SEASONAL PERFORMANCE OF QUERCUS-ROBUR L ALONG AN URBANIZATION GRADIENT

Some seasonal time-course water relation and gas exchange parameters w ere monitored under natural conditions in three groups of 80-year-old Quercus robur L. trees growing along an urbanization gradient, in orde r to explain the positive correlation observed between the degree of d ie-back and urbanization exhibited by these trees. All three groups of trees exhibited the same general pattern of seasonal variation in pre dawn (psi(pd)) and midday (psi(md)) leaf water potential, net photosyn thesis (A), transpiration (E) and stomatal conductance (g), i.e. a dec rease as the season progressed (from spring to winter), The extent of these decreases were, however, more pronounced in the trees growing at the edge of town (group b) and the urban trees (group c). Unlike the rural trees (group a), which were able to maintain their A/E ratios wi thin 50% of that during spring and were typified by both lower A/g and E/g ratios, the trees of groups b and c were unable to maintain their seasonal A/E ratios and had higher A/g and E/g ratios. The trees of g roup a maintained higher A rates (accompanied by higher E) and could i ntercept, more radiation. We propose that the trees of groups b and c were dying back because of a reduced water supply, resulting in the co st to gain carbon (because of smaller leaf area, reduced light interce ption, insect herbivory, stromatal and non-stomatal inhibition) to exc eed the cost in terms of available water.