Seasonal performance of Quercus robur L. along an urbanization gradient

Some seasonal time-course water relation and gas exchange parameters were monitored under natural conditions in three groups of 80-year-old Quercus robur L. trees growing along an urbanization gradient, in order to explain the positive correlation observed between the degree of die-back and urbanization exhibited by these trees. All three groups of trees exhibited the same general pattern of seasonal variation in predawn (psi(pd)) and midday (psi(md)) leaf water potential, net photosynthesis (A), transpiration (E) and stomatal conductance (g), i.e. a decrease 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 within 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 group a maintained higher A rates (accompanied by higher E) and could intercept, more radiation. We propose that the trees of groups b and c were dying back because of a reduced water supply, resulting in the cost to gain carbon (because of smaller leaf area, reduced light interception, insect herbivory, stromatal and non-stomatal inhibition) to exceed the cost in terms of available water.