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Abstract
We examined the relationship between four vegetation indices and tree canopy phenology in an evergreen coniferous forest in Japan based on observations made using a spectral radiometer and a digital camera at a daily time step during a 4 year period. The colour of the canopy surface of Japanese cedar (Cryptomeria japonica) changed from yellowish-green to whitish-green from late May to July and turned reddish-green in winter. The normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and plant area index (PAI) showed no seasonality. In contrast, the green–red ratio vegetation index (GRVI) increased from March to June and then decreased gradually from July to December, resulting in a bell-shaped curve. GRVI revealed seasonal changes in the colour of the canopy surface. GRVI correlated more positively with the evaluated maximum photosynthetic rate for the whole forest canopy, A max, than did NDVI or EVI. These results suggest the possibility that GRVI is more useful than NDVI and EVI for capturing seasonal changes in photosynthetic capacity, as the green and red reflectances are strongly influenced by changes in leaf pigments in this type of forest.
Acknowledgements
The authors thank K. Kurumado, Y. Miyamoto, T. Kojima and Y. Yashiro (River Basin Research Centre, Gifu University) and M. Tomotsune (Waseda University) for their assistance in the field. They also thank all members of the Phenological Eyes Network (PEN) for their assistance. The authors thank the editor and the two anonymous reviewers for their kind and constructive comments. This study was supported by the Japan Society for the Promotion of Science (JSPS) 21st Century COE Programme (Satellite Ecology, Gifu University), the JSPS/NRF/NSCF A3 Foresight Programme, KAKENHI (19688012; Grant-in-Aid for Young Scientists A by JSPS) and a Global Change Observation Mission (GCOM; PI#102) of the Japan Aerospace Exploration Agency (JAXA). H. Muraoka is supported by the Environment Research & Technology Development Fund (D-0909) of the Ministry of Environment of Japan.