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

The broad range in plant responses to chronic O-3 exposure compels a search for integrative, underlying principles. One such approach is the unifying theory proposed by Reich (1987), which combines the O-3 response of contrasting physiognomic classes of plants on the basis of their intrinsic leaf diffusive conductance and, hence, capacity for O-3 uptake. Physiognomic classes differ in the proportional decline in photosynthesis and growth when compared on the basis of cumulative O-3 exposure per unit time, but converge when compared on the basis of O-3 uptake per unit time or cumulative O-3 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 O-3 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 O-3 concentration delivered into the canopy by means of a free-air fumigation system. We accounted for differences in growing season length, leaf longevity and O-3-related effects on leaf diffusive conductance in determining total O-3 uptake over the lifetime of the leaf. On this basis, Norway spruce needles required 5 years to take up as much O-3 as did beech leaves in one growing season. The core of the unifying theory on O-3 sensitivity was substantiated in relation to O-3 exposure and uptake. However, contrary to the unifying theory, which was formulated on the basis of results with juvenile trees, the O-3 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 O-3 sensitivities of leaves according to age class and position within the canopy. Contrary to the theory, photosynthesis was no less sensitive to O-3 in Norway spruce than that of beech, and was reduced in the twice-ambient O-3 regime in the first year of exposure.