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formed between extensive cultivation and poor production since ancient times. In
order to survive from food shortage, local farmers tended to enlarge planting area
using the method of setting on fire and burning mountains, i.e. so-called “slash and
burn cultivation”. At the initial few years following reclamation cultivation, a huge
amount of topsoil was lost, and soil water and fertility conditions were increasingly
worsened, which further resulted in the decline of crop yield. Facing this dilemma,
local farmers tended to abandon the low-yield farmland, and choose another uncul-
tivated land for clearing and farming. This vicious circle was going round and fur-
ther accelerated the shrink and degradation of vegetation cover.
Under weak population pressure before, self-rehabilitating capacity of ecosys-
tem could to some extent compensate the decline of soil quality caused by continu-
ous cropping, and maintain the sustainability of regional ecosystem. With the
increase in population pressure, the sustainability of ecosystem would decline grad-
ually till the collapse of ecosystem at the threshold of pressure. For more food pro-
duction, farmers chose to reclaim the lands which was previously used for natural
restoration. The newly reclaimed lands were used for grazing and cutting down
trees excessively, resulting in continuous deterioration of natural vegetation and
serious soil erosion. With further degradation of soil system, raw black loam in
topsoil layer, accompanying with native vegetation, was quickly lost (Guo and Li
2014 ). Instead, the loess soil was remained at topsoil layer in a large area, with a
very low content of organic matter up to only about 1 % (Li et al. 1998 , 2003 ). In
this case, natural ecosystem suffered much from intensive grazing activities, bring-
ing about a vicious cycle between low agricultural productivity and land degrada-
tion. Therefore, extensive reclamation and decentralized management are major
social-economic characteristics in the society of small household farmer in the his-
torical period, and tended to become the major reasons to cause the degradation of
agricultural ecological environment (Zhang 1989 ). Over 60 years since the founda-
tion of new China, the government enforced several mega projects to focus on soil
and water conservation and agricultural development in the region, which produced
positive impacts on agricultural production, soil erosion, land degradation, and veg-
etation restoration. However, owing to the rapid increase in regional population and
adverse conditions, the shortage of food production could not be fundamentally
resolved over a long period. Vegetation coverage and soil quality were still extremely
deteriorated, which was difficult to reverse fundamentally. The protection of biodi-
versity resources and its cushioning effects on agricultural ecosystem were not
exhibited. At the beginning of this century, the situation of ecological environment
in this region was shown as “more damages than protection, partial improvement
but overall deterioration”(Yang et al. 2001 ).
From the perspective of ecosystem, the problems above mentioned were origi-
nated from strongly weakening assimilation and increasingly activating catabolism
within the system. Those factors including expanding reclamation area under the
condition with destructing vegetation, low agricultural productivity, serious soil
erosion by water and wind, and continuously declining soil fertility, have led to
more active catabolism in the system. The signals of weakened assimilation are the
decline of vegetation cover and the low productivity of agricultural ecosystem and
F.-M. Li et al.