基于卫星遥感的北京城乡植物物候差异

Differences in plant phenology between urban and rural areas of Beijing based on satellite-based remote sensing

利用2002-2009年1 km分辨率的MODIS NDVI数据,通过提取春季NDVI上升最快的日期和秋季NDVI下降最快的日期,对比分析了北京地区城市与乡村植物物候的差异。从空间格局和时间演变两个角度,分别对比分析了核心城区、边缘城区和乡村3个区域的生长季开始日(SOG)、结束日(EOG)以及总日数(TD)3个物候指标。结果表明,城区的SOG早于乡村,其中核心城区最早,比乡村提前约20天;城区的EOG晚于乡村,其中核心城区最晚,比乡村推迟约15天;由此导致城区的TD多于乡村,其中核心城区最多,大约比乡村多35天。2002-2009年,城区的SOG呈现推迟的趋势,EOG呈现提前的趋势,而乡村的SOG呈现提前的趋势,EOG呈现推迟的趋势;由此导致城区TD呈现减少的趋势,乡村则呈现增加的趋势。上述结果量化了北京地区城市与乡村植物物候的差异,揭示了城市环境对植物物候的影响,为研究人类活动的生态环境效应提供了实证。

Urbanization is the most intense human modification on natural environment. To understand the effects of urbanization on terrestrial ecosystem, it is valuable to study the differences in plant phenology between urban and rural areas. Taking Beijing as an instance, this paper attempted to address this issue. The MODIS NDVI （MOD13A2） with the spatial resolution of 1 km × 1 km and at the 8-day interval during 2002-2009 was used. The time point when the NDVI increased most rapidly in spring was defined as the start of growing season （SOG） and the time point when the NDVI decreased most rapidly in autumn was defined as the end of growing season （EOG）. The days from SOG to EOG were the total days of growing season （TD）. We illustrated the climatological spatial variations of SOG, EOG and TD, respectively, in Beijing metropolis area. In addition, we illustrated the temporal variations of SOG and EOG from 2002 to 2009 for urban core area, urban margin area, surrounding rural area, respectively. We found that the SOG advanced and EOG delayed from rural area to urban area. The SOG in urban core area was earlier by -20 d than surrounding rural area, while EOG was later by -15 d than surrounding rural area. As a result, the TD in urban area was 35 d more than that in the surrounding rural area. More importantly, the inter-annual variability of phenology of urban area and rural area was consistent with each other, but the tendency of phenology of urban area and rural area were reverse. From 2002 to 2009, the SOG in urban area delayed gradually and EOG advanced gradually, while in rural area SOG advanced gradually and EOG delayed gradually. As a result, the TD in urban area decreased gradually whereas that in rural area increased gradually. Although the NDVI-derived phenology might have some uncertainties, the spatial variability and temporal variation mentioned above could be verified by ground-based observations. Moreover, the amplitudes of inter-annual variations were different between urban area and rural area. The SOG and EOG in urban area had larger inter-annual variability than those in rural area. The reasons for these differences remain unclear. These findings quantified the differences of plant phenology between urban area and rural area, and thus revealed the modifications on plant phenology by human urbanization. This study shows the value of providing an instance for studying human modifications on environment. In the future, it is needed to study the reasons for the above mentioned differences in phenology between urban and rural. In addition to the urban heat island effect, it is also worthwhile to study the role of other conditions, such as air pollution and relevant modifications on solar radiation, night light, irrigation, and so on. Furthermore, as a potential research direction in the future, it is needed to further quantify the changes in the carbon sink/source resulted from the different plant phenology between urban area and rural areas.