塔里木河上游灰胡杨种群生活史特征与空间分布格局

Abstract

灰胡杨是荒漠地区特有的珍稀渐危植物,主要分布在塔里木河流域,是塔里木荒漠生态系统的关键种之一。在塔里木河上游选择具有代表性群落设立5 hm²研究样地,应用相邻格子法进行每木调查,从年龄结构、生命表特征、存活曲线、生存分析以及种群不同发育阶段个体空间分布格局等方面研究了灰胡杨种群生活史动态特征与空间分布规律。结果表明:塔里木河上游灰胡杨种群结构整体上呈钟型,幼树个体较少,成年个体较多,属暂时稳定型种群;不同生境种群结构不同,随河流走向种群结构由稳定型转为衰退型。种群存活曲线趋于DeeveyⅡ,生存率呈单调下降趋势,中期(Ⅵ-Ⅸ)降幅较高;危险率呈上升趋势,于Ⅸ龄级出现危险率与死亡率、消失率高峰,生命期望单调下降,反映种群生存力下降的趋势。灰胡杨种群空间分布格局总体上为聚集分布,因生境不同空间格局发生变化,这是种群适应环境压力及与环境协同进化的结果;种群空间格局随发育进程而变化,从幼龄到中、大龄,种群分布格局的聚集强度减弱并逐渐转变为随机分布,这是种内竞争强度变化的结果。环境因子与种群分布多度的相关与逐步回归分析表明地下水位是影响种群分布的主要因素。可见,塔里木河上游灰胡杨种群结构与空间分布格局随生境和时间发生变化,环境因素和生物因素是影响灰胡杨种群生态特征的两个主要原因。;<em>Populus pruinosa </em>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 <em>P. pruinosa</em> populations in the upper reaches of the Tarim River, Xinjiang, China, to determine the ecological characteristics of <em>P. pruinosa</em> populations and to learn how biological and environmental factors affect population the distribution and dynamics. The current trends in development of the <em>P. pruinosa </em>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. <br>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 (<em>DBH</em>) of all trees <em>DBH</em> > 2.5 cm, regeneration-sized trees of <em>P. pruinosa</em> 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 <em>P. pruinosa</em> populations to analyze population structure and dynamics. We divided trees into four groups based on twelve <em>DBH</em> size classes to analyze spatial pattern dynamics for different age classes using field data. The results showed that the age structure of <em>P. pruinosa</em> 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 <em>P. pruinosa</em> 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 <em>P. pruinosa</em> 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 <em>P. pruinosa </em>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 <em>P. pruinosa</em> 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<em> P. pruinosa </em>populations should focus on improving habitat conditions and seedling establishment, while reducing human disturbance and enhancing genetic diversity.