The impact of air pollution on terrestrial managed and natural vegetation.

Although awareness that air pollution can damage vegetation dates back at least to the 1600s, the processes and mechanisms of damage were not rigorously studied until the late twentieth century. In the UK following the Industrial Revolution, urban air quality became very poor, with highly phytotoxic SO2 and NO2 concentrations, and remained that way until the mid-twentieth century. Since then both air quality, and our understanding of pollutants and their impacts, have greatly improved. Air pollutants remain a threat to natural and managed ecosystems. Air pollution imparts impacts through four major threats to vegetation are discussed through in a series of case studies. Gas-phase effects by the primary emissions of SO2 and NO2 are discussed in the context of impacts on lichens in urban areas. The effects of wet and dry deposited acidity from sulfur and nitrogen compounds are considered with a particular focus on forest decline. Ecosystem eutrophication by nitrogen deposition focuses on heathland decline in the Netherlands, and ground-level ozone at phytotoxic concentrations is discussed by considering impacts on semi-natural vegetation. We find that, although air is getting cleaner, there is much room for additional improvement, especially for the effects of eutrophication on managed and natural ecosystems. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Carbon and nitrogen assimilation in red oaks (Quercus rubra L.) subject to defoliation and nitrogen stress.

To examine how rates of net photosynthesis and N uptake of red oak seedlings respond to defoliation under contrasting conditions of N availability, nitrogen-deficient plants were grown in sand culture and subjected to partial defoliation and increased N availability under low light conditions. Both photosynthesis and N uptake rates were measured regularly before and after the treatments. Defoliation resulted in elevated rates of net photosynthesis in both low-N and high-N trees, but the high-N trees were able to maintain the high photosynthetic rates for a longer period of time. Nitrogen availability did not affect the photosynthetic rate of the undefoliated plants. Nitrogen uptake was not affected by the defoliation treatment, but was increased by increasing N availability in both the defoliated and undefoliated plants. Nitrogen uptake rates increased less than would be expected on the basis of N availability alone, but the uptake rates were apparently not limited by carbon supply in the short term. Suboptimal concentrations of N in plant tissues resulted in a strong sink for N even in the absence of refoliation.

Atmospheric deposition and canopy interactions of major ions in a forest.

Airborne particles and vapors contributed significantly to the nutrient requirements and the pollutant load of a mixed hardwood forest in the eastern United States. Dry deposition was an important mechanism of atmospheric input to the foliar canopy, occurring primarily by vapor uptake for sulfur, nitrogen, and free acidity and by particle deposition for calcium and potassium. The canopy retained 50 to 70 percent of the deposited free acidity and nitrogen, but released calcium and potassium. Atmospheric deposition supplied 40 and 100 percent of the nitrogen and sulfur requirements, respectively, for the annual woody increment. This contribution was underestimated significantly by standard bulk deposition collectors.

Cloud droplet deposition in subalpine balsam fir forests: hydrological and chemical inputs.

Subalpine forests of the northern Appalachians are subject to significant deposition of water and chemicals via cloud droplet impaction. This deposition has been estimated by a method linking micrometeorological measures of turbulent transfer, a detailed representation of canopy structure, and experimentally derived capture efficiencies. Water inputs from clouds are about 46 percent, and chemical inputs range from 150 to 430 percent of the bulk precipitation.