Daily Carbon Exchange in a Tropical Dry Evergreen Forest: Insights from Eddy Covariance and Multi-Sensor Remote Sensing

Tropical dry evergreen forests retain a closed canopy during long dry seasons, yet their carbon exchange remains poorly measured. We analyzed 20 months (April 2024–December 2025) of eddy-covariance observations at Sakaerat, Thailand, including a single-point CO2 storage term, gap filling, and flux partitioning. We assessed whether tower soil measurements and optical, thermal, and Sentinel-1 radar predictors improved daily net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (Reco) estimates beyond tower meteorology. Estimated monthly NEE was negative throughout the record, including the dry season. Cumulative NEE over 1 January–17 December 2025 was −1043 g C m−2 (GPP 2620; Reco 1635 g C m−2). Uptake was stronger in the wetter, cooler second wet season, consistent with directly measured daytime fluxes. In the later held-out period, soil water content and temperature improved NEE and GPP estimates, while cross-calibrated Sentinel-1 backscatter improved GPP estimates. Optical and thermal products did not improve NEE and degraded GPP. These gains mainly tracked the interannual shift in flux level rather than daily variation. Better constraints on nighttime respiration are needed to confirm the magnitude of the carbon sink.