitigate for O3 impacts on forests depends on emissions and legislation scenarios followed.
The issues around ground-level O3 are not "solved" and new data continue to shed light on more aspects of O3 and its interactions in the global atmosphere. A few issues about O3, such as explaining surface trends, mechanistic understanding of O3 effects, ozone-climate coupling and a better assessment of O3 impacts by evaluating the performance of O3 risk metrics, are still challenging. The knowledge across the range of scales (transboundary issue) is important for dealing with air quality and climate change in a synergistic manner to suggest appropriated O3 critical levels for forest protection.
Current O3 levels are high enough to negatively affect trees by inducing e.g. crown defoliation, visible foliar O3 injury, altered allocation of carbon and decreased fitness. As a result of chain-reactions that plants undergo under the pressure of O3, several consequences may follow which threaten the forest ecosystems and biosphere sustainability. These consequences include alteration of plant and soil fauna biodiversity, litter decomposition, nutrient cycling and carbon pools, trophic interactions etc. Meanwhile, the O3 levels forecasts suggest that the O3 impacts on ecosystems may worsen further.
To assess O3 exposures and O3 impacts on trees and forests both quantitatively and qualitatively, and to set critical levels and reveal potential countermeasures for protecting forests, mechanistic understanding of O3 effects on trees (or of tree response to O3) must be achieved. To date, the mechanistic understanding remains incomplete. It is thus very important to multiple levels, and of high priority, to understand the mechanistic which underlines O3 impacts on trees.