Extension of the growing season due to delayed autumn over mid and high latitudes in North America during 1982-2006

Aim We intend to characterize and understand the spatial and temporal patterns of vegetation phenology shifts in North America during the period 19822006. Location North America. Methods A piecewise logistic model is used to extract phenological metrics from a time-series data set of the normalized difference vegetation index (NDVI). An extensive comparison between satellite-derived phenological metrics and ground-based phenology observations for 14,179 records of 73 plant species at 802 sites across North America is made to evaluate the information about phenology shifts obtained in this study. Results The spatial pattern of vegetation phenology shows a strong dependence on latitude but a substantial variation along the longitudinal gradient. A delayed dormancy onset date (0.551 days year(-1), P = 0.013) and an extended growing season length (0.683 days year(-1), P = 0.011) are found over the mid and high latitudes in North America during 1982-2006, while no significant trends in greenup onset are observed. The delayed dormancy onset date and extended growing season length are mainly found in the shrubland biome. An extensive validation indicates a strong robustness of the satellite-derived phenology information. Main conclusions It is the delayed dormancy onset date, rather than an advanced greenup onset date, that has contributed to the prolonged length of the growing season over the mid and high latitudes in North America during recent decades. Shrublands contribute the most to the delayed dormancy onset date and the extended growing season length. This shift of vegetation phenology implies that vegetation activity in North America has been altered by climatic change, which may further affect ecosystem structure and function in the continent.