关键词: 冠层物候, 物候相机, 植被绿度指数, 气象因子

Abstract: Aim This study aims to investigate the dynamics of spring canopy greenness in a subtropical evergreen broadleaf forest and to identify the key environmental factors influencing these changes. By focusing on the seasonal variations in greenness indices, this study attempt to demonstrate subtle seasonal transitions in subtropical forests contributing to the development of phenological models for such ecosystems. Methods The study was conducted in the Shimentai Forest Monitoring Station in Guangzhou, China, utilizing long-term observational data (2020–2023) from canopy phenology cameras. Greenness indices, including the Excess Green Index (ExG) and Green Chromatic Coordinate (GCC), were extracted from the images. These indices were combined with meteorological data from a flux tower. Analytical techniques such as random forest analysis, regression analysis, and Dynamic threshold method were applied to explore the variations in greenness indices during different phenological stages and their associated environmental drivers, including temperature, soil water content (SWC), relative humidity, and vapor pressure deficit. Important findings We found that: (1) The subtropical forest canopy in the Shimentai area of the Guangzhou region undergoes distinct spring leaf-coloring and leaf-unfolding periods, with canopy greenness remaining markedly lower from February through May; (2) the decline in greenness during the leaf-coloring period is jointly regulated by SWC, relative humidity, vapor-pressure deficit, and photosynthetically active radiation (PAR), whereas the greenness increase during the leaf-unfolding period is governed mainly by temperature; (3) when SWC drops below the critical 16 %–20 % range, the canopy becomes especially sensitive to leaf-coloring changes, providing a foundation for constructing phenological models for tropical forests.

Key words: canopy phenology, phenology cameras, greenness index, meteorological factors