DEPENDENCE OF THE ABOVEGROUND CO2 EXCHANGE-RATE ON TREE SIZE IN FIELD-GROWN HINOKI CYPRESS (CHAMAECYPARIS-OBTUSA)

The CO2 exchange of the aboveground parts for five different-sized 17- year-old (as of 1991) hinoki cypress (Chamaecyparis obtusa) trees grow ing in the field was nondestructively measured over one year, using an open CO2 exchange system. The CO2 exchange of individual trees decrea sed with decreasing tree sizes, such as aboveground phytomass, leaf ma ss and leaf area. However, the CO2 exchange abruptly decreased near th e smallest-suppressed sample tree. The size dependence was well descri bed by a generalized power function. The annual gross photosynthesis o f individual trees was proportional to the square root of leaf mass or leaf area. The dependence of CO2 exchange on annual phytomass increme nt was described by a simple power function with an exponent value les s than unity, suggesting that CO2 exchange per unit of phytomass incre ment was lower in larger-sized trees than in smaller-sized trees. The mean photosynthetic activity of a tree, i.e., gross photosynthesis per unit of leaf area, slightly increased to as highest value with decrea sing leaf area and then decreased abruptly near the smallest sample tr ee. The maximum value of mean photosynthetic activity was estimated to be 2.85 kg CO2 m(-2) year(-1) for a leaf area of 1.56 m(2) tree(-1). The ratio of mean photosynthetic activity to the maximum photosyntheti c activity was the highest in an intermediate tree and decreased gradu ally toward larger-sized trees by ca. 60% and also decreased toward th e smallest suppressed tree by ca. 35%.