Influences of light and humidity on carbonyl sulfide-based estimates of photosynthesis

Carbonyl sulfide (COS) measurements enable quantification of terrestrial photosynthesis, which cannot be directly measured at scales greater than the leaf level. The accuracy of COS-based estimates of gross primary production (GPP) depends on how we relate the COS uptake to that of CO2. This study shows that COS-based GPP estimates will be significantly overestimated if the different environmental responses of COS and CO2 uptake are not taken into account. These findings are relevant for studies that rely on COS to quantify ecosystem to regional scale GPP, and support the use of a COS-based approach to constrain ecosystem flux partitioning. Moreover, the strong stomatal control on COS uptake shown in this study makes COS a suitable tracer for stomatal diffusion.Understanding climate controls on gross primary productivity (GPP) is crucial for accurate projections of the future land carbon cycle. Major uncertainties exist due to the challenge in separating GPP and respiration from observations of the carbon dioxide (CO2) flux. Carbonyl sulfide (COS) has a dominant vegetative sink, and plant COS uptake is used to infer GPP through the leaf relative uptake (LRU) ratio of COS to CO2 fluxes. However, little is known about variations of LRU under changing environmental conditions and in different phenological stages. We present COS and CO2 fluxes and LRU of Scots pine branches measured in a boreal forest in Finland during the spring recovery and summer. We find that the diurnal dynamics of COS uptake is mainly controlled by stomatal conductance, but the leaf internal conductance could significantly limit the COS uptake during the daytime and early in the season. LRU varies with light due to the differential light responses of COS and CO2 uptake, and with vapor pressure deficit (VPD) in the peak growing season, indicating a humidity-induced stomatal control. Our COS-based GPP estimates show that it is essential to incorporate the variability of LRU with environmental variables for accurate estimation of GPP on ecosystem, regional, and global scales