Air pollution and vegetation: hypothesis, field exposure, and experiment

Abstract

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.