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

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.