沙柳(Salix psammophila)是毛乌素沙地典型的优势灌木,对维持沙区生态系统稳定起着重要作用。在自然生长过程中,随着年龄增加,沙柳死亡枝条数增多,植株生命活力衰退,沙区生态系统的稳定受到挑战。研究沙柳丛生枝结构随年龄增加的变化规律对保育沙柳生命活力、保障沙区生态系统稳定具有重要意义。本研究通过对沙柳丛生枝分枝收获,查数年轮计数枝龄,绘制丛生枝的年龄结构图,编制静态生命表,分析存活曲线、生存函数等参数变化,并运用时间序列模型预测丛生枝的年龄结构动态。结果表明:沙柳丛生枝中以3 a生枝条最多,年龄结构呈两头小中间大的纺锤型;当年生的枝条生命期望最高、5 a生枝条的死亡率和消失率最高,存活曲线趋于Deevey-Ⅱ型;枝条生长4 a时死亡密度开始下降,生长5 a后的累计死亡率和危险率稳定,枝条动态为前期衰退、后期稳定;丛生枝动态指数趋近于0,趋于稳定型;在未来的2、4、6、8 a时间后沙柳丛生枝数量逐渐增多,但幼龄枝条补充不足,沙柳稳定性难以长期维持。因此,在沙柳生长4 a后需要通过适当的平茬抚育来调整丛生枝的年龄结构以维持沙柳生命活力。
Salix psammophila is a pioneer species in the Mu Us Sandy Land. Understanding the age structure and survival status of branches can provide insight into the senescence process of S. psammonphila. We surveyed and analyzed 342 branches of S. psammonphila in southern Mu Us Sandy Land. We estimated the survival curve, mortality curve, life expectancy of the branches, and other parameters of age structure. The results showed that the branches were characterized by the abundance of 3-year-old branches, and the age structure of branches was spindle shaped. The stationary life table of S. psammonphila branches indicated that the observed life expectancies were the highest in 1-year-old branches, while the mortality rate and disappearance rate were the highest in 5-year-old branches. The survival curve of S. psammonphila branches tended to be of Deevey-Ⅱ type. The survivability of the branches declined at young age and fluctuated at old age. The dynamic index of population size of Vpi' was close to zero under random disturbance, indicating that the age structure of the branches tended to be stable and was very sensitive to environment disturbance. The time sequence model predicted that the number of individuals increased in the coming 2, 4, 6 and 8 age classes, but the number in young individual deceased. It was assumed that S. psammonphila population would decline from a lack of recruitment of young individuals if appropriate action is not taken to protect and foster these plants. Therefore, we encourage appropriate pruning to rejuvenate S. psammonphila.
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