新疆石河子南山地区表土花粉研究

Abstract

天山作为亚洲大陆最大的山系之一,横贯于新疆的中部,成为分隔南、北疆自然地理区系的山系,它对花粉的传播、保存、搬运与沉积具有重大作用。根据对西北干旱区域新疆石河子南山地区一条沿着海拔高度从2400 m到300 m的样带所采集的23个表土花粉样品的孢粉组合图式和现代植被样方调查资料,探讨了北坡垂直带的植被与表土花粉之间的关系。该区表土孢粉谱可分为4个孢粉组合带,分别对应森林植被带、森林草原植被带、蒿属荒漠带和典型荒漠带。比较特殊的是典型荒漠带被划分为两个亚带,一个亚带是以蒿属、藜科占主要成分的典型荒漠带,另一亚带蒿属、藜科含量较高并含有大量沼泽蕨和芦苇植硅体,兼具典型荒漠和湿地特征。在海拔400 m以上,孢粉组合与现代植被的对应关系较好,带Ⅰ中较高含量的云杉花粉验证了以云杉为主的森林植被带。带Ⅱ中以云杉为主的乔木植物和含量较高的藜科、蒿属和蓼属等草本植物为主的孢粉组合特征与森林-草原植被带的植被特征较为类似。云杉属花粉在海拔低于1350 m的地方即林带下方所占的比例很小,一方面由于距林地的距离较远,另一方面,可能是山风气流对云杉花粉往下搬运的能力较弱所致。带Ⅲ的蒿属花粉含量较高,与该带植被中绢蒿较多有一定的关系,带Ⅳ以藜科为主的花粉组合特征代表了这个植被带的荒漠植被类型。但是在海拔400 m之下,带Ⅳ的亚带Ⅳ₂的高含量的沼泽蕨和芦苇植硅体的孢粉组合在一定程度上还代表了古湿地环境。通过该部分表土花粉组合特征与草滩湖剖面孢粉谱的对比,验证了当地农业种植选址的生态可行性,同时就开垦程度对环境的影响进行了初步探讨。另外,亚带Ⅳ₂的蒿属/藜科(<em>Artemisia</em>/Chenopodiaceae(A/C))比值比亚带Ⅳ₁高,可能与该样点受人为扰动较大有关。;Tianshan Mountains, one of the largest Mountain Ridges in the Asian continent, divide Xinjiang province into two different natural geographical parts: south and north Xinjiang. They display a distinct vertical gradient distribution of vegetation and soil types and play an important role in the dispersal, preservation and deposition of pollen. In order to investigate the relationship between modern vegetation and surface pollen on the northern slopes of the Tianshan Mountains, northern Xinjiang, China, we collected 23 modern pollen samples from Nanshan hill of Shihezi city along an altitudinal gradient from 300 m to 2400 m. In this study, the altitudinal pollen spectra could be subdivided into 4 pollen assemblage zones (forest vegetation, forest-steppe, <em>Artemisia</em> desert and typical desert), and 2 subzones (<em>Artemisia</em>-Chenopodiaceae typical desert and <em>Thelypteris</em>-<em>Phragmites</em>-<em>Artemisia</em>-Chenopodiaceae typical desert), based on modern vegetation data and characteristics of the surface pollen spectra. The results show that surface pollen assemblages at elevations above 400 m provide a good indication of modern vegetation. The most remarkable characteristic of Zone Ⅰ is that it has the highest percentage of spruce and a very high mean vegetation cover of <em>Picea schrenkiana</em>. The pollen samples in Zone Ⅱ are dominated by <em>Picea</em>, Chenopodiaceae,<em> Artemisia</em> and <em>Polygonum</em> and thus represent forest-steppe vegetation. Below 1350 m, pollen percentages of spruce are very low, partly because they are far away from the spruce forest and partly because the wind-driven downhill transport of arboreal pollen to lower elevation is not strong. Higher pollen percentages of <em>Artemisia</em> in Zones Ⅲ are from relatively high vegetation cover of <em>Seriphidiam</em> sp.. The percentages of <em>Artemisia</em> and Chenopodiaceae pollen in the zone are both more than 30%, and thus reflect <em>Artemisia</em> desert vegetation. Mean vegetation covers of desert vegetation are high in Zone Ⅳ, and pollen percentages of Chenopodiaceae are also high. However, surface pollen spectra can not accurately reflect modern vegetation at elevations lower than 400 m. Regional pollen spectra reflect desert vegetation in the Zone Ⅳ₂. But a large proportion of <em>Thelypteris </em>spore and <em>Phragmites</em> fan-shaped phytolith are recorded in the zone, suggesting the local wetland environment. In order to study the surface pollen source areas, we collected surface pollen at different altitude and identified the pollen categories. As was expected, few <em>Thelypteris </em>spore and <em>Phragmites</em> phytolith are indentified at elevations above 400 m, inferring that they were not transported by flowing water and air. When compared with the pollen spectrum of Caotanhu fossil profile, it is able to verify the ecological feasibility of the selection of agricultural sites, and also investigate the influence of cultivation on environment. The <em>Artemisia</em>/Chenopodiaceae (A/C) value is a good indicator of climate change and an index for distinguishing steppe and desert vegetation in semi-arid and arid areas to a certain extent. The value in Zone Ⅳ₂ is higher than in Zone Ⅳ₁. We think that it is not linked to climatic drying but probably reflect human disturbance at Caotanhu. So in some local environments, the value should be used carefully when human disturbance is involved in reconstructing palaeoclimate and palaeoenvironment.