Effect of Stand Density on Damage to Birch (Betula pubescens) Caused by Phytotoxic Air Pollutants

1993

15 Lit. Ang.

Unselbständiges Werk

4570

200048396

Premature yellowing and browning of the foliage of deciduous trees was observed in western Finland up to the Arctic Circle during summer 1991. There is growing evidence that ubiquitous volatile organic air pollutants, especially C2-halocarbons, may be converted to various secondary air pollutants, which are phytotoxic and broadly classed as herbicides. One of these, trichloroacetic acid, is found in considerable concentration 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. An experimental area for tending of young stand in various densities (500-5 100 stems per hectare), located within the damage zone in Rovaniemi, provided an ideal opportunity to investigate the damage and degree of defoliation of birch trees (Betula pubescens Ehrh.). The assessment was made in summer 1992. The proportion of injured trees and their degree of defoliation were inversely coorelated with stand density. The proportion of undamaged trees was much greater where density exceeded 3 000 trees per hectare.