L'observation des forêts à l'échelle pan-européenne

Summary

Forest monitoring as an international initiative was launched in Europe during the 1980s in response to the concern generated by the alleged forest decline caused by transboundary air pollution. Since then, and thanks to the effort put in place by countries, European Union (EU) and United Nations Economic Commission for Europe (UNECE), a unique forest monitoring system has been developed. Such a system consists of:

• two networks of plots (so-called Level I and Level II) distributed all across Europe
• a set of Standard Operating Procedures (SOPs, i.e. the monitoring Manual)
• an international database
• an international network of scientists and experts organized in thematic groups (Expert Panels and Committees).

All the above elements are co-ordinated by the UNECE International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests), probably the largest long-term internationally co-ordinated forest monitoring program in the world ICP Forests organizes data produced by 42 participating Countries and covering different responses (e.g. forest health, growth, diversity, nutrition) in relation to different drivers (e.g. climate, air pollution, deposition, biotic agents) (attention: il manqué une virgule dans le texte en français) and media (soil, vegetation, atmosphere). As such, it is unique also with respect to the possibility to integrate data across spatial, temporal and ecological scales. It has been proven to be essential in reporting, and detecting and understanding changes in the status of European forests.
Although the original aim of the ICP Forests remains linked to air pollution, its potential goes much beyond, and data collected under the ICP Forests are pivotal in many evaluations attempting to understand the condition of European forests with respect to key environmental and forest management issues. These evaluations will be hardly possible on the basis on national initiatives only.

The importance of long-term, fully documented data-series on forest resource is more and more obvious and acknowledged among scientists, but it is not always so among politicians and resource managers. This contrasts with the evidence that only long-term data can help in understanding status, changes and determinants of change of our forests.

When considering the ecological, societal and economic value of European forests, the results obtained up to now and the potential for future scientific and technical development at European and global level, the monitoring system co-ordinated by the ICP Forests is an invaluable asset for forest ecosystem science that should be maintained.

Summary

Detecting changes in forest ecosystems at the European scale supposes measurements to be done in some homogeneous way over decades and accross countries. Samples must be analysed in labs e.g for surveying atmospheric deposition, tree nutrition or soil chemistry. So the evaluation and the improvement of the quality in labs are a key issue.

Over 100 different labs have been analysing water, soil and foliage samples within the monitoring program of ICP Forests. To guarantee the comparability of the analytical results between different laboratories, a quality assurance (QA) program is necessary with participation of all laboratories. The ICP Forests QA program is based on three pillars:

• the use of harmonized, well-defined and documented analytical methods
• an internal quality control (QC) program within each lab
• an external QC program coordinated by the monitoring program organizers.

Analytical data can be comparable only if laboratories are using the same or comparable methods for extracts, digestion and element determinations. Therefore each new digestion or extraction method which is not the mandatory one has to be tested if the results are comparable. This is very important when new instruments or analytical technics come to the market; the comparability has to be tested before these new technics can be allowed for analysing ICP Forest samples.

Proposals for the internal quality control program for each laboratory have been developed by the Working Group QA/QC in Labs and the meetings of the heads of the labs of ICP Forests and are documented in the ICP Forests manual. It consists of different tools to check the plausibility of each measurement in regard to range limits, and to test the consistency between values obtained for different parameters.

For instance for water samples the positive and negative electric charges of the measured ions should be balanced. If not, that means that at least one ion concentration must be wrong. For plausible range checks the monitoring data of different countries have been evaluated to decide which range of results is realistic for the different parameters. If one value exceeds the plausible range, it must be corrected or confirmed by another measurement.

The main part of the external QC program is made of ring tests between all labs. They are planned regularly and generally based on 5 natural samples representing contrasted conditions. The percentage of non-tolerable results in ring tests can be seen as a degree of quality and comparability of results from participating labs.

The Figure 1 shows the results for several parameters of water ring tests within the last 15 years. The development of the quality of the labs, but also the limitation due to different analytical methods can be seen from these results.

Percentage of results outsit to the tolerance level during the ring tests organized to the water analyze by the ICP Forests programme

The results of the ring tests are discussed during the meetings of the Working Group QA/QC and the meetings of the heads of the labs. These meetings offer the possibility to find the mistakes and problems for those parameters with unacceptable ring test results. New methods are discussed and method comparisons are presented. Indeed sharing the knowledge and experience of so many labs is a great opportunity to make quality improve in all of them.

