Temporal Changes of Top-of-Canopy Solar-Induced Chlorophyll Fluorescence in Evergreen Coniferous Forest in Japan

In temperate region, gross primary production (GPP) of terrestrial vegetation shows significant seasonal change, because of the seasonal changes in meteorological conditions, e.g., radiation, precipitation and temperature and phenological changes of the vegetation, e.g., canopy structure and physiological traits. Previous studies suggested the current vegetation indices, e.g., NDVI and EVI showed less accuracy in assessing the temporal changes of GPP in vegetation, especially in evergreen forests. In this decade, another optical index, solar-induced chlorophyll fluorescence (SIF), has been utilized to understand the diurnal and seasonal variations in the photosynthetic activity and/or production. Since the chlorophyll fluorescence is a radiation emitted by photosynthetic process itself, SIF is thought to be directly linked to photosynthetic activity. In present study, we assessed the temporal changes in SIF and GPP of a forest dominated by Japanese cypress (Chamaecyparis obtusa), evergreen conifer at Kiryu Experiment Watershed in Japan (Kiryu site), over four years (2015 2018). Top-of-canopy SIF (TOC SIF) was measured using two spectrometers, PGP-100 (PREDE Co., Japan) and HR4000 (Ocean Insight, USA), fixed on the canopy access tower. Previous study in Kiryu site showed that the seasonal pattern of GPP majorly corresponded to those of solar radiation, air temperature and vapor pressure deficit, with considerable inter-annual fluctuation (Kosugi et al. 2013). We will discuss the diurnal and seasonal changes in the relationship between GPP and SIF, and the applicability of two devices in detecting TOC SIF at Kiryu site.