Abstract: Spatiotemporal expansion of urban area is an important indicator for characterizing urbanization level. In this paper, the information of recent 20 years land use change in the six districts of Tianjin central urban area and four districts around the urban area was extracted by integrating four TM/ETM images from 1987 to 2006, and, combining with socio-economic statistic data and applying GIS spatial analyses and spatial gravity center transfer model, the expansion process, orientation, and driving forces of Tianjin built-up area were analyzed. In 2006, up to 82.0% of construction lands were in the central urban area of Tianjin, and near 24.9% were in the four districts around the urban area. From 1987 to 2006, the built-up area showed a large expansion, with the construction lands increased by 19.1%, and the period 2001-2006 was most evident in built-up area expansion. The increase of construction lands was dominantly from agricultural lands and water area. In recent 20 years, the expansion of Tianjin built-up area was mainly in an export-oriented way, with the six districts in central urban area as the center and an overall southeastward transfer of gravity center. Economic development, especially the rapid growth of GDP and the adjustment of industrial structure, was the most important factor controlling the expansion of the built-up area in recent 20 years. Transportation infrastructure improvement was the impetus for the built-up area expansion and partly affected the expansion orientation, population increase as a result of economy development also accelerated the expansion of the built-up area, and macroscopic policies, in a large degree, determined the expansion pattern and scale of Tianjin City.

Key words: Hyperaccumulator, Heavy metal, Physiological mechanisms, Molecular biological mechanisms