Thermal optimum (Topt) and Aopt of An varied greatly during the growing season (from May to October). Topt and Aopt showed no interaction effect of seed source and month during the growing season (Table 4). Mean Topt varied from 12.6 to 19.5 °C and was highest in June and August and lowest in May (Fig. 5). Mean Aopt was higher in June than in May (Fig. 5). Both Aopt for the northern seed source and Topt for both seed sources were correlated with mean temperature 5 d before measurements (Fig. 4). The basal rate of Rd (Rd10) but not Q10 varied greatly during the growing season and neither of them showed any interaction effect of seed source and month (Table 4). Mean Rd10 was higher in May than in the remaining months excluding October (Fig. 5). Rd10 was negatively and linearly related to mean temperature 5 d before measurements (Fig. 4). In contrast, Q10 was not related to mean temperature 5 d before measurements (Fig. 4).