Land Degradation on the South Side of Africa-LADESA 94

本项目的目的是调查撒哈拉以南非洲土地退化的详细物理过程和空间格局，这种现象统称为“荒漠化”，是气候变化和人类活动的综合影响造成的。在西非苏丹-萨赫勒区域沿沿着的两个样带开展了实地工作。主要研究结果如下：（1）在所调查的区域内，最易退化的地貌类型是红土质的胸甲状高原面和丘陵、高原边缘陡坎坡脚的冰川或山麓。（2）土地退化的发展和恢复的格局和过程与当地土壤条件的关系比与区域降水格局的关系更为密切。(3)As只要能在土壤中保存有活力的种子，就可以在雨水充沛的情况下，在冲积平原和沙丘上迅速重新植被。（4）在“古沙丘和沙席”上，由于过度放牧和过度耕作破坏植被覆盖，激活了流沙，造成年降雨量为600- 700毫米的“古沙丘和沙席”零星和选择性退化。(5)It必须强调的是，流沙造成的土地退化不是撒哈拉沙漠向南移动造成的，而是由于人类活动和持续干旱条件的共同影响，曾经固定的沙丘或沙丘被重新移动造成的。(6)In为了回答与土地和植被的不可逆转性和/或复原力有关的问题，在若干固定地区和样带进行10-20年时间尺度的长期观测是必不可少的。

The present project aims at investigating the detailed physical processes and spatial patterns of land degradation in Subsaharan Africa, the phenomenon collectively called by the term 'desertification', due to the combined effect of climate variations and human activities. Field works were carried out along the two transects in the West African Sudano-Sahelian Region. The main results obtained can be summarized as follows :(1)Throughout the areas surveyed, lateritic cuirass-capped plateau surface and glacis or pediments fringing scarp foot of hills and plateaux are the landform types which are most prone to degradation.(2)Both patterns and processes of progress of and recovery from land degradation, are more closely related with local edaphic conditions rather than regional precipitation pattern.(3)As long as viable seeds are preserved in the soils, rapid revegetation can be achieved over alluvial plains and sand dunes in response to the return of good rains.(4)Over the "fossil sand dunes and sheets", which are located to the for south of the desert boundary and receive an annual rainfall of 600-700mm sporadic and selective degradation has caused by shifting sands activated by the destruction of vegetavive cover due to overgrazing and excessive cultivation.(5)It must be emphasized that the land degradation due to moving sands is not caused by the southward march of the Sahara Desert but by the remobilisation of once fixed sand dunes or sheets owing to the coupled effect of human activities and persistent drought conditions.(6)In order to answer the questions related to the irreversibility and/or resilience in land and vegetation, long-term observations at a time-scale of 10-20 years in several fixed areas and transects are indispensable.