Populus pruinosa Schrenk, a rare and vulnerable plant in arid deserts occurring mainly in the Tarim River Basin, is a key species of the Tarim desert ecosystem. The existence and development of desert riparian forest has controlled the structure and function of the desert ecosystem and eased sustainable development in the region. Since the 1950s, people used soil and water resources in the upper reaches of the Tarim River in an unsustainable manner, resulting in poor forest growth and death of many areas of forest. This directly endangered the sustainable development of the oasis' agricultural system and disturbed the ecological balance in this area. We studied the life history characteristics and spatial distribution patterns of P. pruinosa populations in the upper reaches of the Tarim River, Xinjiang, China, to determine the ecological characteristics of P. pruinosa populations and to learn how biological and environmental factors affect population the distribution and dynamics. The current trends in development of the P. pruinosa forest was studied, with the goal of ensuring the wise use (conservation) and protection (preservation) of this precious tree species in the Tarim River Basin.
The survey area was located in the upper reaches of Tarim River (40°25'-41°10'N, 80°10'-84°36'E). Five 1.0 hm² plots were established and divided into 100 subplots. Diameter at breast height (DBH) of all trees DBH > 2.5 cm, regeneration-sized trees of P. pruinosa and shrubs as well as herbs were identified and measured using a contiguous grid quadrate method. We determined the life table, age structure and survivorship curve of P. pruinosa populations to analyze population structure and dynamics. We divided trees into four groups based on twelve DBH size classes to analyze spatial pattern dynamics for different age classes using field data. The results showed that the age structure of P. pruinosa populations had a bell shape with a low percentage of younger and healthy adult trees, which indicated the population is temporarily stable. The age structures of P. pruinosa populations were different in different habitats and the population structure transitioned from stationary to declining shape as one proceeded down river. The survivorship curve of P. pruinosa populations conformed to Deevey type Ⅱ shape. The survival rate decreased over time as trees aged and survival rate of the medium aged trees (Ⅵ-Ⅸ) decreased more quickly. The hazard rate for P. pruinosa populations increased with developing age and the mortality rate surpassed the survival rate after trees reached the Ⅷ age class. Moreover, the mortality rate, hazard rate and age-specific mortality rate peaked in the Ⅸ age class and life expectancy decreased monotonically during a given tree's life span, which was caused by intraspecific competition for resources and physiological aging. This species generally has a clumped distribution pattern, but spatial distribution patterns differed among plots and growth periods, which implies that the spatial distribution of this species at any one site is the result of adaptation to environmental stress and interaction with the local environment. The spatial distribution patterns of different growth stages of P. pruinosa populations varied as age increased; aggregation intensity gradually decreased and changed over time from a clumped to a random pattern as trees matured from young to middle-age to old trees, which was a result of changes in the intensity of intraspecific competition. Correlation and stepwise regression analysis between environmental factors and population density showed that groundwater level was the main factor affecting the population distribution. As a result of variations in the availability of groundwater, the population structure and spatial pattern varied with different growth periods and different habitats, which revealed that biological and environmental factors were the two main types of factors affecting the population's ecological characteristics. We conclude conservation strategies designed to protect P. pruinosa populations should focus on improving habitat conditions and seedling establishment, while reducing human disturbance and enhancing genetic diversity.