Onset of photosynthesis in spring speeds up monoterpene synthesis and leads to emission bursts.

Emissions of biogenic volatile organic compounds (BVOC) by boreal evergreen trees have strong seasonality, with low emission rates during photosynthetically inactive winter and increasing rates towards summer. Yet, the regulation of this seasonality remains unclear. We measured in situ monoterpene emissions from Scots pine shoots during several spring periods and analysed their dynamics in connection with the spring recovery of photosynthesis. We found high emission peaks caused by enhanced monoterpene synthesis consistently during every spring period (monoterpene emission bursts, MEB). The timing of the MEBs varied relatively little between the spring periods. The timing of the MEBs showed good agreement with the photosynthetic spring recovery, which was studied with simultaneous measurements of chlorophyll fluorescence, CO2 exchange and a simple, temperature history-based proxy for state of photosynthetic acclimation, S. We conclude that the MEBs were related to the early stages of photosynthetic recovery, when the efficiency of photosynthetic carbon reactions is still low whereas the light harvesting machinery actively absorbs light energy. This suggests that the MEBs may serve a protective functional role for the foliage during this critical transitory state and that these high emission peaks may contribute to atmospheric chemistry in the boreal forest in springtime.

OCO-2 advances photosynthesis observation from space via solar-induced chlorophyll fluorescence.

Quantifying gross primary production (GPP) remains a major challenge in global carbon cycle research. Spaceborne monitoring of solar-induced chlorophyll fluorescence (SIF), an integrative photosynthetic signal of molecular origin, can assist in terrestrial GPP monitoring. However, the extent to which SIF tracks spatiotemporal variations in GPP remains unresolved. Orbiting Carbon Observatory-2 (OCO-2)'s SIF data acquisition and fine spatial resolution permit direct validation against ground and airborne observations. Empirical orthogonal function analysis shows consistent spatiotemporal correspondence between OCO-2 SIF and GPP globally. A linear SIF-GPP relationship is also obtained at eddy-flux sites covering diverse biomes, setting the stage for future investigations of the robustness of such a relationship across more biomes. Our findings support the central importance of high-quality satellite SIF for studying terrestrial carbon cycle dynamics.