| Sub-alpine natural forests are distributed in the region of Hengduan Mountains, which are the transition from north-west to south-east from Qinghai-Tibet Plateau to Western Sichuan basin. They are distributed the origin part of the upper reaches of Yangtse River, and have great and important ecological functions such as biodiversity conservation, water holding and soil erosion controlling. But due to long-term disturbance of irrational human activities, especially the long-term and extensive lumbering, sub-alpine natural forests have degraded badly, and seriously ecological effects have arose. It was eager to take effective measures to accelerate the speed and quality of restoration, to enhance the restoration of health, stability, biodiversity of the degraded sub-alpine forests, to gradually increase the ecological effect, to ensure regional ecological security and sustainable development.Using the method of "time by space", spruce (Picea asperata) plantations of 6 different age stages about 10 yr (year), 20 yr, 30yr, 40 yr, 50 yr and 70 yr, naturally restoration series of 4 stages including secondary shrubs about 10yrs, secondary red birch (Betula albo-sinensis) forests about 30yrs, secondary mixed fir {Abies faxoniana)-red birch (Betula albo-sinensis) forests about 40yrs and natural old fir (Abies faxoniana) forests older than 180yr, were chosen to set fixed plots (arbor plots 20X20 m2, shrub plot 10X10 m2 ) by GPS to study the restoration process, including plant species diversity, community structure and soil nitrogen. And other typical types of vegetation and land-uses including oak (Quercus aqufolioides) forest (20 X 20 m2 ), sup-alpine meadow (10X10 m2 ), cultivated land and forest landconvened from cultivated land were also chosen for study. The altitude, aspect, slope, slope position, and the status of circumstance of every plot were recorded. After community investigation separately for arbor layer, shrub layer and herb layer, soil samples were get by 6 layers (0-10, 10-20, 20-30, 30-40, 40-60 and 60-80 cm) in 3-5 profiles of every plots to analysis the soil water conditions, soil nitrogen contend (nitrate nitrogen (use fresh soil), available nitrogen and total nitrogen). The main results are as follows:The plant species diversity of plantations fluctuated from 10 yr to 70 yr, during which the highest plant species diversity appeared at 40 yr plantations, secondly appeared at 10 yr plantations, and the lowest appeared at 50yr plantations. During the naturally restoration series, red birch forests had the highest plant species diversity, mixed fir-red birch forests had the lowest. Naturally restored forests had higher plant species diversity than the plantations of same age.From 20 yr to 30 yr, was the period that the community structure of plantations changed most intensively. The average tree height, DBH increased the most quickly. The differentiation of inner structure of plantations after the canopy completely closure at about 30 yr, reflected the dynamics of the interact of self-thinning and species inbreaking of outer communities. Average tree height, DBH, coverage, height range with live branch of natural old fir forest were higher than all aged plantations and all stages of secondary forests. The dynamics of the structure of height classes and DBH classes of plantations from 10 yr to 70 yr revealed the process of differentiation and dwindling during the increasing of height and DBH; but that of either red birch forest or mixed fir-red birch forest was an increasing one (like the shape of inverse "J"); and that of natural old fir forest reflected the characteristics of stable, big height, large size of mature forest. