The relationship between the temperature optimum at which maximum net CO₂ uptake occurred and the average maximum air temperature of the 10 days preceding the measurements obtained in fall (open symbols) and summer (closed symbols). Points were fit by linear regression: fall: y = 0.98x − 3.7, r² = 0.97; summer: y = 1.05x − 2.3, r² = 0.98.