Combining branch-scale COS/CO2 exchange and canopy-scale SIF measurements to disentangle the effects of high VPD and low soil moisture in mature pine trees under field conditions

During the regular seasonal drought conditions in our semi-arid pine forest, soil water content decrease below the 16% threshold of no transpirable soil water availability, and VPD increase to values of >5 kPa. Soil drought in one forest plot was eliminated by using supplemental drip irrigation during summer. We used automatic branch chambers to measure CO₂/H₂O exchange together with laser-based COS exchange, and retrieving canopy sun induced fluorescence (SIF) using a high-resolution spectrometer above the canopy that was moved between the control and irrigated plots on a weekly basis. Using these research tools, we investigated the ecophysiological response (including rates of gas exchange, conductances, and photochemical response) of the mature pine trees to the differential effects of soil and atmospheric droughts. Leaf relative uptake (LRU) ratio of COS to CO₂ fluxes was used to constrain estimates of carbon assimilation (An) and changes in the An/gl ratio (where gl is leaf conductance), and leaf COS and H₂O exchange fluxes were also used to partition leaf conductance. We will report on the first seasonal cycle of COS and CO₂ fluxes, LRU and SIF_A of mature pine trees under field conditions in the semi-arid forest, which includes the summer dry stress period, the recovery during the transition to the winter wet season, and the spring peak activity period.