Strong and stable relationships between tree-ring parameters and forest-level carbon fluxes in a Siberian larch forest

The tree-ring width index (RWI) and satellite-derived vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), are used as long-term indicators of the past forest carbon uptake. However, fundamental questions remain with respect to what is represented by the RWI and NDVI at the ecosystem level. To address this question, we compared tree-ring parameters (RWI and the carbon isotope ratio: delta C-13) and NDVI products with forest ecosystem CO2 fluxes estimated using the eddy covariance method, at a larch forest in eastern Siberia. The RWI and tree-ring delta C-13 correlated well with the ecosystem gross primary production (GPP), and their temporal stabilities were high during 2004-2014. However, the NDVI products did not show any temporally stable relationship with the GPP. This could be ascribed to significant changes in the understory vegetation in this forest, i.e., from dense cowberry to shrubs and moisture-tolerant grasses, because of an excessively moist environment during 2007-2008. Changes in the understory vegetation could be reflected by the NDVI products but not by the GPP. Our results indicate that it is more feasible to study forest carbon uptake using tree-ring parameters than using satellite-derived vegetation indices in such larch-dominated forest ecosystems in eastern Siberia.