Detection of Bio-Meteorological Year-to-Year Variation by Using Digital Canopy Surface Images of a Deciduous Broad-Leaved Forest

Clarification of the year-to-year variations and long-term trends of the timings of the start of leaf-expansion (SLE) and end of leaf-fall (ELF) is an important and challenging task because these timings affect spatial and temporal variations in water, heat and carbon cycles. Here, (1) we examined the relationships between daily mean air temperatures and the timings of SLE and ELF by using digital camera images in a cool-temperate deciduous broad-leaved forest in Japan from 2004 to 2011, and constructed a simple statistical phenology model based on their relationships, and (2) we then evaluated year-to-year variations and long-term trends in the timings of SLE and ELF over the past 51 years (1961-2011) by using that phenology model. We found that (1) the year-to-year variations in daily mean air temperatures over 2 degrees C and below 18 degrees C well affected those in the timings of SLE and ELF, respectively, and (2) a significant long-term linear trend in the timing of ELF (it moved later; 2.7 days decade(-1)) and the length of the leafy period were observed (it prolonged; 3.4 days decade(-1)), but no long-term trend was evident in the timing of SLE.