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