Response of tree-ring width to rainfall gradient along the Tianshan Mountains of northwestern China

By comparing the long-term tree-ring growths at various geographic scales, we can make clear the effects of environmental variations on tree growth, and get an understanding of the responses of forest ecosystems to the possible changes in global and regional climate. Radial tree-ring growth of Picea schrenkiana and its relationship to air temperature and precipitation were investigated across longi- tude transects on the north slopes of the Tianshan Mountains in northwestern China. Tree-ring samples were collected and residual chronologies were developed for three different regions along a gradient of decreasing precipitation from west to east. Response-function analysis was conducted to quantify the relationships between tree-ring chronologies and climate variables, such as monthly mean temperature and monthly precipitation from 1961 to 1998, using the PRECON software program. The statistical characteristics of the chronologies showed that the three chronologies constructed in this study con- tained significant environmental signals and were well suitable to reveal the impacts of climatic change on tree growth and forest productivity. Annual ring-width variations were similar among the three sites, but the variability was greatest in the east. This research showed that the growth trends of Picea schrenkiana in the Tianshan Mountains have not followed a uniform pattern. Response-function analy- sis indicated that there were significant correlations between tree growth and climatic factors in all the three regions, among which precipitation was the principal. With decreasing precipitation, the response of tree-ring widths to increasing temperature changed from a positive to a negative correlation. As for precipitation, the positive relationship to tree-ring width always dominates. It could be expected that with increased temperature and decreased precipitation, the importance of precipitation to tree growth would increase, and the response of tree growth to environmental changes would also increase. This study emphasizes the importance of regional-scale investigations into the biosphere-climate interac- tions. The results of this research indicated a substantial increment of tree-ring radial growth as a re- sult of warmer and wetter climate in the eastern regions. However, climate change will have less effect on forest growth and primary production in the western regions.

The discovery of surface runoff in the megadunes of Badain Jaran Desert,China,and its significance

The Badain Jaran Desert exhibits the greatest difference in altitude of all of the world's deserts.On the slopes of megadunes in the desert,there are physical and chemical deposits produced by surface runoff.In addition,we have observed rarely-seen infiltration-excess surface runoff in the megadune depressions as well as spring streams at the base of megadunes.We used electron microscopy,energy spectrum analysis,infiltration experiments,moisture content determinations and grain-size analysis to study the mineral and chemical composition of the runoff precipitates,and grain-size of the deposits associated with the runoff,together with the hydrological balance in the megadune area,and the atmospheric precipitation mechanism responsible for groundwater recharge and for supplying water to lakes.The observations of shallow runoff and infiltration-excess surface runoff indicate the occurrence of strong and effective precipitation in summer,which would provide an important source for groundwater recharge.Several lines of evidence,such as the physical and chemical deposits resulting from shallow subsurface runoff,spring streams,infiltration-excess runoff,and gravity capillary water with a moisture content of 3-6%,demonstrate that precipitation reaches the base of the megadunes through infiltration and subsequently becomes groundwater.The chemical deposits,such as newly-formed calcite and gypsum,and gray-black physical deposits,as well as different stages in the development of fan-shaped landforms resulting from shallow subsurface runoff,indicate that groundwater recharge in the area is the result of long-term precipitation,rather than intermittent individual major rainfall events.Fine sand layers with a low infiltration capacity lead to subsurface runoff emerging at the ground surface.Five factors play an important role in maintaining a positive water balance and in replenishing groundwater via rainfall:effective rainfall as a water source,the high infiltration capacity of the sands enabling rainfall to rapidly become capillary water in the dunes,low evapotranspiration rates due to the sparse vegetation,the fact that the depth of the sand layer influenced by evaporation is shallow enough to maximize the deep infiltration of rainfall,and rapidly-moving gravity capillary water in the sandy dunes.These five factors together constitute a mechanism for groundwater recharge from rainfall,and explain the origin of the groundwater and lakes in the area.Our findings represent a significant advance in research on the hydrological cycle,including groundwater recharge conditions and recharge mechanisms,in this desert region.