Interannual variations and trends of gross primary production and transpiration of four mature deciduous broadleaf forest sites during 2000–2020

The interannual variations of gross primary production (GPP) and transpiration (T) in deciduous broadleaf forests reflect how the forest responds to climate change. However, our knowledge remains limited due to lack of multi-decadal data. In this study, we selected four mature deciduous broadleaf forest sites in the United States of America and Canada from 2000 to 2020 to investigate decadal trends in atmospheric CO2 concentrations, climate, vegetation indices, phenology, and carbon and water fluxes. The Vegetation Photosynthesis Model (VPM v3.0) and the Vegetation Transpiration Model (VTM, v2.0) were used to estimate GPP and T at the four sites. The GPP from the VPM simulations (GPPVPM) is highly consistent with that from the eddy flux tower sites (GPPEC) (R2 range from 0.77 to 0.89). The interannual trends of carbon and water fluxes during the period from 2000 to 2020 varied by sites, ranging from increases (CA-Cbo) to no trend (US-Ha1, US-MMs, US-WCr sites), dependent upon the temporal changes in atmospheric CO2 concentration and climate. Spring phenology at these sites had no significant trends due to the lack of an interannual trend of air temperature during 2000–2020. In those years with hotter winter and early spring season (WESS), the start of the season (SOS) advanced and the growing season length (GSL) extended, but there were little changes in the end of the season (EOS) and annual GPP. This study highlights the value of long-term measurements at forest eddy flux tower sites and the skill of VPM v3.0 and VTM v2.0 models and the ERA5 climate dataset for simulations over the past two decades, which could be used to do regional and global simulations of deciduous broadleaf forests and assess their responses to climate change.