The urban forest in Beijing and its role in air pollution reduction (China)

Tree planting has been proposed by the municipal government as a measure to alleviate the air pollution in Beijing, the capital of China. This study examines that proposal. It is based on the analyses of satellite image and field surveys to establish the characteristics of current urban forest in the central part of Beijing. The influence of the urban forest on air quality was studied using Urban Forest Effects Model (UFORE).; The results showed that there were 2.4 million trees in the central part of Beijing. The diameter distribution of the trees was skewed toward small diameters. The urban forest was dominated by a few species. Condition of trees in the central part of Beijing was not ideal, about 29% of trees were classified into a poor condition class. The trees in the central part of Beijing removed 1261.4 tons of pollutants from the air in 2002. The air pollutant that was most reduced was PM10 (particulate matters with an aerodynamic diameter smaller than 10 mum) amounted to 772 tons. The carbon dioxide (CO 2) stored in biomass form by the urban forest amounted to about 0.2 million tons.; After studying the current urban forest, new planting was designed with the goal of achieving better air pollution reduction benefits. The potential planting sites were located by using remote sensing and GIS to analyze the satellite image. Nineteen tree species were selected based on their ranking of air pollutants reduction ability and suitability for planting. A multiobjective optimization procedure was used to produce the numbers of each species that should be planted. Cost and benefit analysis was conducted for the proposed new planting. It was found that the cost to air pollution reduction benefit ratio was about 0.05%. However, if other benefits of trees were quantified, and innovative cost-saving methods were taken into consideration, the final benefit to cost ratio of tree planting would be higher. Preserving existing trees and changing the current pruning practice were recommended to improve the efficiency of the urban forest in air pollution reduction. Future research directions to improve our understanding of the role of individual tree species in air pollution reduction are discussed.