Ming Xu

Abstract Soil‐surface CO 2 efflux and its spatial and temporal variations were examined in an 8‐y‐old ponderosa pine plantation in the Sierra Nevada Mountains in California from June 1998 to August 1999. Continuous measurements of soil CO 2 efflux, soil temperatures and moisture were conducted on two 20 × 20 m sampling plots. Microbial biomass, fine root biomass, and the physical and chemical properties of the soil were also measured at each of the 18 sampling locations on the plots. It was found that the mean soil CO 2 efflux in the plantation was 4.43 µmol m −2 s −1 in the growing season and 3.12 µmol m −2 s −1 in the nongrowing season. These values are in the upper part of the range of published soil‐surface CO 2 efflux data. The annual maximum and minimum CO 2 efflux were 5.87 and 1.67 µmol m −2 s −1 , respectively, with the maximum occurring between the end of May and early June and the minimum in December. The diurnal fluctuation of CO 2 efflux was relatively small (< 20%) with the minimum appearing around 09.00 hours and the maximum around 14.00 hours. Using daytime measurements of soil CO 2 efflux tends to overestimate the daily mean soil CO 2 efflux by 4–6%. The measurements taken between 09.00 and 11.00 hours (local time) seem to better represent the daily mean with a reduced sampling error of 0.9–1.5%. The spatial variation of soil CO 2 efflux among the 18 sampling points was high, with a coefficient of variation of approximately 30%. Most (84%) of the spatial variation was explained by fine root biomass, microbial biomass, and soil physical and chemical properties. Although soil temperature and moisture explained most of the temporal variations (76–95%) of soil CO 2 efflux, the two variables together explained less than 34% of the spatial variation. Microbial biomass, fine root biomass, soil nitrogen content, organic matter content, and magnesium content were significantly and positively correlated with soil CO 2 efflux, whereas bulk density and pH value were negatively correlated with CO 2 efflux. The relationship between soil CO 2 efflux and soil temperature was significantly controlled by soil moisture with a Q 10 value of 1.4 when soil moisture was <14% and 1.8 when soil moisture was >14%. Understanding the spatial and temporal variations is essential to accurately assessment of carbon budget at whole ecosystem and landscape scales. Thus, this study bears important implications for the study of large‐scale ecosystem dynamics, particularly in response to climatic variations and management regimes.

It is commonly believed that greenhouse‐gas‐induced global warming can weaken the east Asian winter monsoon but strengthen the summer monsoon, because of stronger warming over high‐latitude land as compared to low‐latitude oceans. In this study, we show that the surface wind speed associated with the east Asian monsoon has significantly weakened in both winter and summer in the recent three decades. From 1969 to 2000, the annual mean wind speed over China has decreased steadily by 28%, and the prevalence of windy days (daily mean wind speed > 5 m/s) has decreased by 58%. The temperature trends during this period have not been uniform. Significant winter warming in northern China may explain the decline of the winter monsoon, while the summer cooling in central south China and warming over the South China Sea and the western North Pacific Ocean may be responsible for weakening the summer monsoon. In addition, we found that the monsoon wind speed is also highly correlated with incoming solar radiation at the surface. The present results, when interpreted together with those of recent climate model simulations, suggest two mechanisms that govern the decline of the east Asian winter and summer monsoons, both of which may be related to human activity. The winter decline is associated with global‐scale warming that may be attributed to increased greenhouse gas emission, while the summer decline is associated with local cooling over south‐central China that may result from air pollution.

Abstract This study extends upon previous analyses and details near‐surface wind speed change in China and its monsoon regions from 1969 to 2005, using a new dataset consisting of 652 stations. Moreover, causes of wind speed changes are examined. Major results show that most stations in China have experienced significant weakening in annual and seasonal mean wind during the study period. The averaged rate of decrease in annual mean wind speed over China is − 0.018 ms −1 a −1 . Decrease in seasonal mean wind differs. The largest rate of decline is in spring at − 0.021 ms −1 a −1 and the least is in summer at − 0.015 ms −1 a −1 . Spatially, large declines are found in northern China, the Tibetan Plateau and the coastal areas in east and southeast China, while central and south–central China have the least change in their wind speed. Significant weakening of wind speed has occurred primarily in strong wind categories. Decreases in light wind categories are trivial, and light wind has even increased slightly in parts of central China. These changes indicate reduced fluctuations in wind and wind storms in recent decades, contributing to decreased frequency and magnitude of dust storms. The trivial changes in summer winds in east and southeast China suggest fairly steady monsoon winds over the decades. A main cause of the weakening wind is shown to be the weakening in the lower‐tropospheric pressure‐gradient force, a result pointing to climate variation as the primary source of the wind speed change. Superimposed on the climate effect is the urban effect. While analysis of winds between urban and rural stations reinstate the urban frictional effect, a peculiar stronger increase in wind at urban stations than at rural stations after the abrupt urbanization since 1990 indicates a new aspect of the urban effect on wind speed. Copyright © 2010 Royal Meteorological Society