The process-based growth model, BIOMASS, was modified to incorporate low-temperature effects on photosynthetic production in Norway spruce (Picea abies) stands growing in northern Sweden. The low-temperature features incorporated in BIOMASS made it possible to simulate and estimate the reduction in photosynthetic rates caused by boreal conditions. The following four simulation-scenarios were used: (i) 'potential' photosynthesis without boreal restrictions; (ii) reduction caused by a frozen soil; (iii) reduction caused by incomplete recovery of photosynthetic capacity during spring as a result of damage caused by low winter temperatures; and (iv) reduction as an effect of frost-induced autumn decline. Annual photosynthetic production (or gross primary production (GPP)) was simulated for three calendar years, 1990-1992, for stands with low (control) and high (irrigated and fertilized) nutrient availability. The reduction of 'potential' GPP, caused by the low-temperature effects, ranged from 35-44% for control (C) and from 34-42% for irrigated-fertilise (IL) stands, respectively. The most pronounced loss of 'potential' GPP originated from reduced photosynthetic capacity, in spring and early summer, which led to losses of 21-28% for C and 19-26% for IL stands. The variation between years differed mainly as an effect of differences in spring temperatures, which resulted in different rates of recovery of photosynthetic capacity. Reductions caused by frozen soil and low photosynthetic capacity during winter were similar in C and IL stands (12-13%), as were the losses resulting from severe autumn frosts (3-4%). It is concluded that, unless the effects of frozen soils and reduced photosynthetic capacity during spring and early summer are considered, large errors (ca. 40%) will be introduced into estimates of the annual photosynthetic production of boreal conifer forests. (C) 1998 Elsevier Science B.V.
