EFFECT OF STAND DENSITY ON DAMAGE TO BIRCH (BETULA-PUBESCENS) CAUSED BY PHYTOTOXIC AIR-POLLUTANTS

Premature yellowing and browning of the foliage of deciduous trees was observed in western Finland up to the Arctic Circle during summer 199 1. There is growing evidence that ubiquitous volatile organic air poll utants, especially C-2-halocarbons, may be converted to various second ary air pollutants, which are phytotoxic and broadly classed as herbic ides. One of these, trichloroacetic acid, is found in considerable con centration in the foliage of northern forest trees. If uptake from the atmosphere is the prevailing process in tree exposure to herbicides, stand density of a forest should influence the extent of the damage. A n experimental area for tending of young stands in various densities ( 500-5 100 stems per hectare), located within the damage zone in Rovani emi, provided an ideal opportunity to investigate the damage and degre e of defoliation of birch trees (Betula pubescens Ehrh.). The assessme nt was made in summer 1992. The proportion of injured trees and their degree of defoliation were inversely correlated with stand density exc eeded 3 000 trees per hectare.