喜马拉雅山脉中段南坡孢粉垂直分布特征及影响因素

研究现代孢粉沿海拔梯度分布、传播特征及其驱动因素, 有助于提高基于孢粉重建古植被和古气候的可靠性。喜马拉雅山是全球中纬地区落差最大的山脉之一, 其中段南坡植被垂直带谱完整, 目前尚缺乏该垂直带谱孢粉现代过程研究。本研究通过喜马拉雅山脉中段南坡不同海拔26个表土样品(海拔梯度为3551~4444m)的孢粉分析并整合前人表土孢粉数据(n=138, 海拔梯度涵盖132~5000m), 系统分析了孢粉(包括组成、百分比含量、浓度及多样性)的垂直分异规律及其与气候、土地利用之间的关系。研究结果显示孢粉组合垂直分布特征能很好地代表植被垂直分布特征, 年降水和暖季气温是影响孢粉分布的重要气候要素。孢粉丰富度沿海拔梯度无明显分异特征; 而孢粉均匀度随海拔升高而显著降低(特别是海拔3500m以上区域), 能够代表植被群落随海拔的变化特征。常见乔木植物花粉(松属、云杉属、桦木属和栎属等)存在明显的向高海拔传播现象, 且不同类型间传播能力差异较大; 而柏科花粉传播能力差, 能很好地指示局地植被。表土样品孢粉浓度随海拔升高呈现降低趋势, 且高海拔地区较低的孢粉浓度一定程度上放大了由低海拔传播而来的乔木植物花粉对孢粉组合的影响。研究结果提示我们应注意乔木植物花粉传播能力差异对地层孢粉谱解释的可能影响。

Vertical distribution of modern pollen assemblages and potential driving factors on the southern slope of the middle Himalayas

Studies on the dispersal characteristics of modern pollen and their potential driving factors can improve the reliability of pollen-based past vegetation and climate reconstructions. The Himalayas Mts. has the largest elevation range in the mid-latitude region with complete vertical vegetation zones on its southern slope, hitherto no study on modern pollen processes has been carried out for the vertical vegetation zones completely. In this study, 26 soil-surface samples were collected from different elevations (3551~4444ma.s.l; 26.71°~29.28°N and 88.00°~90.75°E) and utilized in pollen analysis, and a modern pollen dataset was established for the southern slope of the middle Himalayas by combining our pollen data with previous soil-surface pollen data(in total 138 samples, 132~5000m a. s. l.), to investigate the vertical pattern of modern pollen assemblages(including pollen compositions, percentage abundances, concentration, and diversity, etc.)and their relationships with vegetation, climate, and human activities. Results show that the vertical distribution patterns of pollen assemblages can represent the vertical distribution characteristics of the vegetation zones well, both the annual precipitation and temperature in the warm season are important climatic determinants of pollen distribution. The richness of pollen taxa has no obvious variation along the elevation gradient, however, their evenness decreases significantly when elevation increases(particularly in the areas above 3500m a. s. l.), which represents the different vegetation communities well. Some arboreal pollen taxa(such as Pinus, Picea, Betula, and Quercus)can be transported to higher elevations by wind, and their transportation ability differs markedly; while Cupressaceae pollen has poor transportation ability and indicates the local occurrence of the plant well. Pollen concentration decreases following the increasing elevation generally, and the low pollen concentration in the high-elevation area might amplify the effect of arboreal pollen grains transported from low-elevation areas on pollen assemblage. The difference in transportation ability of arboreal pollen should be paid more attention when explaining fossil pollen data.