Atmospheric aridity and soil moisture fluctuations regulate GPP in a temperate peat bog

Despite only covering ~3% of the land area, peatlands store more carbon (650 gigatons (Gt) of C) than global terrestrial vegetation (409 GtC). However, this C is vulnerable to climate warming and drainage. Plants mediate land-atmosphere C exchange and its coupling with water by regulating stomatal opening and root water intake during droughts. Stomatal regulation and photosynthesis are dependent on soil water content (SWC), air temperature (Tair), and vapour pressure deficit (VPD). However, the role of SWC on gross primary productivity (GPP) is still not straightforward, as evidenced by several contradictory literature. Considering this, we asked whether there is a threshold at which SWC drawback starts to regulate GPP in a peat bog in Vancouver, Canada. We used weekly time step eddy covariance data spanning five years (2016-2020). Our analysis suggests that stomatal regulation in response to increased VPD caused a reduction in GPP during the 2016 drought (~2.5gC m^-2 day^-1). On the other hand, an absence of stomatal regulation in 2017 and 2018 (to maximise C assimilation) following the initial drought caused the peat surface to dry out in 2019. This resulted in SWC regulating GPP more than VPD by 2019. We report a SWC threshold of 82.5% (-8 cm water table depth), below which it starts to regulate GPP at this site. The interaction between energy and water limitations on GPP is expected to intensify with the projected increase in the frequency of drought events across the northern hemisphere.