黄土高原-青藏高原过渡带小流域立地类型划分及植被优化配置

Site type division and optimal vegetation allocation in small watershed in the Loess Plateau-Tibetan Plateau transition zone

为了探讨黄土高原-青藏高原过渡带上小流域立地分异性及其植被适宜性问题,以青海大通安门滩小流域为例,选取40个典型样地,分别调查其海拔、坡度、坡向、坡位、土壤密度、土壤水分、植被类型等因子,运用聚类分析对小流域内典型样地进行汇总归类划分立地类型,采用主成分分析确定主导因子。研究划分出5种立地类型,并采用立地主导因子分级组合法依次命名为阳平缓坡低海拔立地类型(T1)、阴陡坡低海拔立地类型(T2)、阴斜坡低海拔立地类型(T3)、阴平缓中海拔立地类型(T4)、阴陡坡高海拔立地类型(T5)。得出主成分主导因子为海拔、坡度、坡向。通过对各立地类型的植被现状分析,归纳其存在的生态问题和原因,遵照适地适树原则,提出了植被优化建议:立地类型(T1)中油松、祁连圆柏纯林补植柠条等灌木优化为混交林;立地类型(T2)对青杨华北落叶松混交林实行择伐,补植青海云杉幼苗;立地类型(T2和T3)华北落叶松、白桦纯林适当疏伐后,补植青海云杉幼苗,加速自然演替。立地类型(T3和T4)应适当疏伐青海云杉纯林以扩大林窗。

[Background] The Loess Plateau-Tibet Plateau transition zone is the most vulnerable and fastest-changing region in the world ecological environment. The local land degradation is serious,and the plantations after returning farmland to forest generally have a single stand and an excessive density.How to optimize vegetation within a small watershed,and how to optimize vegetation has become an essential issue. The purpose of this article is to explore the division of site types in the transition zone and the vegetation optimization models under each site type. [Methods] This study focused on the small watershed of Anmen Beach. Forty typical sample plots were selected in the watershed. The slope and slope of each plot were measured using a handheld compass,and the longitude,latitude,and altitude of each plot were determined using handheld GPS. The vegetation types of each plot were recorded. After digging the soil profile in each site,the original soil samples were taken in layers using a ring knife with a volume of 100 cm3. Each sample was repeated three times. The soil bulk density and moisture were obtained by drying. Factors such as altitude,slope,aspect,slope position,soil bulk density,soil moisture,and vegetation type were summarized as indexes for 40 sample plots. Cluster analysis was used to summarize and classify plots in small watersheds to obtain site type classification. Principal component analysis was used to obtain the variance contribution of the main factors and determine the dominant factors. [Results] 1) When the clustering coefficient is 6 and the number of categories is 5,the clustering coefficient curve becomes noticeably slower,and 40 plots are divided into 5 categories: Low altitude sunny and gentle slope site( T1),low altitude shady and steep slope site( T2),low altitude shady slope site( T3),medium altitude shady and gentle slope site( T4),and high altitude shade and steep slope( T5). 2) The first three principal components by the principal component analysis have synthesized 68. 484% of all the information. The three principal component principal factors are altitude,slope,and aspect,and the coefficients are-0. 687,0. 696,and 0. 744,respectively. And we speculate that soil moisture is a potential dominant factor affecting site type division. [Conculsions] In site type( T1),the pure forest of Pinus tabuliformis and Juniperus przewalskii should be optimized as mixed forest by adding Caragana korshinskii and other shrubs. Site type( T2) mixed forest of Populus cathayana and Larix principis-rupprechtii should be selectively cut and Picea crassifolia seedlings should be replanted. In the site types( T2 and T3),the pure forests of L. principis-rupprechtii and Betula platyphylla in North China should be subjected to proper thinning,followed by P. crassifolia replacement to accelerate the natural succession. Site type( T3 and T4) should be in appropriate thinning P. crassifolia pure forest to expand the forest window.