The effects of simulated urban pollution on trees and shrubs
In urban situations, plants are exposed to atmospheric pollutants derived from vehicle emissions in conjunction with hostile conditions such as low water availability, elevated temperatures and poor, compacted soils. This study used an experimental fumigation system for exposing trees and shrubs to simulated polluted urban atmospheres under near-natural environmental conditions. It also examined the interactions in plant responses to pollution and other stresses commonly encountered by plants in the urban environment. Initial screening studies of 19 broad-leaved tree and shrub species revealed variation between species in responses to urban pollution mixtures. Stomatal conductance was generally suppressed in responding species. In further studies, the'pollution regime was found to afford protection against drought by delaying wilting in Ligustrum ovalifolium. However, its effect of decreasing the root : shoot ratio in this species might impair drought resistance in the longer-term. In several species, urban pollution accelerated the onset of leaf senescence, which might be expected to have implications for long-term growth and survival in perennial species. In Quercus robur, there was a marked detrimental effect of exhaust gas pollution on surface waxes of the leaf cuticle, with associated changesin the water-repellency of leaves.A study of the common fimgal disease of sycamore, tarspot, showed infection to be drastically impaired by urban pollution at the concentrations used in this fumigation. Overall, there were surprisingly subtle effects on physiology in these species of a quite severe pollution treatment that represented the atmosphere of a polluted urban canyon. It may be that slow-growing perennial tree and shrub species such as those studied here are highly pollution-tolerant, or perhaps effects of pollution become apparent only over several growing seasons.