Leaf gas exchange and its temperature response were measured to assess temperature acclimation within tree canopies in thermally contrasting genotypes of Acer rubrum L. Over the course of two 50 day continuous periods, growth temperature was controlled within tree crowns and the steady-state rate of leaf gas exchange was measured. These data were then modeled to calculate the genotype variation and vertical distribution of physiological activity. Maximum RuBP saturated rate of carboxylation (V_cmax), the maximum rate of electron transport (J_max), leaf dark respiration rate (R_d), maximum photosynthesis (A_max), and the CO₂ compensation point increased with temperature during both (i) a constant long-term (50 d) daytime temperature or (ii) ambient daytime temperature with short-term temperature control (25 - 38 ^oC).  In addition, within-crown variation in the temperature response of photosynthesis and R_d was influenced by acclimation to local microclimate temperature gradients.  Results indicated that carbon uptake estimates could be overestimated by 22-25% when temperature gradients are disregarded.  Temperature is the major factor driving photosynthetic acclimation and within-crown gas exchange variation.  Thus, the importance of including both spatial acclimation to temperature and provenance, ecotype, and/or genotype specific parameter sets into carbon uptake models is established.