Constraining terrestrial gross primary productivity

The magnitude of the terrestrial carbon sink is determined by the ratio of gross primary productivity (GPP), that is the primary rate at which plants take up carbon dioxide from the atmosphere, to ecosystem respiration (ER), that is the release of carbon dioxide from all living beings in an ecosystem. At present, the terrestrial sink removes roughly one quarter of the carbon dioxide emitted into the atmosphere by human activities and thus significantly slows down anthropogenic global warming. Unfortunately, GPP cannot be directly quantified, but instead must be estimated through a combination of measurements and models. At present, this is typically done by partitioning the net ecosystem carbon dioxide exchange into the underlying GPP and ER components, which has a number of acknowledged limitations. The resulting uncertainty of GPP and ER estimates considerably hampers efforts aimed at projecting the magnitude of the terrestrial carbon sink to likely future climatic and land-use scenarios. The objective of the proposed project is to reduce the uncertainty of present-day GPP (and as a further consequence, ER) estimates by means of the joint application of two novel approaches, carbonyl sulfide exchange and sun-induced fluorescence measurements. The advantage of carbonyl sulfide over carbon dioxide is that it is taken up by leaves in a similar fashion, but in contrast to carbon dioxide no emission of carbonyl sulfide has been observed, making it a proxy for GPP. Sun- induced fluorescence refers to light absorbed by chlorophyll molecules that is emitted at a longer wave length. A relationship between fluorescence and photosynthesis exists because both processes compete for the same energy. The hypothesis underlying the proposed project is that because the two approaches yield complementary information about GPP carbonyl sulfide targets the diffusive and sun-induced fluorescence the energy provisioning component of photosynthesis - their joint use will allow reducing the uncertainty of GPP derived from carbon dioxide flux measurements. So far, these two novel approaches have been used in isolation by applying them jointly, the proposed project aims at capitalizing on the strengths of both approaches. To this end we propose a combined approach including joint measurements of carbon dioxide and carbonyl sulfide fluxes and sun-induced fluorescence during two field campaigns in two contrasting ecosystems, a mesocosm experiment, and process-based simulation modelling making use of the experimental data.

The magnitude of the terrestrial carbon sink is determined by the ratio of gross primary productivity (GPP), that is the primary rate at which plants take up carbon dioxide from the atmosphere, to ecosystem respiration (ER), that is the release of carbon dioxide from all living beings in an ecosystem. At present, the terrestrial sink removes roughly one third of the carbon dioxide emitted into the atmosphere by human activities and thus significantly slows down anthropogenic global warming. Unfortunately, GPP cannot be directly quantified, but instead must be estimated through a combination of measurements and models, which at present are associated with a number of uncertainties. This considerably hampers efforts aimed at projecting the magnitude of the terrestrial carbon sink to likely future climatic and land-use scenarios. The objective of this project was to reduce the uncertainty of present-day GPP (and as a further consequence, ER) estimates by means of the joint application of two novel approaches, carbonyl sulfide exchange and sun-induced fluorescence measurements. The advantage of carbonyl sulfide over carbon dioxide is that it is taken up by leaves in a similar fashion, but in contrast to carbon dioxide no emission of carbonyl sulfide has been observed, making it a proxy for GPP. Sun-induced fluorescence refers to light absorbed by chlorophyll molecules that is emitted at a longer wave length. A relationship between fluorescence and photosynthesis exists because both processes compete for the same energy. To this end we conducted, within the three-year project period, two extensive field campaigns and a mesocosm experiment and analysed the resulting data using mathematical models. The major result of this project is that carbonyl sulfide and sun-induced chlorophyll fluorescence, in isolation but especially in combination, show great potential for estimating the GPP of terrestrial ecosystems. Our results, however, also show that both approaches at present suffer from shortcomings which, also on the basis of future research, need to be addressed. We were able to show that the ratio between carbonyl sulfide and carbon dioxide uptake appears to vary between different ecosystem types/climate zones and that the unique relationship between GPP and sun-induced chlorophyll fluorescence breaks down when plants experience stress conditions.