Seasonal changes in the relationship between photochemical reflectance index and photosynthetic light use efficiency of Japanese larch needles

Abstract

We investigated seasonal changes in the photochemical reflectance index (PRI) and its relation to the diurnal profile of photosynthetic light use efficiency (LUE) in mature Japanese larch (Larix kaempferi Sarg.) forest throughout the growing season from June to October 2003. The daily mean value of needle PRI showed seasonal variation, strongly correlated with the chlorophyll concentration and carotenoid/chlorophyll ratio of the needles. During the green period from early June to late September, the hourly values of both PRI and LUE showed significant midday depression, and were positively correlated. In late October, however, because the PRI of yellowing needles tended to increase slightly at midday in contrast to the LUE, this correlation became negative. Even before autumn senescence, the sensitivity of PRI to LUE changed with the season. Correlation analysis indicated that the slope and intercept of the regression line of the PRI–LUE relationship increased during summer, with peaks in July and August, respectively. The seasonal change in slope was strongly correlated with the foliar photosynthetic pigment concentration, nitrogen concentration, air temperature and the daily mean value of the normalized difference vegetation index (NDVI). The value of the intercept was positively correlated with the daily mean PRI. These results suggest that although diurnal change in LUE cannot be estimated quantitatively from PRI on its own throughout the growing season, the combined use of PRI and other variables such as foliar pigments or NDVI could improve the remote evaluation of seasonal changes in LUE of deciduous tree leaves.

Acknowledgments

The authors are greatly indebted to Professor T. Koike (Hokkaido University) and Professor Y. Mukai (Gifu University) for their invaluable advice and technical support for the gas exchange measurement and chemical analysis. We also thank Dr S. Tsuchida (National Institute of Advanced Industrial Science and Technology), Dr T. Takeda (National Institute for Environmental Studies) and Dr S. Kobayashi (Tokyo University) for their technical support and valuable discussion.