Abstract

Keywords

Ozone; Forest trees; ozone symptoms; phenole; chlorophyll pigments

Abstract

During periods of hot and dry conditions, as experienced in Europe during 2003, not only climatic conditions but also the concentrations of air pollutants such as tropospheric ozone can change drastically. Beech trees are sensitive to ozone and show specific symptoms in their leaves even below the AOT40 critical threshold for forest trees. As a consequence of dry weather and soil moisture conditions stomata are closing leading to reduced ozone uptake of the leaf. In this study, we assessed beech trees within stands or along forest edges of seven Swiss long-term forest ecosystem research plots (LWF/ICP-Forests Level II plots), where site and climate conditions have been monitored since 1998. Drought and ozone symptoms were analyzed on branches of the upper crown which were sampled from several, randomly selected trees per plot. Visible foliar symptoms were validated using microscopical analyses and quantified with color charts. The underlying changes in the content of tannins and pigments were measured with spectrophotometric techniques. During the 2003 vegetation season and in comparison to averages based on seven years of meteorological data, temperature increased by 1.7 degrees, precipitation was reduced by 44 % and the ozone dose was 1.5 times higher. Symptoms indicating drought stress were frequent and those induced by ozone were found at all investigated sites. With increasing symptom intensity, the photosynthetic pigment content of leaves was reduced and that of tannins increased. Smaller and biologically more active tannin oligomers were inlayed along the water pathway in the veins and lower mesophyll of drought-symptomatic samples and in the upper mesophyll of those with ozone symptoms. These findings suggests that 1) drought and ozone stress significantly injured the light exposed foliage, 2) the leaves showed visible and biochemical responses to both stress factors, 3) the leaf biochemical reactions can be distinguished at a tissue level.

LWF Classification

Network: LWF, Category: non-reviewed,