The construction of stone terraces to minimise soil erosion is common throughout Guizhou. This technique, however, has high inherent risk. Stone terraces are usually much higher than those with earth risers. While they trap a greater thickness of soil on the slope they increase the risk of slope failures, reduce moisture and nutrient availability to plants, and thin more soil up slope. The stone risers also threaten long-term productivity. When a riser collapses debris is deposited over the terrace below making farming difficult. These breaches in the terrace focus surface runoff leading to gully formation and increased sediment transport down slope. Artificial drainage systems, often used in conjunction with terracing, compound the risk. These channels prevent precipitation from soaking into the soil. This limits groundwater and soil moisture recharge which reduces the availability of water for crops and the length of the growing season. The rapid drainage of water from the slope reduces the time of concentration of the catchment resulting in an increase in flood activity. Floods are caused by smaller rainstorm events. They arrive faster and peak quicker and higher than before the channels were constructed. Engineering solutions to soil erosion must therefore be used in conjunction with, and are not as a substitute for, good land management strategies. Furthermore, despite changes in land use practices, and the application of new technologies, there is a maximum amount of production that can be obtained from this land on a sustainable basis.
The dry-hot valleys (DHV) are located mainly in the deeply incised v alleys along the upper streams of several international and domestically rivers, like Yangtz, Zhu, Lanchang, Hong, and Nu rivers. This paper briefly described t he reasons of formation of DHV from view of climate and geographical conditions, and by referring to great deal of documents, analyzed the historical case and p resent status of the vegetations in DHV. The environment in DHV is facing the se rious vulnerable period in the history due to its nature situation of half-year dry period, fragile geological structure and shallow soil, and its social situat ion of over dense population and over farming. The primary vegetation is broad l eaf forest and it was denuded in the history. The current local vegetation is th e degraded secondary vegetation: savanna and succulent thorny shrub. Since the e nvironmental situation in valley influenced directly the water body of river, th e soil erosion control and re-vegetation in DHV is the most urgent task in the p rocess of environmental harness along the rivers. Quite a few pilot research pro jects have been carried out.on demonstrating new silviculture techniques for re- vegetation in DHV, but there still exist great difficulties in carrying out larg e-scale afforestation engineering.
