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SynopsisUnravelling the subtle effects of air pollution on vegetation requires adherence to the experimental method for testing hypotheses. Three experimental approaches are described.Field releaseof pollutants causes minimal disturbance of other aspects of the environment but is difficult to control and to operate continuously.Closed chambers, such as glasshouses, are furthest removed from field conditions but many aspects of their environments can be controlled. There is scope for the more sophisticated use of computer controlled glasshouses to investigate responses of stands of crop plants and natural/semi natural communities.Open-top chambers(OTCs) are a popular research tool, but results from major studies such as the U.S. National Crop Loss Assessment Network are of uncertain general value. Incursion of air into the tops of OTCs creates vertical pollution gradients. Evaporation, and the stomatal control of transpiration in OTCs may be very different from that in the field. Uptake of pollutant gases in OTCs may also differ from that in the field, directly because of differences in air movement, and/or indirectly through differences in the distribution of temperatures and moisture.The development and design of a U.K. research programme on red spruce is used to illustrate (i) the need to develop hypotheses from a wide range of observations, (ii) the advantages of using a range of experimental approaches and (iii) the requirement to synthesise results before reaching general conclusions.

Forest ecosystems have distinctive microclimates, characterised particularly by their efficient absorption of solar radiation, and the turbulence generated by air flow over their foliage. Light quantity and quality on the forest floor influence plant development. Strong coupling to the atmosphere minimizes mean temperature gradients in the air within and above forests, but foliar temperatures may differ considerably from air temperatures in sheltered environments. The sensitivity of the stomata of many tree species to atmospheric humidity controls the energy balance of dry forest canopies. Evaporation of intercepted rain is a large term in the hydrological balance of many forest ecosystems.

Declining growth rates and injury to foliage have been observed in forests at high elevations in areas of Europe and North America1,2; in such areas the trees are regularly enveloped in cloud and mist. Concentrations of dissolved chemicals in cloud drops are often much larger than in rain, so the capture of cloud droplets by leaves may give rise to significant rates of ‘occult deposition’3 of acidic pollutants and heavy metals on forests3,4. I point out here that evaporation of intercepted cloudwater from forest canopies, proceeding simultaneously with occult deposition, can lead to chemical concentrations on leaf surfaces that are substantially larger than those measured in cloud drops themselves. These large chemical concentrations may be a cause of the observed injury in forests where wind-driven cloud is common.

Deposition of materials from the atmosphere onto the earth’s surface involves many complicated processes. Gases, liquids, and solids are deposited over time periods of minutes to months after they are placed into the atmosphere. Once airborne, matter can change its chemical and physical characteristics. Constituents in rain that confer acidity such as H+, NO3 −, and SO4 2−, as well as gaseous pollutants such as O3, SO2, and NOX are of concern because they may have affected ecosystems in North America and Europe.

The capture of wind-driven cloud by vegetation provides a pathway for pollutant deposition that has only recently been identified, and remains only poorly quantified. This paper reviews current knowledge of three aspects of the pathway: measurement and modelling the rates of deposition of cloud water to various Vegetation types; techniques for monitoring the concentrations of soluble and solid material in cloudwater; and potential mechanisms for injury to Vegetation by the deposited material.

In a brief review article it is not possible to cover all aspects of a topic such as this, nor can a full sense of historical perspective be given. We have attempted to survey the major sources of pollutants, pollutant uptake or deposition on plants and the physiological and biochemical responses to pollutant exposure. Interactions with diseases and pests or with plant nutrition are scarcely treated, and the interested reader is referred to articles by Heagle (1982) or Cowling and Koziol (1982) for those subjects.

Measurements of the composition of aerosol in the Sudan Gezira were made in southerly and northerly airstreams during September and October 1973. By using iron as a normalizing element and calculating enrichment factors it is shown that natural dust is the source of most elements. Exceptions are chlorine, bromine, sulphur and nitrogen which appear to have gaseous sources. Substantial concentrations of oxidized nitrogen and sulphur compounds were measured and are probably produced photochemically. Concentrations of sulphur and nitrogen compounds were lower than in Europe but suggest that anthropogenic sources are having equal impact on their respective natural cycles. Finally, comparisons of aerosol in the Sudan and at Harwell show that the natural Sudanese aerosol is potentially basic whereas anthropogenic European aerosol is acid. DOI: 10.1111/j.2153-3490.1979.tb00908.x

(1) Stands of Salix viminalis L. (clone Bowles hybrid), growing near Edinburgh, were studied in two successive years. In 1984, the stands grew from coppiced stumps at 0 5 m spacing in the field. In 1985, the stands grew from cuttings in containers of compost, at 0 3 m spacing, supplied with trickle irrigation and nutrients. (2) Interception of total solar radiation by the canopies was measured using tube solarimeters. Regular harvests were made to determine above-ground dry matter in 1984 and both above-ground and root dry matter in 1985. Leaf area index (L) was also determined, and total stem surface area was estimated in 1985. (3) Accumulated dry matter was linearly related to accumulated intercepted radiation until heavy leaf fall. The slopes of the relationships for above-ground dry matter were 0 99 g MJ-I in 1984 and 1 38 g MJ-1 in 1985. This difference in light-use efficiency (rather than a difference in amounts of light intercepted) was primarily responsible for a difference between years in above-ground dry matter production (11 t ha-1y-I in 1984 and 14 t ha y1 in 1985). Greater efficiency in 1985 was attributed to better water and nutrient supplies and lower levels of incoming radiation. (4) L reached 2 4 in 1984 and 4 5 in 1985. The relationship between L and fractional interceptance (f) was