Summary

In Europe, forest productivity has increased over the last few decades due to the combined effect of increasing CO2 concentrations in the atmosphere and atmospheric nitrogen deposits. Nitrogen deposits have remained high in different parts of Europe and with atmospheric CO2 concentrations continuing to increase, productivity could increase further, unless it is slowed by the limited availability of other resources, most notably nutrients.

The objectives of the present study were to describe leaf nutritional status for the main forest species in Europe (beech, sessile and pedunculate oak, spruce, Scots pine, silver fir), to identify growth-limiting nutrient levels for each of these species, and to detect changes in foliar nutrition over time.

The study used foliar analysis data collected from 1992 to 2009 on plots in the level II forest monitoring network of the IPC Forests programme. This unique data set covers two decades of data for the whole of Europe, collected according to standardised methods respected by the different countries involved.

The nitrogen nutrient levels for broadleaves were good (sub-optimal to optimal), but around half of the coniferous plots were below deficit levels. For phosphorus, on the contrary, a deficiency was found for a considerable portion of the plots regardless of species (from 22 to 74%). Furthermore, all species showed deficiencies in base cations (calcium, magnesium, potassium) though the proportion of plots concerned was lower (from 5 to 40%).

Analysing trends over time revealed a significant increase in foliar mass for beech (leaves) and Norway spruce (needles). Concerning foliar nutrient concentrations, a decreasing trend was detected in almost all cases (20 out of 22).

Among the trends of highest concern, most species showed a clear degradation in phosphorus nutrition; this confirms previously reported evidence on plots in France, Wallonia and Luxemburg (Jonard et al., 2009). Nitrogen, sulphur and potassium concentrations are also significantly declining in certain species. Concerning calcium and magnesium, trends declined for broadleaves and increased for conifers (cf. table).

Temporal evolution of foliar nutrients of the main European forest species

Several explanations for these trends are possible. Declining concentrations could reflect a dilution effect due to increasing foliar mass.

Indeed, the increased productivity of European forests (fertilisation effect of increasing atmospheric CO2 concentrations and nitrogen deposits) implies stronger demands for nutrients, which the soil may not be able to supply. The decrease in atmospheric sulphur deposits and, to a lesser extent, in nitrogen deposits certainly contributes somewhat to the decrease in the foliar concentrations of these elements. Since sulphates and phosphates are absorbed by the same type of exchangers in the soil, the decrease in sulphate concentrations in the soil solution (a consequence of the reduction in sulphur deposits) could cause phosphates to remain more strongly bonded to the solid phase of the soil, thus decreasing their bioavailability.

Stand age, though possibly also involved, cannot explain such a degree of decline.

This study shows that tree mineral nutrition is deteriorating in Europe and that the response of our forest ecosystems to global changes may become more and more constrained by nutrient availability. It is therefore crucial to consider these nutritional constraints in global carbon balance modals in order to avoid overestimating our forests' capacity to capture and store carbon.

Summary

Tropospheric ozone (O3) is well known to be an air pollutant causing injury to plants (Novak et al., 2003). Ozone pollution leaves no elemental residue in plant tissues that can be detected by analytical techniques; therefore, visible injury on leaves and needles is the only easily detectable indication in the field.

Although visible symptoms do not include all the possible forms of injury to vegetation (i.e. physiological changes, reduction in growth, etc.), observation of typical symptoms on foliage has turned out to be a valuable tool for the assessment of the impact of ambient ozone concentrations on sensitive plant species (Gottardini et al., 2017a).The assessment of ozone visible injury serves therefore as a means to estimate the ozone potential risk for European ecosystems, and is very relevant in the context of ICP Forests (Schaub et al., 2010; Gottardini et al. 2017b).

The main objective of assessing ozone visible injury is to contribute to an ozone risk assessment for European forest ecosystems. In this paper, we provide first, comprehensive results on occurrence of ozone visible injury over space and time in Europe (see Gottardini et al., 2016). The potential impact of tropospheric ozone on forest ecosystems will be discussed and a possible approach for assessing the ozone effect on forest growth will be presented.