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Bioclimatology of Canopy Photosynthesis in Temperate European Forests

Final Activity Report Summary - BCP-TEF (Bioclimatology of Canopy Photosynthesis in Temperate European Forests)

The effect of anthropogenic carbon dioxide (CO2) emissions on global climate is currently mitigated through net CO2 uptake by terrestrial ecosystems. Temperate forests are among the strongest terrestrial CO2 sinks.

The European Union Marie Curie project 'Bioclimatology of canopy photosynthesis in temperate European forests' (BCP-TEF) analysed CO2 and water (H2O) flux magnitudes and factors controlling canopy photosynthesis (Pg) in a Danish beech forest, namely the CarboEurope-IP site Sorø. We investigated Pg at leaf scale and at the whole forest scale with gas exchange measurements and with the eddy correlation technique, respectively. The three-dimensional canopy model Maestra was used to scale up the leaf scale measurements to the entire stand and to analyse the sensitivity of Pg to model parameters. The model output was directly compared with the turbulent flux measurements above the canopy. The fellow developed his skills in turbulent flux data processing and ecophysiological field measurements.

We developed a method to correct the stand scale flux data for low-pass filtering by the closed-path eddy correlation system which had considerable effect on the estimated CO2 and H2O vapour fluxes. The new correction was then applied to the existing 10 years' record of continuous turbulent flux measurements. The recalculated flux data were made available to a broader scientific community via European and global data bases.

Parameters of a biochemical canopy physiology model were taken in several field campaigns and yielded a data set that described the responses of leaves in different canopy positions to photosynthetically active radiation (Q), leaf temperature and vapour pressure deficit exceptionally well. Sun crown leaves reached up to three and four times higher potential rates in photosynthesis and dark respiration, respectively, than shade crown leaves. These large differences among different leaves could be accurately described with linear functions to leaf nitrogen concentrations as a proxy for the shade modification of leaves.

Together with measured canopy structure data and leaf optical properties, the leaf scale physiology was used to simulate canopy scale fluxes. Comparison with the measured and corrected turbulent CO2 flux data yielded excellent agreement, despite the fact that the model was not calibrated. This result showed that canopy photosynthesis could be accurately predicted from sub-canopy scale processes using measurable parameters, i.e. the model was prognostic. Sensitivity studies were performed to better understand the parameter requirements for accurate Pg prediction and model error estimation.

BCP-TEF provided data and analyses to measure, understand and predict the gross CO2 uptake in European forests as a consequence of tree physiology, forest structure and environmental conditions. The prognostic model opened the prospective to extrapolate the findings in space and time using regionally available remote sensing products and meteorological data. This would directly contribute to the goal of describing the current and future CO2 flux in forests at regional and continental scales.