A novel approach combining canopy flow analysis and stable isotopes to understand and quantify turbulent carbon exchange in forests

We present a novel, cross-disciplinary approach combining turbulence observations, wavelet analysis, and high time resolution stable isotope ratio measurements to directly quantify component fluxes of carbon dioxide at the atmosphere-vegetation interface, and to improve our understanding of important pathways for energy and mass transport in forests. The approach is based on identifiable changes in the correlation structure of carbon dioxide and water vapor concentrations in the canopy air during updrafts. A recently developed conditional sampling approach and wavelet analysis are both applied to high-frequency observations of the two trace gases and velocities. The two methods are combined to evaluate the hypothesis that short-term excursions from similarity theory predictions of the correlation structure between carbon dioxide and water vapor are associated with the vertical transport through coherent structures. Although these short-term excursions have been demonstrated to carry the quantifiable fingerprint of sub-canopy ecosystem respiration primarily from the forest floor in a previous study, questions on the exact transport mechanisms and the variability of this signal were left unanswered. In this study, we present preliminary results from the in-depth analysis of the canopy flow using a state-of-the-art wavelet decomposition tool to refine the conditional sampling approach and reduce uncertainty of flux estimates of ecosystem respiration, and explore the use of concurrent high time resolution measurements of the natural abundance stable isotope ratios ¹³C and ¹⁸O of carbon dioxide to explain the variability of the conditionally sampled signal. The working hypothesis is that the stable isotope signatures can be traced back to different sinks and sources of water vapor and carbon dioxide in the canopy, and subsequently be integrated into the conditional sampling scheme to directly quantify ecosystem-scale photosynthesis and respiration fluxes. Such a method would provide perspective on bridging trace gas exchange observed at plant level with that at the ecosystem level. The data used in this ongoing study represent forests of different canopy architectures and were collected over several weeks.