Testing the unifying theory of ozone sensitivity with mature trees of Fagus sylvatica and Picea abies

Abstract

The broad range in plant responses to chronic O3 exposure compels a search for integrative, underlying principles. One such approach is the unifying theory proposed by Reich (1987), which combines the O3 response of contrasting physiognomic classes of plants on the basis of their intrinsic leaf diffusive conductance and, hence, capacity for O3 uptake. Physiognomic classes differ in the proportional decline in photosynthesis and growth when compared on the basis of cumulative O3 exposure per unit time, but converge when compared on the basis of O 3 uptake per unit time or cumulative O3 uptake over the entire lifetime of the leaf. The theory is based on observations on a large number of contrasting plant species, relying primarily on studies of juvenile trees subjected to short-term O3 exposure. To test the applicability of the unifying theory to mature trees, broadleaf deciduous European beech (Fagus sylvatica L.) and the evergreen conifer Norway spruce (Picea abies (L.) Karst.) in a mature mixed stand were exposed to either ambient air (control) or air with twice the ambient O3 concentration delivered into the canopy by means of a free-air fumigation system. We accounted for differences in growing season length, leaf longevity and O3-related effects on leaf diffusive conductance in determining total O3 uptake over the lifetime of the leaf. On this basis, Norway spruce needles required 5 years to take up as much O3 as did beech leaves in one growing season. The core of the unifying theory on O3 sensitivity was substantiated in relation to O3 exposure and uptake. However, contrary to the unifying theory, which was formulated on the basis of results with juvenile trees, the O3 response of mature trees in a natural stand was more complex. The increased complexity was attributed to additional environmental stressors, stress compensation at the whole-tree level, and differential O3 sensitivities of leaves according to age class and position within the canopy. Contrary to the theory, photosynthesis was no less sensitive to O3 in Norway spruce than that of beech, and was reduced in the twice-ambient O 3 regime in the first year of exposure. © 2006 Heron Publishing.