Moisture-Driven Phenology Regulates Seasonal Canopy Dynamics and Carbon Uptake in a Tropical Deciduous Forest

Abstract

Phenology sets the seasonal window over which terrestrial ecosystems take up carbon, and in drought-deciduous tropical forests this window is governed by rapid canopy transitions difficult to capture from satellites alone. We address this gap using daily near-surface imagery from a PhenoCam co-located with an automatic weather station in the Similipal Biosphere Reserve, eastern India, from January 2023 to May 2024. Daily Green Chromatic Coordinate (GCC) was analyzed with co-located meteorology and surface (5 cm) and subsurface (20 cm) soil moisture and temperature. Interannual comparison focused on the leaf-off (LP1), leaf-initiation (LP2), and leaf-growth (LP3) phases common to both years. In 2023, the start of season (SOS) fell on day-of-year (DOY) 82, peak greenness on DOY 154, and senescence onset on DOY 211. In 2024, SOS advanced to DOY 61, yet greening was slower and weaker: the leaf-off minimum occurred about 12 days later and GCC remained below 2023 for most of the window. The two years were compared over their shared DOY 1–150 window. Machine-learning models identified subsurface soil moisture as the dominant control on LP1 (R² ≈ 0.67), subsurface temperature and moisture as joint drivers of LP2 (R² ≈ 0.88), and atmospheric humidity as an increasing constraint on LP3. Granger-precedence analysis revealed rapid (≤6-day) responses to moisture and radiation superimposed on a ∼25-day subsurface thermal memory. The Sentinel-2 enhanced vegetation index, trained on 2023, reproduced the 2024 GCC trajectory (R² ≈ 0.88). These results imply that 10–15-day phenological shifts can substantially alter the seasonal carbon-uptake window and pre-monsoon fire risk.