The consequence of tree pests and diseases for ecosystem services

Trees and forests provide a wide variety of ecosystem services in addition to timber, food, and other provisioning services. New approaches to pest and disease management are needed that take into account these multiple services and the different stakeholders they benefit, as well as the likelihood of greater threats in the future resulting from globalization and climate change. These considerations will affect priorities for both basic and applied research and how trade and phytosanitary regulations are formulated.

Deposition and solubility of airborne metals to four plant species grown at varying distances from two heavily trafficked roads in London

In urban areas, a highly variable mixture of pollutants is deposited as particulate matter. The concentration and bioavailability of individual pollutants within particles need to be characterised to ascertain the risks to ecological receptors. This study, carried out at two urban parks, measured the deposition and water-solubility of metals to four species common to UK urban areas. Foliar Cd, Cr, Cu, Fe, Ni, Pb and Zn concentrations were elevated in at least one species compared with those from a rural control site. Concentrations were, however, only affected by distance to road in nettle and, to a lesser extent, birch leaves. Greater concentrations of metal were observed in these species compared to cypress and maple possibly due to differences in plant morphology and leaf surfaces. Solubility appeared to be linked to the size fraction and, therefore, origin of the metal with those present predominantly in the coarse fraction exhibiting low solubility.

Deposition velocities to Sorbus aria, Acer campestre, Populus deltoides X trichocarpa 'Beaupré', Pinus nigra and X Cupressocyparis leylandii for coarse, fine and ultra-fine particles in the urban environment

Trees are effective in the capture of particles from urban air to the extent that they can significantly improve urban air quality. As a result of their aerodynamic properties conifers, with their smaller leaves and more complex shoot structures, have been shown to capture larger amounts of particle matter than broadleaved trees. This study focuses on the effects of particle size on the deposition velocity of particles (Vg) to five urban tree species (coniferous and broadleaved) measured at two field sites, one urban and polluted and a second more rural. The larger uptake to conifers is confirmed, and for broadleaves and conifers Vg values are shown to be greater for ultra-fine particles (Dp < 1.0 microm) than for fine and coarse particles. This is important since finer particles are more likely to be deposited deep in the alveoli of the human lung causing adverse health effects. The finer particle fraction is also shown to be transported further from the emission source; in this study a busy urban road. In further sets of data the aqueous soluble and insoluble fractions of the ultra-fines were separated, indicating that aqueous insoluble particles made up only a small proportion of the ultra-fines. Much of the ultra-fine fraction is present as aerosol. Chemical analysis of the aqueous soluble fractions of coarse, fine and ultra-fine particles showed the importance of nitrates, chloride and phosphates in all three size categories at the polluted and more rural location.

Urban woodlands: their role in reducing the effects of particulate pollution

In recent years a substantial research effort has focused on the links between particulate air pollution and poor health. As a result the PM10 value has been set as a measure of such pollutants which can directly cause illness. Due to their large leaf areas relative to the ground on which they stand and the physical properties of their surfaces, trees can act as biological filters, removing large numbers of airborne particles and hence improving the quality of air in polluted environments. The role of vegetation and urban woodlands in reducing the effects of particulate pollution is reviewed here. The improvement of urban air quality achieved by establishing more trees in towns and cities is also illustrated.

The uptake of particulates by an urban woodland: site description and particulate composition

Woodlands may improve local air quality by increasing the uptake rates of gaseous, particulate and aerosol pollutants from the atmosphere and can also act as relatively permanent sinks for some pollutants. Rough Wood, Walsall was selected for a study of the material which accumulates on tree foliage because of its location in a densely populated urban area, and its proximity to a motorway with high traffic flow (the M6) and to other pollutant sources. Methods were developed for leaf washing to allow determination of the quantity of dust and the identification of the dust particles present on oak leaves. Elemental analysis of particles was also undertaken using scanning electron microscopy coupled with electron probe microanalysis. A large proportion of particles were organic in origin. Of the inorganic particles, the majority contained silicon and aluminium in varying proportions suggesting that they were soil derived. Some particles were clearly identified as the products of combustion, and sea or road salt was present on leaf surfaces. Some particles contained copper, tin and titanium which may reflect the proximity of Rough Wood to local metal workings. The number of particles counted on leaf surfaces decreased as distance from the motorway increased.

Predisposition of trees by air pollutants to low temperatures and moisture stress

Air pollution can have direct effects on trees. It can cause visible injury to foliage and a disruption of physiological processes, such as photosynthesis and carbon allocation, leading to losses in growth and productivity. This review suggests that of equal or greater importance is the potential of air pollutants to indirectly affect tree growth and vitality by predisposing them to injury from other abiotic and biotic stresses. Predisposition by air pollutants can be the result of a disruption in biochemical processes, such as enzyme activity or production, or physiological factors (e.g. stomatal closure, carbon allocation). Air pollutants such as SO(2), O(3) and acidic mists have been implicated as predisposing agents to two of the most important of these stresses: low temperature and soil moisture. Probable mechanisms, as well as implications of predicted changes in global climate will be discussed.

The control of ozone uptake by Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. during drought and interacting effects on shoot water relations

Exposure to O₃ alone has not yet been shown to reproduce the symptoms of the various types of spruce decline which have been identified in Europe. However, there is increasing evidence that this pollutant has physiological effects which interact with those of other environmental factors in ways which may be important in determining tree condition and growth. The effects of O₃ episodes and drought on O₃ uptake, gas exchange and water relations of Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. were investigated in two experiments. In the first a rapidly drying soil mixture was used, and seedlings of P. abies were exposed to short (2 h) daily episodes of O₃ at 80 nl 1^-1 on each day of a 5 d drought. Photosynthesis (A) and stomatal conductance (g_s ) were significantly decreased (P= 0.01) by water deficit and as a consequence, uptake of O₃ by the plants was also significantly decreased. Exposure to O₃ did not affect A or g_s for this species. In the second experiment a soil mixture designed to give a slower development of water deficit was used and 1 + 1 transplants of P. sitchensis were exposed to a single O₃ episode (up to 100 nl 1^-1 for 3 h) after water had been withheld for 7 or 14 d. Hofler diagrams showed that mild water deficits did not affect shoot water relations. However, O₃ significantly increased solute potential (Ψ_s ) after 7 d of drought, an effect which was lost after 14 d of drought. Flux of O₃ to the watered plants was greater than to the unwatered plants at all concentrations, the effect being more marked at higher concentrations. This effect was partly attributable to the greater stomatal conductances recorded for the well watered plants, but was also partly due to stomatal opening caused by O₃ , an effect which was diminished or reversed for unwatered plants.

Ozone flux to Picea sitchensis (Bong) Carr and Picea abies (L) Karst during short episodes and the effects of these on transpiration and photosynthesis

Sitka spruce and Norway spruce were grown in controlled environments and then exposed to ozone (O3) for short periods as in mid-afternoon episodes experienced in the forest. For concentrations of between 20 and 300 nl litre(-1) there were linear relationships between exposure concentration and O3 uptake rates. Increasing photon flux densities increased rates of photosynthesis and transpiration, the increases being larger in actively growing than dormant seedlings. Physiological condition (dormancy or active growth), species and photon flux density were found to influence O3 flux via their effects on stomatal conductance. Exposure to 80 nl litre(-1) O3 resulted in consistent increases of stomatal conductance and there were also indications that water-use efficiency was decreased.

Rapporteurs' report on discussions at the workshop on excess nitrogen deposition, Leatherhead, September 1987

This paper reports major points from the discussions which took place during the Workshop on Excess Nitrogen Deposition.