The model was fitted using non-linear regression techniques (Proc NLIN, SAS). To fit the model, the measured dark respiration (Rd) values were used as proxy for Rday in order to reduce the number of parameters estimated by the model. Respiration occurring in daylight (Rday), which is assumed to be primarily mitochondrial respiration, was assumed to approximate dark respiration (Rd), as observed for black spruce by Way and Sage (2008b). The values at 25 °C used for Kc and Ko were 272 µmol mol−1 and 166 mmol mol−1, respectively (Sharkey et al., 2007), and its temperature dependency is given by: