A method using δ¹³C records of annual growth rings has been proposed for assessing environmental stresses in mature forest trees. First, we calculate the change with time in the difference (C_a-C_i) between the CO₂ concentration in the atmosphere and the leaf intercellular spaces from the δ¹³C records in tree cellulose based on a carbon isotope fractionation model for C₃ plants. The C_a-C_i value is dependent primarily on the mesophyll capacity for photosynthesis and the supply of CO₂ through the stomata. Next, stomatal limitation of photosynthesis is evaluated from the ratio of ring area to C_a-C_i which is possibly indicative of leaf conductance. These parameters for the trees from a declining grove of Japanese cedar (Cryptomeria japonica) in the Kanto Plain, Japan were suggested to respond sensitively to the changes in air pollution (mainly SO₂) and/or dry atmospheric conditions. This method offers a potentially useful research tool for evaluating possible causes for forest decline.