陕北黄土丘陵区撂荒群落排序及演替

根据2003和2005年分别对36和34块撂荒群落及其生境的调查结果,分析了撂荒群落的排序演替规律、演替速度及演替趋同或趋异性。结果表明：（1）对陕北黄土丘陵区撂荒群落影响较大的因子主要有撂荒年限、海拔、土壤水分含量及变异量、有机质、全氮和速效氮、速效磷等,而坡度、坡向等对撂荒群落分布及动态影响较小;（2）排序图也反映了撂荒演替过程中主要环境因子的变化,即随着撂荒年限的增加,土壤水分含量减少,变异量增加。土壤养分方面,除速效磷呈减少趋势外,土壤有机质,全N、P、K,速效N、K都有增加的趋势;（3）农田撂荒后,群落演替速度有先降低后增大的趋势。提出一个关于撂荒演替速度的分阶段循环往复式的理论模型,认为植被演替从总体上讲是逐渐变慢的,可根据生长、生活型和优势植物种等分为几个阶段,每个阶段初期演替速度较快,以后逐渐变慢达到阶段稳定状态。之后,随着下一阶段主要植物的入侵,阶段稳定状态被打破,演替进入下一阶段。（4）演替趋同、趋异性方面,两年的结果不一致,与样地选择有关。

The ordination of abandoned old-field communities and secondary succession rate, successional divergence or convergence in the Loess Hilly Region of Northern Shaanxi Province

In 2003 and 2005, 36 and 34 abandoned old-fields communities and its factors of habitat were investigated. After that, ordination of communities was conducted; succession rate and successional divergence and convergence were analyzed. The results show that: (1) Factors of abandoned age, altitude, soil water content and its fluctuation, soil organic matter, total and available nitrogen, available phosphorus, etc., show greater impact on old-field communities in loess hilly region than slope degree and exposure. (2) The changes of soil water and nutrients were also reflected in the ordination plot, i.e., as succession go on, soil water and available phosphorus tend to decrease, while soil organic matter, total nitrogen, phosphorus and potassium, available nitrogen and potassium have the increasing tendency. (3) Initially, the secondary succession rate show a falling tendency, lately it rebound at the bottom and began to increase. Based on the results, we put forward a multi-stages cyclic theoretical model of secondary succession rate: succession is carried by stages, in whole stage, the succession rate show slower and slower tendency, the whole succession stage can be divided into several stages based on the growth form, life form or the dominated species. In the divided stages, the previous period have a quick succession rate, lately it become slow and reach a stable state. When species of the next stage begin to colonize excessively, the stable state was broken and the next succession stage starts. (4) About the succesional divergence or convergence, there is discordance deduced from the results of 2003 and 2005, it need further research.