The role of air and soil temperature in the seasonality of photosynthesis and transpiration in a boreal Scots pine ecosystem

Photosynthesis and transpiration in boreal forests are restricted by air temperature (Ta) and soil temperature (Ts), especially in spring after the dormant period, but the extent to which the recovery process is regulated these factors is still uncertain. To examine the role of air temperature and soil temperature, years with three types of typical patterns of temperature rises were identified from 13 years of continuous flux measurements for a Scots pine ecosystem in Hyytiälä, southern Finland. By combining a process-based ecosystem model (CoupModel) with an uncertainty estimation procedure (GLUE), the role of regulating factors was explored and 45 of 51 parameters were found to have reduced uncertainty after calibration. Air temperature was the major limiting factor for photosynthesis in early spring, autumn and winter, but soil temperature was a rather important limiting factor in late spring. Especially during warm spring with a large delay of soil temperature rise both water uptake and photosynthesis was strongly reduced due to low soil temperature. Soil moisture and nitrogen showed indications of being more important for regulating photosynthesis in the summer period. It proved possible to replace the soil temperature acclimation function on photosynthesis and transpiration with a corresponding air temperature function only during warm years with a small delay between Ta and Ts. Fluxes of photosynthesis and transpiration showed a sensitivity to the carbon footprint representation, as expected from the high spatial variability in soil temperature during the spring of a warm year with a large delay between Ta and Ts.