Terrain-induced cold-air pooling reverses typical elevational patterns of deciduous vegetation phenology in a mountainous landscape

Plant phenology is commonly interpreted in relation to regional climate, but plants experience thermal conditions at much finer spatial and temporal scales, particularly in complex mountainous terrain. Cold-air drainage and cold-air pooling can generate strong local temperature inversions, causing basin and valley-bottom areas to become cooler than surrounding slopes. Such topographically driven microclimates may alter the timing of spring leaf flush and autumn coloration independently of elevational gradients. Here, we examined how deciduous vegetation phenology varies across cold-air-pool (CAP) landscapes in the Oku-Nikko area, central Japan. We used 30-m optical satellite data from Harmonized Landsat and Sentinel-2 (HLS) to estimate the start of growing season (SOS) and end of growing season (EOS), and 250-m nighttime land surface temperature (LST) data from JAXA’s GCOM-C satellite to characterize local thermal conditions from 2023 to 2025. Basin and valley-bottom areas around Senjogahara Marshland showed lower nighttime LST than adjacent slopes, consistent with the formation of CAPs. These low-LST areas showed delayed SOS and earlier EOS compared with surrounding slope areas, despite their relatively lower elevation. Although the typical elevational phenological pattern was reversed in CAP areas (CAPA), phenological timing remained closely associated with LST: cooler spring conditions were associated with delayed SOS, and cooler autumn conditions were associated with earlier EOS, although the LST–phenology relationship was clearer for SOS than for EOS. These results suggest that topographically driven microclimates are closely linked to local variation in vegetation phenology. Our findings highlight the importance of considering fine-scale thermal heterogeneity when interpreting satellite-derived phenology and assessing climate-change impacts in mountainous landscapes.