Modelling Net CO₂ Assimilation of Two Sphagnum Species From Temperature and Water Content Response

Photosynthesis and respiration respond differently to the combined effects of temperature and water status. Quantifying their responses is crucial to predict the carbon balance of Sphagnum peatlands in different scenarios of climate change. A first approach was done for two Sphagnum species inhabiting a boreal peatland in Finland. Gas exchange at different temperatures and moss hydration were measured to model net assimilation using simultaneous measurements of photosynthesis and dark respiration. In addition, measurements of moss surface temperature at different water content were performed in the field, covering natural conditions of sun exposure and air temperature. We also accounted for the interaction effect between moss canopy temperature and air temperature, radiation, and water content. Our model accurately predicted net assimilation and was used to estimate net primary productivity based on meteorological inputs and moss water content. The two Sphagnum species presented optimum temperatures for net CO₂ assimilation around 25°C, with minimum changes at other temperatures. In contrast, dark respiration increased exponentially with temperature, which makes losses of carbon during the night and the duration of dark conditions key determinants in the carbon balance of Sphagnum. The modeled net primary productivity revealed an enhancement of CO₂ fixation under warming conditions (averaged +10°C), concomitant to the expected transformation of peatlands from sink to source of CO₂. Our model highlighted the importance of respiration restriction in ensuring positive assimilation in Sphagnum. Therefore, day and night temperature oscillation and short night photoperiods are more important than the optimum temperature of photosynthesis for carbon balance.