Radiation contributed more than temperature to increased decadal autumn and annual carbon uptake of two eastern North America mature forests

The eastern North America mature deciduous forests are increasing their carbon (C) sink, which is believed to be due to a longer growing season. In this study, we investigated the impacts of land surface phenology (LSP) and carbon uptake phenology (CUP) on the net ecosystem productivity (NEP) of the two longest-running flux tower sites in the region. Our results show that there is no trend in the start (SOS), end (EOS), and length of growing season (LOS) at both sites; nor do they explain the interannual and long-term trend in NEP. We found no evidence for a changing growing season and cannot attribute the increasing C sink to growing season length. However, there is strong trend in end (ECU) and length of net positive carbon uptake (LCU) period. ECU is delaying and LCU is getting longer, and they both explain the interannual and long-term trends in NEP. There is increasing trend of photosynthetically active radiation (PAR) at both sites in line with increasing NEP. PAR is contributing the most both to NEP and ECU. CUP is affected more by the increased photosynthetic activity partially due to increased PAR, but not by timing of spring onset and autumn senescence of leaves. There is also significant (p < 0.01) reductions in emissions of sulfur oxides (SOx), nitrogen oxides (NOx), total nitrate (HNO3 + NO3) and ozone since 1992. The reductions in gaseous and particulate emissions imply the occurrence of direct aerosol mediated brightening and therefore increased PAR and enhanced C uptake by eastern North America mature forests.