Remotely sensed assessment of urbanization effects on vegetation phenology in China's 32 major cities

Urbanization-induced phenological shifts may provide evidence on how vegetation will respond to global warming. However, the effects of urbanization on vegetation phenology are poorly understood in urban environments. Using MODIS data between 2007 and 2013, we investigated the trends of the phenological metrics (i.e., start, end, and length of growing season: SOS, EOS and GSL) of individual cities and across cities relative to rural areas for China's 32 major cities. We found that the effects of urbanization on phenology decayed exponentially toward rural areas, and were closely related to the land surface temperature (LST) for more than half of the cities. The phenological sensitivity to temperature were 9-11 days SOS advance and 6-10 days EOS delay per 1 °C increase of LST. On average across all cities, the growing season started 11.9 days earlier and ended 5.4 days later in urban zones compared to rural counterparts. The urbanization effects increased with increasing latitudes, following the pattern of urban heat island effects in general. Our study suggests the value of urban environments in studying the phenological responses to future global change. However, the urbanization impacts are complex and need more direct observations, experimental manipulations, and cross-boundary inter-comparison studies.