Intra-Crown Microclimatic Heterogeneity and Phenological Buffering: A High-Resolution UAV Study of Flowering and Autumn Leaf Senescence

While climate change shifts plant phenology, conventional satellite-based studies often overlook intra-individual variations due to spatial averaging. This study utilized high-resolution UAV imagery and Digital Surface Models (DSMs) to investigate how intra-crown microclimatic heterogeneity affects the spatiotemporal patterns of flowering and autumn leaf senescence. We monitored Rhododendron yedoense f. poukhanense (RY) and Acer triflorum (AT) at the Korea National Arboretum, acquiring 23 time-series images between April and November 2025. Cumulative solar duration was calculated for 0.5 m intra-crown grids, and phenological events were detected using derivative analysis of vegetation indices (Red Chromatic Coordinate [RCC] and Green Chromatic Coordinate [GCC]). The results confirmed asynchrony in phenological events within single individuals depending on crown sectors. As intra-crown microclimatic heterogeneity increased, RY exhibited extended phenological duration, supporting the "Phenological Buffering Hypothesis," whereas AT maintained relatively higher synchrony. Furthermore, AT showed a "Phenological Velocity" gap, where sunlit sectors experienced senescence approximately 1.12 days later than shaded areas. This research demonstrates that phenological responses can be spatially dispersed even within an individual, and the buffering mechanisms against environmental variability differ by crown structure and growth form. These findings highlight the necessity of individual-level spatial resolution in understanding plant responses to climate change.