Impacts of rapid urbanization on carbon fixing and oxygen production of vegetation in The Pearl River Delta

Abstract

The Pearl River Delta which has an area of 4.17 × 104 km2 and a population of 4078.06 × 104 is a typical urbanizing area in China. There are 28 cites and 434 towns and the mean distance between the towns is less than 10 km. The Pearl River Delta has been the most rapidly urbanizing area in China and the world for the last 20 years. Urbanization is an important factor for land use change, which has an impact on the eco-environment due to vegetation destruction etc. However, very few studies focus on the impact of land use change on the eco-environment by estimating the loss of biomass and net primary production (NPP) of vegetation due to rapid urbanization. The loss of NPP has implications for carbon fixing and oxygen production. The tree biomass of forest was estimated from tree trunk volume. The biomass of undergrowth in forests was estimated based upon a case study of the Liuxihe forests of Guangzhou. The NPP of forest is related to biomass, condition and age of forest. Therefore, the NPP of forest was estimated by formulae relating NPP and biomass. The biomass and NPP of shrubland, sparse woodland, orchard and pasture were estimated by their mean values for the region. The biomass of a given crop was calculated by economic production and a relative economic coefficient. The NPP of all crops was estimated from biomass times multiple crop indices. Carbon content is calculated from biomass using a factor of 0.45. Land use has been changed with rapid urbanization in the Pearl River Delta. The area of cultivated land has decreased and the area of residential and industrial land has increased. Woodland, pasture and unused land has also decreased, and area of transport infrastructure, orchard and water bodies increased. The area of cultivated land, sparse woodland and shrubland, and pasture decreased by 304578 hm2, 77093 hm2 and 604 hm2 respectively from 1990 to 2000, because large amounts of ecological land were used for urban expansion in the Pearl River Delta. The 'lost' areas of cultivated land, sparse woodland and shrubland, and pasture in 10 the year period were equal to 28.78%, 30.15% and 38.57% respectively of those in 1990. The regional vegetation area was decreased by 8.00%. The ability for a balance of carbon and oxygen for the ecosystem of the Pearl River Delta was decreased by 8.68%. The amount of CO2 fixed and O2 production decreased by 4000137 t/a and 2909200 t/a respectively. Dongguan was one of the most rapidly urbanizing cities. Its area of residential and industrial land and transport infrastructure increased by 1.12 times and 1.4 times respectively in 10 years. Vegetation area in Dongguan was decreased by 25.35%, of which cultivated land, orchard, forest, sparse woodland and shrubland, and pasture decreased by 21370 hm2, 13214 hm2, 6979 hm2, 473 hm2 and 41 hm2 respectively. The biomass and NPP of Dongguan decreased by 1213961 t and 471419 t/a, respectively. The biomass and NPP of various vegetation types declined. The biomass and NPP of cultivated land, orchard, forest, sparse woodland and shrubland, and pasture decreased by 143820, 313172, 747213, 9346, 410 and 249815 t/a, 121569, 94573, 5150, 312 t/a, respectively. Carbon fixing and oxygen production of vegetation were decreased by 26.76%. It was shown that the ability of vegetation to fix carbon and produce oxygen would be reduced by 2/3 if the suburban area had been changed to an urban area according to a case study in urban Guangzhou. The study indicates that the impacts of land use change on the eco-environment cannot be neglected or ignored.