3) Dr. Rafat Qubaja

Future terrestrial carbon cycles need deep understanding of the carbon exchange between plant and atmosphere. Carbonyl sulfide (COS) is a multi-scale tracer to separate photosynthesis and respiration from observations of the carbon dioxide (CO2) flux. Plant COS uptake is used to infer photosynthesis through the leaf relative uptake (LRU) ratio of COS to CO2 fluxes. However, little is known about variations of LRU under changing environmental drivers and in different phenological stages for deriving robust estimates of photosynthesis. From a long-term field manipulation experiment in Yatir, Israel, we disentangle the effects of high atmospheric water demand and low soil moisture in mature Aleppo pine forest during four seasons 2020–2021. We present COS and CO2 fluxes and LRU from branch-scale measurements under field conditions and investigate if these fluxes respond similarly to environmental changes. Despite big difference between the values of treatments fluxes, the LRUs maintain similar values, and respond similarly to environmental drivers. LRU decrease with high light or atmospheric water demand, and low humidity. Also, we derive a set of LRU models inferred from the CO2 fluxes or the environmental drivers. We derived theoretical links between leaf relative uptake and models of stomatal conductance. These models reproduced the observed LRU with high agreement.