塔里木河中游水文影响下的胡杨种群格局动态

在利用环境同位素技术与多年径流量数据分析塔里木河中游的水文过程基础上,采用聚集度指标与种群结构动态量化法,研究了水文条件变化下胡杨种群结构特征与时空格局动态,预测了胡杨种群未来发展趋势。结果表明:(1)自1998年以来,中游年径流耗损量逐年减少,依靠地表水补给的地下水日益匮乏,具体表现为地下水埋深呈直线下滑趋势,这导致堤防外植被生长所需的生态用水日益紧缺。(2)在分布格局上,胡杨种群借助集群分布的格局抵抗恶劣的生态环境,其聚集强度随个体发育不断减弱,并有向随机分布发展的趋势;在种群结构上,胡杨种群除2003年为稳定型,其他年份下均属衰退型,表现出结构老龄化且长期衰退的特征。(3)2002年中游水量最为充沛,由于生态对环境变化的迟滞性特征使2003年地下水位抬升,导致胡杨种群变为稳定型,且聚集强度最大,种群增长趋势最强。种群动态指数与地下水埋深的回归模型显示,随地下水埋深的下降,种群增长趋势降低,验证了胡杨种群对水文变化的响应。(4)种群动态预测表明,未来2a,仅幼树数量呈增加趋势,其余径级均为减少趋势;未来5年a大树(或老树)数量将大幅减少。可见,地表水与地下水的逐年减少影响了胡杨种群的分布格局,阻碍了种群的更新发育,水文条件是影响中游胡杨生态特征的直接因素。

The distribution pattern dynamics of Populus euphratica are influenced by hydrology in the middle reaches of Tarim River

Populus euphratica is a member of the Salicaceaefamily, and is an important tree species of the desert riparian forest. P. euphratica has some unique abilities such as withstanding sandstorms, curbing desertification, maintaining ecological balance, protecting biodiversity in arid areas, and safeguarding the production of oasis agriculture. The population distribution pattern refers to the level of the population and the distribution status of individuals of the same population within a certain range of a horizontal area. The population distribution pattern is systematically affected by biological characteristics, interspecies relationships, and environmental conditions. Ascertaining the population distribution pattern and its dynamic changes contributes to elucidating the ecological characteristics of a species for further studies. Thirty natural P. euphratica plots were established on the outer side of water conveyance embankments in the middle reaches of Tarim River, and field data were obtained with a typical sampling method. With the use of environmental isotope technology in combination with the monitoring data of annual runoff in Yingbazha and the Kala hydrometric station, we analyzed the hydrological changes in the middle reaches of Tarim River. We divided the samples into a size-class structure to apply aggregation intensity indices and conducted a quantitative analysis for plant population structural dynamics. The dynamics of the distribution pattern and characteristics of population structure influenced by hydrological conditions were analyzed, and then time-series analysis was used to predict the population development trend in the future. This analysis is important to provide a theoretical basis for ecological restoration of the natural P. euphratica forest. Since 1998. the runoff consumption in the middle reaches has decreased and groundwater supplied by the surface water has become increasingly scarce. Groundwater depth showed a linear downward trend, indicating that ecological water for vegetation growth out of the embankment was deficient. With regard to the distribution pattern, P. euphratica showed a clumped pattern in each size-class structure to resist the harsh ecological environment, and the aggregate intensity index was reduced with individual development. The height class size showed a tendency to change from a clumped to a random pattern. With regard to the population structure, P. euphratica showed a stable population type in 2003, and a declining population pattern for the other years. These results showed that the population characteristics were structured by ageing and showed a long-term pattern of decline. The runoff of the middle reach was the most abundant in 2002. Because of the hysteretic characteristic of ecology to environmental change, the groundwater level was promoted in 2003. These changes led to the largest aggregate intensity and the best population growth of P. euphratica in 2003. The regression model between the population dynamics index and groundwater depth showed that population growth had a decreasing trend with a reduction in groundwater depth. Therefore, the response of the P. euphratica population to hydrological changes was verified. Finally, population dynamics prediction indicated that the number of saplings of P. euphratica is expected to increase, whereas the number of trees in other size classes is expected to decrease in the next two years and the number of large (or old) trees will likely be rapidly reduced in the next five years. The decline in surface water and groundwater influenced the distribution pattern and hindered population development. We conclude that hydrological conditions directly influence the ecological characteristics of P. euphratica in the middle reaches of Tarim River.