Transpirations of three dominated tree species, namely Mongol Scotch Pine (Pinus sylvestris var. mongolica Litvin), White elm (Ulmus pumila) and Gansu Poplar (Populus gansuensis Wang et Yang) in oasis shelter forest (Linze site) and of two dominated tree species,namely Euphrates Poplar (Populus euphratica Oliv.) and Russia olive (Elaeagnus angustifolia Linn.) in lowland desert (Erjinaqi site) have been estimated using measured sapflow in summer,autumn and winter, 2002 and in spring, 2003. An ENVIS System was used for each site to measure microclimate variables, soil moisture and sapflow every half an hour, and the study time scale is one day. In the 104 days of observation during the growing season at the Linze site, the average daily sapflow of Gansu Poplar is 9.93L·d-1,and the average transpiration per unit leaf area is 1.99mm·d-1.For White elm tree,the daily average sapflow is 4.08L·d-1,while the daily average transpiration per unit leaf area is 0.49mm·d-1.The values for Mongol Scotch Pine are 3.91L·d-1 and 0.25mm·d-1,respectively.In the total 73 days of observation during the growing season at the Erjinaqi site, the daily average sapflows of Russia olive and Euphrates Poplar are 12.1 and 20.97L·d-1,respectively,and the average transpirations per unit leaf area are 0.22 amd 0.31mm·d-1,respectively.In the observation period of the growing season,tree conductances of Mongol Scotch Pine, White elm, Gansu Poplar or Russia olive show an exponential relationship with the daily average air temperature or vapour pressure deficit, but the relationship is not so obvious between tree conductance and global radiation. The transpiration process of each tree species is affected by all the observed four environmental variables. The response of tree conductance to different climatic factors changes with tree species. The effect of the same factor to the same tree species is also variable in different growing stages. The sapflow of every tree species is relatively large in later spring to early summer, and low in summe
Atmospheric CO2 concentration （CC） near land surface and meteorological variables have been measured at four sites, named Yeniugou （alpine meadow and permafrost）, Xishui （mountainous forest）, Linze （oasis edge） and Ejina （lower desert）, respectively, in Heihe River Basin, northwest China. The results showed that, the half hourly CC at night was larger than in daytime, and the daily averaged CC was the largest in winter. The averaged CC of 932 d at the Linze was about 418 ppm, was about 366 ppm in the 762 d at the Ejina. In the same period from September 23 to November 9, 2004, the averaged CC was about 625,334, 436 and 353 ppm, at Yeniugou, Xishui, Linze and Ejina, respectively. The linear relationship between daily averaged CC and air temperature T was negative, between CC and relative humidity （RH） was positive. The linear CC-atmospheric pressure （A P） relationship was negative at the Linze and Yeniugou, was positive at the Ejina. The relationship between CC and global radiation R was exponent, and soil temperature Ts was negative linear, and soil water content was complex. The correlation between CC and wind speed was not existent. Using meteorological variables together to simulate CC, could give good results.
CHEN Ren-sheng KANG Er-si JI Xi-bin YANG Jian-ping ZHANG Zhi-hui YANG Yong
Based on yearly precipitation and Φ20 evaporation pan data during 1951 to 1999 of 295 stations,the aridity index is calculated in this paper.According to the aridity index,the climatic regions in China are divided into three types:the arid zone,the semi-arid zone and the humid zone. Isoline 0.20 is the boundary between arid and semi-arid zones.Isoline 0.50 is the boundary between semi-arid and humid zones.The fluctuations of dry and wet climate boundaries are very substantial,have greatly regional difference,and have the features of the whole shifting along the same direction and of the opposite moving along the contrary direction over the past 50 years.The semi-arid zone is a transitional zone between humid and arid zones,a border belt of monsoon,and a susceptible zone of environmental evolution in China. In the period of the late 1960s to the early 1970s,remarkable change had occurred for dry and wet climate in China.It manifests significantly that climate is from wetter into drought in most regions of northern China.Moreover,drought has an increasing trend.The fluctuations of climatic boundaries and the dry and wet variations in climate have substantial inter-decadal features. The main factors affecting the dry and wet climate boundary fluctuations and the dry and wet variations of climate in China are East Asian summer monsoon,Indian Monsoon,plateau monsoon in the Tibetan Plateau,westerly circulation,and West Pacific subtropical high.The different types of circulations and the strong and weak combinations of these circulations result in the regional differences of dry and wet climate changes in China.Inter-decadal variations of the dry and wet climate boundary fluctuations and of the arid and humid climate result from the inter-decadal changes of East Asian summer monsoon,Indian Monsoon,plateau monsoon,westerly circulation, and West Pacific subtropical high.The anomalous general atmospheric circulation in the Northern Hemisphere during the late 1960s to the early 1970s is the causes of arid and humid climate
