Ecological Study On Picea Schrenkiana Forest Along An Altitudinal Gradient In The Central Tianshan Mountains

Relationships between tree growth and environment are complicated. One hand, tree rings could provide reliable records of past climatic conditions, tree ring chronologies are therefore the most widely used proxy for reconstructing annual variations in climate that extend back several centuries to millennia. On the other hand, population structures in the treeline ecotones are also found to be good indicators of climate change by many studies on treeline dynamics. By using phytopopulological and dendroecological methods, this paper has studied the correlation between climate change and the age structure or tree-ring width from different altitudinal ecotones. Primary results of this paper were summarized as following:1). To study the size distribution and age structure, the Picea schrenkianu (Fisch. et Mey.) forest of the central Tianshan Mountains was divided into three transects, i.e.. lower limit (1500-1700m). mid-altitude (1800-2400m) and upper treeline (2500-2700m). A reverse-J shape age structure was found in the mid-altitude transect, self-thinning and density dependent competition between trees was probably the most important influence factor in this ecotone. The low altitude transect was characterized by the dominance of young trees, precipitation may be the important factor to survival and population age structure of P. schrenkiana forest in this transect. However, a bell shape age structure was found with infrequent recruitments and the <60a age class accounted for 14% of total individuals in the cold high altitudinal treeline. Low temperature may be the major limiting factor to population dynamics of this high altitudinal ecotone.2). Three chronologies were developed at three different altitudes, i.e., lowforest border (1600-1700m a.s.l.), interior forest (2100-2200m a.s.l.). and upper treeline (2600-2700m a.s.l.). to study the correlation between tree-ring width and climatic change. Trends in annual ring-width variations were similar among the three sites but variability was greatest at the low forest border site. The results showed that tree radial growth positively correlated with precipitation but negatively with temperature even at the cold high-elevation treeline site. With decreasing altitude, air temperature increased and the importance of precipitation on tree growth increased. Trees most sensitive to drought were found near the arid low forest border. Precipitation during the growing season also influenced tree-ring growth patterns, but the lag effect was the dominant limiting factor at sites of higher elevation.3). P. schrenkiana from the treeline of central Tianshan Mountains were disaggregated into different age class to discuss the relationships between tree-ring width and climate by using correlation coefficients and response function. The results suggested that the radial growth of young P. schrenkiana were significantly correlated with temperature and precipitation from the current growth period: but the tree-ring width from the aging trees were more influenced by the climate of the end of previous growth period or by the temperature and precipitation prior to current growth period. Chronologies responsed to climate differently if they concluded different age-class tree ring cores. The wider age class of age-independent tree ring models one chronology contains, the less accurate reconstructions of past climate this chronology could give.4). Both the climatic records and dendroecological data confirmed climatic warming in recent several decades in the central Tianshan Mountains, but the regeneration has decreased in the recent several decades. Tree radial growth was positively correlated with mean February temperature, whereas reproduction and establishment of seedlings were mainly influenced by climatic fluctuation of intra-year or interannual temperature. High temperature is not the only factor to cause treeline rising, it could not rise even with global warming if climatic fluctuation of intra-year or interannual temperature increased. It is inferred that if the regeneration...