long term observation of vegetation indices and eddy covariance fluxes during winter leaf reddening in a temperate Japanese cypress forest

Winter leaf reddening is a phenomenon that evergreen conifer species leaf changes colour from green to red before or during winter due to the accumulation of red pigment (rhodoxanthin in case of Japanese cypress). This phenomenon might be a photoprotection strategy for plant leaves to deal with excess light stress. In gymnosperms, the xanthophyll cycle (VAZ cycle) and accumulation of rhodoxanthin work together to protect the photosynthetic apparatus from photodamage during winter. However, the joint role of these two processes in corresponding with canopy photosynthesis under winter excess light stress has not been further studied. And how the canopy gas exchange, vegetation indices, and environmental conditions change during this phenomenon was also a question. Hence, the objectives of this study are to clarify whether: (1) the low temperature is the dominant factor affecting the phenomenon and ascertain the air temperature conditions when the winter leaf reddening phenomenon occurs; (2) the accumulation of rhodoxanthin, the VAZ cycle, and low light-use efficiency (LUE) correspondingly occurs in low air temperature conditions during winter season. We utilized the photochemical reflectance index (PRI) as a tool to reflect the VAZ cycle. The automated system with a digital camera was used to monitor the canopy phenology changes. The RGB channels of image data were extracted to calculate the Red-Green vegetation index (RGVI) to track the colour changes of canopy leaves. The canopy CO2 flux was measured with the eddy covariance method, which can be used to calculate LUE. Micrometeorological data were also monitored. Our findings demonstrated that low air temperature in winter plays a leading role in the occurrence of the winter leaf reddening phenomenon. The winter leaf reddening phenomenon occur along with the lower level of PRI and LUE. It suggests that the accumulation of rhodoxanthin and VAZ cycle played a collaborative role in adjusting with low LUE under a combination of the chilling temperature and high solar irradiances conditions. This mechanism leads to the occurrence of winter leaf reddening. Key word: winter leaf reddening, Japanese cypress, photochemical reflectance index (PRI), Red-Green vegetation index (RGVI), phenological analysis, digital camera, light-use efficiency