全新世东北林草交错带植被变迁的植硅体证据

Northeast forest-steppe ecotone is an ideal study area for global change. Studying the change of the ecotone borderline in Holocene is of great significances, which will enrich the outcomes of paleovegetation reconstruction. Also, it will provide basic suggestions for the ecological protection. In this project, more than 100 kinds of typical plants in this region are collected. And then the phytoliths are extracted and identified under the microscope. Using morphometry and image analysis，we target to get the shape factors of the woody and herbaceous phytoliths. Then we build a discriminant function between the woody and herbaceous phytoliths based on shape factors. In addition, the woody and herbaceous phytoliths in top soil are recognized with the discriminant function. Next their quantity is counted, and corrected by the biomass. After this，the range of some character ratio of the top soil phytoliths assembles under different kinds of vegetation(such as the ratio between the woody and herbaceous phytoliths, the percentages of herbaceous phytoliths, etc.) is determined with mathematical methods. Furthermore, based on character values, we will set up an empirical function to identify different regional vegetation types. After all, we would like to apply the character values and this empirical function to the phytolith assembles extracted from the profile in Holocene. And this will not only reconstruct the evolution history of the tree-herb borderline, but also help us to understand the change of the vegetation pattern since Holocene in detail. At the same time, the period analysis and spectral analysis technology are adopted to probe into the period and amplitude of the paleovegetation transition in Holocene. These results will also provide a new idea for improving the accuracy of the regional paleovegetation reconstruction.

东北林草交错带是开展全球变化研究的理想区域，对其变迁历史的研究无论在区域古植被格局重建的理论方面还是关于林草交错带生态保护等应用领域，均具有重要意义。本项目以植硅体分析为主要研究手段，拟对东北区100余种典型植物植硅体进行鉴定，结合形态测量和图像分析方法获取木本和草本植硅体的形状系数，建立木本和草本植硅体的判别函数。通过判别函数对表土植硅体组合中草本、木本植硅体的识别并对其进行群落生物量校正的基础上，利用数理统计方法确定该区现代表土特征植硅体比值（如木本植硅体与草本植硅体的浓度比、草本植硅体的百分含量等）的值域。建立适合该区植被型判别的经验函数，进而将交错带表土特征植硅体比值和该经验函数应用于选定的全新世地层剖面中重建林草交错带变迁过程，恢复古植被格局。同时，利用周期分析、频谱分析等方法探讨该区全新世植被变迁的周期和幅度，从而为提高区域古植被重建的精度，提供新思路。

东北地区林草交错带地处森林-草原过渡地带，是开展古气候研究的理想区域。重建其变迁历史对区域古植被格局的重塑和林草交错带生态保护，均很重要。项目组依据项目实施方案， 在东北地区共布设了196个采样点，采集植物单种131种，调查和采集群落样品392个，同时，采集表土样品607个，钻取沉积剖面13个，土壤剖面110个。对野外采集的植物、表土和沉积物样品进行了植硅体分析，获取植硅体数据6133个。对90个样点的表土及土壤剖面表层进行了土壤理化性质的测试，获得数据947个。对沉积剖面选择49个样品进行AMS14C年代测定，对沉积剖面进行了孢粉分析、粒度分析、有机质分析、腐殖化度分析和同位素分析，分别获得相关数据765、691、1006、229和54个。.项目组使用多种数理统计分析手段，对实验结果进行深入分析讨论，取得了丰富的理论成果。首先，项目组研究了东北不同生物气候带典型群落的草本和木本植物植硅体的形态类型，定量刻化了它们的形态差异，建立了识别标准，并将其应用于表土中，判别表土中植硅体来源。其次，项目组在对表土植硅体的迁移规律以及代表性的探讨的基础上，提出了识别典型植物群落的表土植硅体指数S/L、Ro/La和W/G并进行了校正，S/L指数可用于判别草地（S/L>1.65）和森林（S/L<1.65）；Ro/La指数可判别森林区的林下草地（Ro/La>0.89）和处于过渡带的非林下草地（Ro/La<0.23）；利用W/G指数可定量区分东北地区的植被状况，草原区表土中植硅体的W/G数值介于0-0.3，森林区W/G值普遍高于0.6，林草交错带的W/G值则位于0.3-0.6之间。最后，项目组对泥炭和沙丘-古土壤剖面进行了植硅体、孢粉和粒度分析，从沉积环境、生物因素等多个角度来还原了林草交错带的变迁历史：在10.0 cal. ka B.P.林草交错带的边界位于东北地区的最西，随后逐渐向东部退缩并在6.0 cal. ka B.P.退缩至全新世期间最东的位置，其东西摆动最大幅度约200km，然后向西偏移，并在2.0 cal. ka B.P.最西的位置，其界限与10.0 cal. ka B.P.时林草交错带的界限比较接近。全新世期间，林草交错带存在约210年、550年、750年、1000年和1800年的变化周期。