Assessing the 20-Year Changes in Leaf Phenology of Temperate Deciduous Forests in Japan Using in-situ and Satellite-GRVI

In the temperate region, inter-annual variation of air temperature affects leaf phenology, i.e., timings of leaf emergence and growth in spring and defoliation in autumn. These changes have significant impacts not only on the canopy of dominant trees of forest ecosystems, but also on the seasonal light environment within the forest understory which further influences the growth and survival of tree seedlings, shrubs, and herbaceous species. Consequently, global warming is expected to influence biodiversity by altering species-specific growth responses to the environmental shifts, affecting primary production and hence the progress of vegetation succession. Therefore, in order to comprehensively monitor and assess the state and changes in forest ecosystems across wide geographical and decadal scales, it is important to observe leaf phenology at both the species and ecosystem scales, which is considered one of Essential Biodiversity Variables (EBVs).

The objective of this study is to investigate the decadal-scale change of the leaf phenology in deciduous forest in Japan. We examined 20-year changes of the dates of leaf emergence, leaf area index (LAI) reached its maximum, and defoliation by using in-situ and satellite data. The in-situ remote sensing has been conducted by a spectroradiometer and automated digital cameras on a canopy tower since 2003 at a deciduous forest in Takayama site, located in the cool-temperate region in the central Japan. The system is part of the Phenological Eyes Network (PEN). We estimated the dates of leaf emergence, maximum LAI, and defoliation based on the seasonal pattern on the Green-Red Vegetation Index (GRVI). These dates exhibit notable inter-annual variations, and notably, the date of maximum LAI occurrence tended to shift earlier over the 20-years period from 2004 to 2023. Those inter-annual variations in the leaf phenology were strongly related to the air temperature. Based on the knowledge gained at the Takayama site, we then examined the spatial distribution and annual changes of phenology of the deciduous forests in Honshu Island with satellite-GRVI. We will discuss the spatial and temporal changes in phenology along the environmental gradient and rising air temperature due to global warming, and evaluate the sensitivity or tolerance of these forests by focusing on species composition and geographical characteristics.

The authors thank PEN for sharing the data of spectral reflectance and canopy images.