Ontogenetic patterns of leaf CO ₂ exchange, morphology and chemistry in Betula pendula trees

In order to explore ontogenetic variation in leaf-level physiological traits of Betula pendula trees, we measured changes in mass- (A(mass)) and area-based (A(area)) net photosynthesis under light-saturated conditions, mass- (RS(mass)) and area-based (RS(area))leaf respiration, relative growth rate, leaf mass per area (LMA), total nonstructural carbohydrates (TNC), and macro- and micronutrient concentrations. Expanding leaves maintained high rates of A(area), but due to high growth respiration rates, net CO ₂ fixation occurred only at irradiances >200 μmol photons m ^-2 s ^-1 . We found that full structural leaf development is not a necessary prerequisite for maintaining positive CO ₂ balance in young birch leaves. Maximum rates of A(area) were realized in late June and early July, whereas the highest values of A(mass) occurred in May and steadily declined thereafter. The maintenance respiration rate averaged ≃8 nmol CO ₂ g ^-1 s ^-1 , whereas growth respiration varied between 0 and 65 nmol CO ₂ g ^-1 s ^-1 . After reaching its lowest point in mid-June, leaf respiration increased gradually until the end of the growing season. Mass and area-based dark respiration were significantly positively correlated with LMA at stages of leaf maturity, and senescence. Concentrations of P and K decreased during leaf development and stabilized or increased during maturity, and concentrations of immobile elements such as Ca, Mn and B increased throughout the growing season. Identification of interrelations between leaf development, CO ₂ exchange, TNC and leaf nutrients allowed us to define factors related to ontogenetic variation in leaf, level physiological traits and can be helpful in establishing periods appropriate for sampling birch leaves for diagnostic purposes such as assessment of plant and site productivity or effects of biotic or abiotic factors.