An increase in anthropogenic activities since the industrial revolution, primarily due to burning of fossil fuels and changes in land cover, has resulted in a steady increase in the global mean atmospheric CO2 concentrations. While there is unequivocal scientific evidence on global warming and its multidimensional impacts on natural and human systems, uncertainties on the magnitude of future warming persist, propagating from uncertainties in the response of terrestrial plants to changing climates. This is partially due to our inability to directly measure photosynthesis beyond the leaf level.
Measurements of carbonyl sulfide OCS have recently been shown to provide an independent and direct estimate of plant productivity. OCS is the most abundant reduced sulfur gas present in the atmosphere, with a mean atmospheric concentration of ~ 500 ppt (parts per trillion) and is emitted into the atmosphere from oceans via direct emissions or oxidation of CS2, and taken up by leaves of actively photosynthesizing leaves.
The focus of this dissertation was to further assess OCS as a proxy for ecosystem-scale stomatal conductance and photosynthesis in a temperate old growth coniferous forest. This dissertation is composed of three studies conducted at the Wind
Ecosystem Photosynthesis and Forest- Atmosphere Interactions Inferred from Carbonyl
River Experimental forest located within the Gifford Pinchot National Forest in southwest Washington state, USA (45°49ʹ13.76ʹʹ N; 121°57ʹ06.88ʹʹ W; 371 m above sea level).
In the first study, co-authors and I find significant ecosystem uptake of OCS using measurements of mixing ratios of this gas within and above the canopy. We find that diurnal patterns of OCS are influenced by a combination of ecosystem uptake and mixing of air from the overlying atmosphere. We quantify the magnitude of mixing and demonstrate how OCS measurements can be used to estimate entrainment and mixing between forests and the atmosphere. We also find that the forest surface and epiphytes consume OCS, and that uptake strength is linked to moisture status.
In the second study, we develop a simple empirical model to predict ecosystem-scale OCS fluxes from mixing ratio measurements at the canopy top. OCS uptake was found to scale with independent measurements of CO2 fluxes at hourly and monthly timescales across the growing season in 2015. OCS fluxes tracked changes in soil moisture, and were strongly influenced by the fraction of downwelling diffuse light. Fluxes were also strongly affected by sequential heat waves during the growing season.
In the third and final study of this dissertation, OCS flux estimates obtained in the previous study were used to determine GPP for the old growth forest. GPP estimated from OCS flux showed similar seasonal and diurnal patterns as that obtained from flux partitioning of NEE at the site. However, we find that the magnitude of GPP obtained using this method was significantly higher, and similar to independent estimates obtained from measurements of sap flow in trees and the isotopic composition of leaf CO2, and a process-based model. We conclude that flux partitioning of net ecosystem exchange of CO2 results in a systematic underestimation of both source and sink terms, likely due to an underestimation of respiratory fluxes.
Our results support the growing body of work that suggests ecosystem-scale OCS uptake is controlled by stomatal dynamics. These studies have implications for carbon cycling in forested ecosystems, particularly in dense and wet forests with extensive epiphyte cover, which are widespread in the humid tropics.