Silver nanoparticles enter the tree stem faster through leaves than through roots.

A major environmental pollution problem is the release into the atmosphere of particulate matter, including nanoparticles (NPs), which causes serious hazards to human and ecosystem health, particularly in urban areas. However, knowledge about the uptake, translocation and accumulation of NPs in plant tissues is almost completely lacking. The uptake of silver nanoparticles (Ag-NPs) and their transport and accumulation in the leaves, stems and roots of three different tree species, downy oak (Quercus pubescens Willd.), Scots pine (Pinus sylvestris L.) and black poplar (Populus nigra L.), were assessed. In the experiment, Ag-NPs were supplied separately to the leaves (via spraying, the foliar treatment) and roots (via watering, the root treatment) of the three species. Uptake, transport and accumulation of Ag were investigated through spectroscopy. The concentration of Ag in the stem was higher in the foliar than in the root treatment, and in poplar more than in oak and pine. Foliar treatment with Ag-NPs reduced aboveground biomass and stem length in poplars, but not in oaks or pines. Species-specific signals of oxidative stress were observed; foliar treatment of oak caused the accumulation of H2O2 in leaves, and both foliar and root treatments of poplar led to increased O2- in leaves. Ag-NPs affected leaf and root bacteria and fungi; in the case of leaves, foliar treatment reduced bacterial populations in oak and poplar and fungi populations in pine, and in the case of roots, root treatment reduced bacteria and increased fungi in poplar. Species-specific mechanisms of interaction, transport, allocation and storage of NPs in trees were found. We demonstrated definitively that NPs enter into the tree stem through leaves faster than through roots in all of the investigated tree species.

Variation of gas exchange within native plant species of Switzerland and relationships with ozone injury: an open-top experiment.

Gas exchange and ozone-induced foliar injury were intensively measured during a 6-day period in mid-August 1998 on leaves of Acer pseudoplatanus, Betula pendula, Corylus avellana, Fagus sylvatica, Fraxinus excelsior, Morus nigra, Prunus avium, Prunus serotina, Rhamnus cathartica, and Viburnum lantana at a forest nursery site in Canton Ticino, Switzerland. Plants were grown in four open plots (AA), four open-top chambers receiving carbon-filtered (CF) air, and four receiving non-filtered (NF) air. Significant variation in gas exchange (F > 12.7, P < 0.001) was detected among species with average net photosynthesis and average stomatal conductance differing by a factor of two. Species also varied significantly in foliar injury for those leaves for which we measured gas exchange (F = 39.6, P < 0.001). Fraxinus excelsior, M. nigra, P. avium, P. serotina, R. cathartica, and V. lantana showed more injury than A. pseudoplatanus, B. pendula, C. avellana, and Fagus sylvatica. Plants grown in CF chambers had significantly higher net photosynthesis (A) and stomatal conductance to water vapor (gwv), and lower foliar injury than plants grown in NF chambers and AA plots; interactions between species and ozone treatments were significant for all variables (F > or = 2.2, P < 0.05) except gwv (F = 0.7, P > 0.1). Although A and gwv decreased and foliar injury increased with leaf age, the magnitude of these changes was lower for plants grown in CF chambers than for plants grown in NF chambers and AA plots. Neither ozone uptake threshold (r = 0.26, P > 0.20) nor whole-plant injury (r = -0.15, P > 0.41) was significantly correlated with stomatal conductance across these species. It appears that the relationships between stomatal conductance and foliar injury are species-specific and interactions between physiology and environments and leaf biochemical processes must be considered in determining species sensitivity to ambient ozone exposures.

Air pollution and forest decline in Central Europe.

The term 'Waldsterben' was introduced in the early 1980s to describe the progressive death of forests that was believed to be occurring in Central Europe as a result of air pollution. Subsequent surveys and investigations have failed to confirm that forests are dying or are even declining over large areas of Central Europe, defined here as consisting of Germany, Switzerland, southeastern France (Alsace), the Czech Republic, northern Italy and Austria. Foliar injury by air pollutants, together with mortality, has occurred, but is generally restricted to specific locations in the Czech Republic and in eastern Germany, such as the Fichtelgebirge. Where foliar damage has been recorded, it can often be attributed to high concentrations of sulphur dioxide, often acting in combination with other stresses (e.g. frost or insects). Outside areas affected by local sources of pollution, there is little, if any, evidence that the crown condition of trees has been adversely affected by pollution over large areas. Instead, climate appears to have a major effect on the crown condition and growth of trees. Measurements and surveys have revealed a very different picture to that forecasted in the mid-1980s. Growth rates of trees and stands in Central Europe are currently higher than have been recorded at any time in the past; the reasons for this are uncertain, although increases in forest area have not substantially contributed to the observed trends. Although declines in individual species in specific areas have been recorded, past records indicate that these do not represent a new phenomenon. Consequently, the terms 'Waldsterben' (forest deaths) and 'neuartige Waldschäden' (novel type of forest damages) should not be used in the context of the phenomenon reported in Central Europe in the 1980s. Instead, different problems should be described separately and the term forest decline used only when there is clear evidence of a general deterioration in the condition of all tree species within a forest.

Influence of air pollution on the foliar nutrition of conifers in Great Britain.

