古尔班通古特沙漠西部梭梭种群退化原因的对比分析

古尔班通古特沙漠植被的建群种梭梭在沙漠的低平地和小沙丘(高度<5 m)区域发生大面积退化死亡,而在大沙丘(高度>10 m)生境中梭梭却长势良好。对此分析了梭梭林枯死植株的空间分布、活株和死株的年龄结构,研究了地下水埋深以及对比低平地、小沙丘和大沙丘3种生境土壤理化性质对梭梭生存的影响。发现梭梭死亡植株在观测的19个样点中的15个呈显著的集群分布,表明梭梭植株死亡并非是种内自疏。梭梭各年龄级植株均在死亡,说明死亡与植株年龄关系不大。退化区地下水埋深过浅或过深均对梭梭生长不利,表明地下水埋深对梭梭生长产生显著影响。低平地和小沙丘区土壤电导率高,土壤水分入渗速率慢,且0-90 cm深度范围梭梭侧根数和成活的梭梭幼苗幼株数较少,与梭梭正常生长的大沙丘区明显不同。综合分析认为梭梭退化主要是由自身的根系分布特性,以及地下水位、土壤盐分、土壤水分入渗变化等综合作用,实质是梭梭水分利用受限和种群更新不良引起的。

A comparative study on reasons of degenerated of Haloxylon ammodendron population in the western part of Gurbantunggut desert

Haloxylon ammodendron (saxaul, a yellow-flowered shrub) is a keystone species in China's Gurbantunggut Desert. The population of the species has degenerated in low-lying areas and small dunes (height <5m), while nearby in large dunes (height >10m) the plants grow normally. In this study we investigated the spatial distribution of dead plants, the ages of living and dead plants, and the condition of the shallow lateral roots of H. ammodendron, as well as the depth of groundwater and the physical and chemical properties of the soil in low-lying areas, small dunes, and large dunes. Dead plants showed a highly significant cluster distribution in 15 out of the 19 observation plots. We found that intraspecific self-thinning was not the cause of the decline of H. ammodendron: there were dead plants in all age groups, suggesting no relationship between population age and degeneration. The relationship between the survival index of H. ammodendron populations and the groundwater level was significantly negative, indicating that groundwater has a profound effect on the viability of the populations; however, the relationship was reversed for shallow water levels, indicating that water tables need to be maintained within a certain appropriate range for the species to thrive. We found a significant negative relationship between the electrical conductivity of soil (at depths from 0-30cm to 90-120cm) and the numbers of H. ammodendron seedlings (height <40cm), with greater electrical conductivity in the low-lying areas and small dunes where the species population is degraded. Infiltration of groundwater in degraded areas was slower and there were fewer lateral roots at depths of 0-90 cm in the degenerated areas, compared with the large dunes. The numbers of lateral roots were inversely correlated with soil electrical conductivity, significantly so at depths of 0-30cm and 60-90cm. The numbers of lateral roots were positively correlated with soil saturated hydraulic conductivity at depths of 0-30cm and 30-60cm. In summary, the slow infiltration of groundwater, excessive soil salinity, and the resultant reduction in the distribution of lateral roots and the depth of the groundwater, have all severely restricted the availability of water to H. ammodendron in the degraded area. Hence, a great number of H. ammodendron plants have perished in the affected areas due to water shortage. The renewal of populations is also restricted by excessive soil salinity, so that recruitment of H. ammodendron seedlings is also low. These factors lead to the conclusion that natural regeneration of the affected areas is unlikely.