Forests play a leading role in regional and global carbon (C) cycles. Detailed assessment of the temporal and spatial changes in C sinks/sources of China's forests is critical to the estimation of the national C budget and can help to constitute sustainable forest management policies for climate change. In this study, we explored the spatio-temporal changes in forest biomass C stocks in China between 1977 and 2008, using six periods of the national forest inventory data. According to the definition of the forest inventory, China's forest was categorized into three groups: forest stand, economic forest, and bamboo forest. We estimated forest biomass C stocks for each inventory period by using continuous biomass expansion factor (BEF) method for forest stands, and the mean biomass density method for economic and bamboo forests. As a result, China's forests have accumulated biomass C (i.e., biomass C sink) of 1896 Tg (1Tg=1012g) during the study period, with 1710, 108 and 78 Tg C in forest stands, and economic and bamboo forests, respectively. Annual forest biomass C sink was 70.2 Tg Ca-1 , offsetting 7.8% of the contemporary fossil CO2 emissions in the country. The results also showed that planted forests have functioned as a persistent C sink, sequestrating 818 Tg C and accounting for 47.8% of total C sink in forest stands, and that the old-, mid- and young-aged forests have sequestrated 930, 391 and 388 Tg C from 1977 to 2008. Our results suggest that China's forests have a big potential as biomass C sink in the future because of its large area of planted forests with young-aged growth and low C density.
Global surface temperature has dramatically increased in the past decades.It is critical to evaluate such a change using appropriate approaches.The previous studies for assessment of the change usually used overall trends of temperature series(i.e.slopes of simple linear regression of temperature versus year based on a least-square analysis) for entire study period.Temperature trends,however,differ among different periods,i.e.there are often breakpoints in the temperature series.Therefore,the overall linear trend of a temperature series may conceal some of the temporal characteristics of the temperature change.To precisely characterize the temporal and spatial patterns of air temperature change in China,we analyze the annual mean temperature series between the year of 1961?2004 for 536 meteorological stations across China,using piecewise linear regression approach.We found remarkable breakpoints in the annual mean temperature during the study period across the country.The annual mean temperature started to increase in 1984 at a rate of 0.058°C/a at the country level.The year when warming started appeared to be gradually later from the north to the south:temperature increased since the 1970s in the north(north of 40°N),and did not rise until the 1980s in most areas of the south(south of 40°N),with warming starting in 1983 in the Tibetan Plateau.The trends in annual mean temperatures showed a large spatial heterogeneity across China:a relatively small rising with a rate of 0.025-0.05°C/a in the Sichuan Basin,Central China and South China;the greatest increase in some parts of northwest China(i.e.Xinjiang) with up to a rate of 0.1°C/a;and rising at a rate of >0.05°C/a for most regions of the country.The feedbacks of cold waves and snow may be responsible for such regional differences in the timing and rates of warming in China.
