内蒙古高原等地湖泊表层沉积物孢粉与植被覆盖度定量关系研究

Lake central surface sediment-based pollen-vegetation cover transfer functions and its application in Inner Mongolia Plateau and adjacent area

过去植被覆盖度重建在长尺度气候模拟和陆地生态环境演变机制研究等方面具有重要意义。基于现代过程的孢粉—植被覆盖度转换函数研究,可为利用孢粉地层数据重建过去植被覆盖度变化提供一条新的途径。以内蒙古高原等地39个湖泊和7个水库中心的表层沉积物孢粉组合与归一化植被指数(NDVI)为研究对象,利用加权平均偏最小二乘法(WA-PLS)、局部加权加权平均法(LWWA)和最佳类比法(MAT)分别建立了孢粉—NDVI转换函数;通过留一交叉检验法、自助法和空间自相关性等检验,筛选出最优模型并应用于古地层孢粉数据,实现定量重建过去植被覆盖度变化。结果表明:MAT和WA-PLS模型均受到空间自相关性的显著影响,而LWWA模型则是建立孢粉—NDVI转换函数的最优模型。与已有类似转换函数表现能力的对比表明,本研究建立的转换函数具有较强的可靠性和应用潜力。基于本研究转换函数重建的内蒙古辉腾锡勒(HTL)区域全新世植被覆盖变化与孢粉重建的植被变化具有很好的一致性,进一步印证了本研究转换函数的较强潜力,均表现出对降水变化的响应更为显著。

Vegetation cover was an important factor in palaeo-climate simulation.The study of pollen-vegetation cover transfer function based on modern processes provides a way to reconstruct the past vegetation cover change by using pollen stratum data.The Inner Mongolia Plateau and its adjacent area,which is located at the marginal monsoon area of northern China and has a large climatic and vegetal gradient,provides us an ideal area to study the pollen-vegetation cover transfer functions.In this paper,we investigated pollen assemblages of lake central surface sediment from 46 lakes and reservoirs,and developed transfer functions between lake surface sediment pollen assemblage and Normalized Difference Vegetation Index(NDVI) using three different methods which include weighted average partial least squares(WA-PLS),local weighted average method(LWWA) and modern analogue technique(MAT).We used leave-one-out cross-validation,bootstrapping and autocorrelation test to evaluate the performance of those three pollen-NDVI transfer functions and select the most reliable to reconstruct the Holocene vegetation cover history of Huitengxile Plateau of central Inner Mongolia.As results,the redundancy analysis(RDA) was conducted on the pollen data and environmental data according to the linear data type of the pollen data,as indicated by the gradient length(i.e.,2.047) of the detrended correspondence analysis(DCA).The RDA plot displays that the NDVI points to the positive of the first axis where forest and forest steppe sites clustered,and has smaller angles with the Pinus and Betula and larger angles with the desert pollen type such as Nitraria,Ephedra and Tamaricaeae,which suggested that the NDVI index was closely related to the spatial distribution of pollen and thus the pollen-NDVI transfer functions could be reliably developed.The leave-one-out cross-validation and the bootstrapping test showed that the LWWA_Inv(R^2jack=0.57,R^2boot=0.57) and WA-PLS-3(R^2jack=0.56,R^2boot=0.54) models have better performance than the MAT(R^2jack=0.53,R^2boot=0.58) model.The autocorrelation test suggests that both MAT model and WA-PLS model are greatly affected by spatial autocorrelation,and thus not suitable to develop pollen-NDVI transfer function.Therefore,the LWWA Inv model is suitable to reconstruct the past vegetation cover history(NDVI) for its overall good performance,and was used to reconstructed the NDVI history of Huitengxile Plateau of Inner Mongolia by applying to a fossil pollen record of a lacustrine sections from a unnamed small crater lake.The comparison shows there are broad consistence between pollen reconstructed NDVI and other pollen-based vegetation(i.e.,biome score in this paper),which further demonstrated the reliability of our pollen-NDVI transfer functions.Furthermore,we compared those regional vegetation evolution with those non-pollen based Holocene temperature and precipitation sequences.The result shows that the pollen-based vegetation sequences were correlated well with the non-pollen based precipitations but not correlated with regional temperature sequences,which possibly indicated the Holocene vegetation change was closely responded to precipitation but temperature in marginal monsoon area of northern China.