The Afforestation Effect on Regional Climate: Take Xuzhou Region as an Example

Abstract

Forest coverage has been increasing in the Xuzhou region, Jiangsu Province, China, since the 1960s, increasing from about 1–31.8% in 2013, or expanding by more than 30 times in the past 50 years. Normalized Differential Vegetation Index data which represents ‘the degree of green’ in 1998–2008 also showed an obvious increasing trend in the Xuzhou region. Even though the climate is affected by a population increase and urbanization, the effects of forest vegetation on the regional climate are still very significant in the Xuzhou area. Using statistical analysis, remote sensing and numerical simulation, the main effects of climate on vegetation are as follows. (1) Reduced range of annual temperatures and increased warming rate. Based on meteorological observation data, the mean high temperature showed a declining trend over the 50 year study period, while the mean lowest temperature did increase, and the annual mean temperature range declined from 24.3 °C in the 1960s to 22.0 °C in the 2000s. For the four seasons, the minimum winter temperature increased moderately and the maximum temperature decreased significantly. Simulation results from Regional Climate Model, version 3 (RegCM3) also suggested that with the increase of forest coverage the warming rate decreased over this entire region. (2) Increasing humidity. Based on single point meteorological observations, the mean water vapor pressure rose from 13.4 hPa in the 1960s to 14.2 hPa in the 2000s; the rising trend in winter was the most significant. In addition, results from the RegCM3 model showed coincident conclusions. In the entire Xuzhou region, the water vapor mixing ratio rose from 9.75 kg/kg in the 1960s to 10.35 kg/kg in the 1990s. Furthermore, the relative humidity was 80.5% in the 1980s, and 81.3% in the 1990s, suggesting an increasing trend for atmospheric humidity exists under the warming conditions. (3) Significantly reduced wind speed at and near the earth’s surface. Based on observations, wind speed decreased from 10.99 m/s in the 1960s to 7.59 m/s in the 2000s; in addition, wintertime wind speeds decreased significantly. Data from the RegCM3 model clearly show wind speed at the surface fell from 4.72 m/s in the 1960s to 1.4 m/s in the 1980s, to 1.2 m/s in the 1990s, then to 0.6 m/s in the 2000s in the entire Xuzhou region. (4) Reduced duration of surface sunshine. Weather station data showed that the monthly mean sunshine hours were 198.4 h in the 1960s, but only 165.5 h in the 2000s.