关键词: 平原绿化, 环境卫星, 农田林网, 作物物候

Abstract:

The term “afforestation” in this paper generally represents all kinds of non-crop vegetation in crop-dominated plain area, including but not limited to the vegetation in crop land (e.g., orchard, herb, nursery garden, fast-growing and high-yielding timber, farmland shelterbelt, and artificial turf), residential green, urban vegetation landscapes, trees planted alongside the roads and rivers, and the wind break and sand fixation forest. Accurate and timely information about afforestation in the plain region is useful because it can reflect the degree of agricultural diversity and the environmental health. However, getting the information about afforestation using remote sensing is hindered by some factors: afforestation are spatially scattered, temporally and spectrally overlapped with some crop lands. In addition, as a land use type, afforestation in plains often shares common spaces with other types of land use, such as croplands and roads. As a result, there is little, if any, data about afforestation in plains, let alone the afforestation change monitoring. As a follow-up to UN Millennium Ecosystem Assessment, the Chinese government is currently conducting a similar assessment at the provincial level, covering years of 2000-2005-2010, and using the traditional (FAO) land cover/land use system. It is beyond the expectation that the “forest” - as the major indicator of a good environment, in the province such as Henan, has only experienced a negligible increase. This is simply because the recent afforestation in plains was classified as other land cover types. With the advent of Chinese satellite HJ-1 A/B in September 2008, there may be a chance to extract information about afforestation in plains since its CCD sensor has Red and NIR channels, with a 30m spatial resolution and a 4-day temporal resolution. To test this potentiality, three steps were taken to extract afforestation in this study: (1) extract the plain information using 2010 Land Cover Map; (2) eliminate the double-crop (winter wheat) land using NDVI_June data when all the wheat had been harvested; (3) eliminate the single-crop land using NDVI_April data. Afforestation was extracted by calculating ((NDVI_April≥0.22)∩(NDVI_June≥0.35)). Validation was conducted using correlation with the statistics of 26 major cities, resulting in a significant R²=0.9166. The algorithm performed well in the wheat region, yet it did poor in the rice and rice-wheat transition regions, which are mainly distributed in the southern part of Xinyang. Aside from Xinyang, afforestation area extracted from HJ-1 is 6909.8 km². This accounts for 20.53% of the total forest in Henan Province. Future study should make full use of HJ-1 high temporal resolution data by conducting zoning according to the climate and the soil.

Key words: plain afforestation, HJ-1 A/B CCD, NDVI, farmland shelterbelt, crop phenology