The nutritional status of needles from Sitka spruce, Norway spruce and Scots pine in a total of 108 stands was assessed. There was little evidence of nutritional deficiency, although potassium levels were frequently quite low. Analysis of some heavy metals (lead and copper) failed to reveal any likely toxicity problems. Sulphur, nitrogen and iron levels in/on the foliage were all related to various measures of sulphur and nitrogen pollution, determined using improved deposition models that take into account cloud deposition and the seeder-feeder mechanism. The analysis strongly suggested that direct air pollution has a greater effect on sulphur, nitrogen and iron foliar analyses than indirect pollution (wet deposition). The relationships were identified for levels of pollution that were generally lower than those seen in traditional gradient studies.

Theoretical and practical criteria for the selection of ecosystem monitoring plots in Swiss forests.

A great deal of attention has been paid to the selection of nature reserves. These are important from a conservation viewpoint but, for long-term evaluations, it is important to monitor ecosystems. The need for long-term monitoring plots has been recognized for some time in forest ecology. Of the natural ecosystems, forests are some of the most difficult to monitor because of the time-scales involved in the life-spans of the dominant organisms (100 → 1000 years). The selection of long-term forest ecosystem monitoring plots is a critical process involving decisions that need to remain valid for many years. Traditional sampling theory suggests that some form of systematic or random sampling may be appropriate, but this is usually inappropriate for the selection of ecosystem monitoring plots. Instead, the selection of plots more closely resembles some of the procedures that are used in the selection of nature reserves. In Switzerland, a monitoring programme has been established which uses a number of criteria for the selection of sites. These include site homogeneity, the abundance and sensitivity of the plant communities to change and the presence of pre-existing data series or monitoring equipment. In addition, the human factor is incorporated by selecting sites from throughout the country, with the willingness of the local forest managers to help with the project being an important factor influencing the final choice of plots. In contrast to most inventories, statistical representativeness is not a requirement for the programme, as the plots are treated as a series of case studies.

Climatic sensitivity of temperate forests.

Climatic change and associated global changes are of major interest to foresters, both in terms of forest ecology and of future forest production. Predicting the likely effects of global change on forests is extremely difficult due to the critical lack of information on regional changes in meteorological factors relevant to forests. However, existing models of forest production and forest distribution fail to take adequate account of what is already known. Climate and carbon dioxide concentrations have shown substantial changes over the last 100 years. Although the rate of change is likely to increase, recent proposed and implemented control strategies, together with better climatic models, are tending to suggest that the rate of change will be less than initially thought. This means that past changes may provide an increasingly useful source of information. In particular, information on the impact on forests of both long-term climate change and short-term climatic events is rapidly increasing. Such information should be built into future forest response models.

Reliability of differing densities of sample grids used for the monitoring of forest condition in Europe.

Concern about the possible deterioration of forest health led to the establishment in the 1980s of inventories of forest condition throughout Europe. International standardisation of the programmes was sought and a number of recommendations were made concerning sampling and assessment procedures. One of the most important rulings was that the assessment should be made on a systematic grid, the minimum density of which was 16×16 km. However, many countries adopted denser sampling grids, with 4×4 km being used in several countries and 1×1 km being used in the Netherlands. With five or more years of monitoring completed, there is a growing belief that a rapid and irreversible decline in forest health is not occurring. Consequently, some countries/regions are seeking to reduce their annual investment in forest health monitoring.The precision of national/regional estimates of forest health can be directly related to the sample size. As the sample size decreases, so also does the precision of the estimates. This is illustrated using data collected in Switzerland in 1992 and using grid densities of 4×4 km, 8×8 km, 12×12 km and 16×16 km. The value of the data is dependent on the sample size and the degree to which it is broken down (by region or species). The loss of precision associated with most subdivisions at the 8×8 km grid level remains acceptable, but a sharp deterioration in the precision occurs at the 12×12 km and 16×16 km grid levels. This has considerable implications for the interpretation of the inventories from those countries using a 16×16 km grid. In Switzerland, a reduction from the current 4×4 km grid to an 8×8 km grid (i.e. 75% reduction in sample size) would have relatively little impact on the overall results from the annual inventories of forest health.

Consistency of observations of forest tree defoliation in three European countries.

The crown densities of 186 trees of five common European tree species (Norway spruce (Picea abies), silver fir (Abies alba), Scots pine (Pinus sylvestris), oak (Quercus robur) and beech (Fagus sylvatica) were assessed simultaneously by observation teams from France, Germany and the United Kingdom. Major differences in the scores existed, with the maximum difference on any one tree being 45%. Differences tended to be consistent, with the French team scoring more lightly than the German team and the German team more lightly than the UK team. The differences throw into question the value of international comparisons of forest condition, particularly the use of comparative tables of the extent of "forest decline" in individual European countries.

Forest health surveys--a critique.

Surveys of forest health, as presently undertaken, provide information on the general condition of forests. They integrate the effects of all factors, biotic and abiotic, of which pollution is but one, that affect trees. Most surveys are based on the assessment of crown density and crown discoloration. These assessments are subjective and serious errors can occur. In some cases, the sampling design that has been adopted is not the most suitable for the forest area under investigation. The blanket application of a systematic sampling design based on a 16 by 16 km grid is particularly questionable in view of the variation in the nature of Europe's forests. Comparisons, either between regions or through time, may or may not be valid, depending on how the surveys are conducted. Although much progress has been made, standardisation within Europe has not yet been achieved and it is therefore difficult to make comparisons between the results obtained by different countries.