Seven Frozen Trees in Sodankyla: Relating ASCAT slope to water and carbon processes over a Boreal forest using in-situ, model and reanalysis data

Abstract

Combining data from in situ measurements, remote sensing and models can provide new insights on global vegetation dynamics, specifically on the role of vegetation in the carbon and water cycles. Here we will demonstrate the benefits of combining Metop Advanced SCATterometer (ASCAT) C-band radar backscatter observations with in-situ and model data for monitoring vegetation dynamics and constraining parameters in terrestrial carbon stock and flux simulations. The slope of the relation between backscatter and incidence angle of Metop ASCAT data is sensitive to vegetation dynamics over the Amazon region and North-American grasslands, as demonstrated in previous studies by Petchiappan et al. (2022) and Steele-Dunne et al. (2018).  Here we use the slope in combination with in-situ observations to analyze vegetation dynamics over the ICOS site in Sodankyla. Results from this boreal forest region in Northern Finland show that slope dynamics are influenced by freezing temperatures and snow, hindering monitoring of vegetation dynamics during these times. During periods without freezing temperatures and snow, the slope reveals phenological changes both in terms of seasonal changes and anomalies. During the 2018 drought, positive anomalies in slope were found, consistent with results found by Bastos et al., (2020), who demonstrated that increased temperature, drier than average conditions and increased radiation led to increased vegetation growth as modelled with several vegetation models and observed with SMOS Vegetation Optical Depth.To benefit terrestrial carbon cycle modelling and science, ASCAT slope can be assimilated directly into land surface models to constrain states and parameters related to the fast and slow water and carbon fluxes. Results from the ESA Land Carbon Constellation project will be presented to demonstrate that the measurement operator required for assimilation can be determined using several approaches. Bastos, A., Ciais, P., Friedlingstein, P., Sitch, S., Pongratz, J., Fan, L., Wigneron, J.P., Weber, U., Reichstein, M., Fu, Z., Anthoni, P., Arneth, A., Haverd, V., Jain, A.K., Joetzjer, E., Knauer, J., Lienert, S., Loughran, T., McGuire, P.C., Tian, H., Viovy, N., Zaehle, S., 2020. Direct and seasonal legacy effects of the 2018 heat wave and drought on European ecosystem productivity. Science Advances 6, eaba2724. https://doi.org/10.1126/sciadv.aba2724Petchiappan, A., Steele-Dunne, S.C., Vreugdenhil, M., Hahn, S., Wagner, W., Oliveira, R., 2022. The influence of vegetation water dynamics on the ASCAT backscatter-incidence angle relationship in the Amazon. Hydrology and Earth System Sciences 26, 2997–3019. https://doi.org/10.5194/hess-26-2997-2022Steele-Dunne, S.C., Hahn, S., Wagner, W., Vreugdenhil, M., 2019. Investigating vegetation water dynamics and drought using Metop ASCAT over the North American Grasslands. Remote Sensing of Environment 224, 219–235.