Phenology controls on CO2 exchange in a northern peatland: insights from a decade-long record of phenocam imagery and eddy-covariance data

Northern peatlands are recognised as important long-term carbon sinks. However, measurements from a number of peatland sites reveal a large amount of interannual variability in their net carbon dioxide (CO2) balance. Differences in both weather conditions and plant phenology (i.e. the seasonal development of the vegetation canopy) between years are thought to be key here. Timing of the growing season (i.e. start, end, length) regulates the period over which vegetation can actively photosynthesise. Hence, a longer growing season is often related to increased seasonal CO2 uptake for example. At the same time, meteorological conditions (e.g. air temperature, water-table depth) affect not only plant physiology, but also its phenological cycle. Our current understanding of the complex interplay between these two main drivers of peatland carbon dynamics has been limited by a lack of long-term phenology studies. This work explores a unique, decade-long record of phenocam and eddy-covariance data from Deger Stormyr, a northern Swedish peatland. We used structural equation modelling (SEM) to identify the pathways regulating CO2 uptake, and found that phenology plays an important mediator role over the growing season. Our analysis of the interannual and seasonal variability in the drivers of CO2 uptake further suggest that increases in vegetation greenness are linked to increased CO2 uptake over the growing season. These findings provide valuable insight on the controls of peatland carbon dynamics, and its feedbacks with future climate change.