Accelerating Phenology of Apple Trees in Japan as Influenced by Rising Air Temperature

Abstract

Recent studies show hastening of the onset of the plant growing season and delay in the end of the growing season for the last several decades. With the well-established dependence of plant phenology on temperature, the phenological trends are interpreted as an indication of global warming. However, for several decades, other changes, e.g., those in varieties and agronomic practices, may also have contributed to the phenological trends. In this study, we analyzed the records of temperature and phenological events (budding, flowering and harvest) in the apple (Malus pumila var. domestica) from 1977 to 2004 at two locations (Kuroishi and Fukushima) in northern Japan. To each time series, we fitted a smoothing spline to obtain the long-term trend, and calculated the anomaly of deviations in yearly observations from the long-term trend. We found acceleration of budding by 0.23 d/y (Kuroishi and Fukushima), and that of flowering by 0.24 d/y (Kuroishi) and 0.27 d/y (Fukushima). The timing of budding was closely correlated with mean air temperature in March at a sensitivity of -3.9 d/°C (Kuroishi) and -4.3 d/°C (Fukushima). The temperature sensitivity showed no significant difference between the long-term trends and the anomalies. Flowering results were very close with one exception in Kuroishi: the temperature sensitivity in the long-term trend (-3.4 d/°C) was significantly less than that for the anomaly (-5.2 d/°C). The date of harvest showed no significant temporal trends or correlation with air temperature. These results indicate that the acceleration of budding in the apple can be regarded as a signal of a temperature increase, and that the acceleration of flowering may be a result of the temperature increase despite in an exceptional case.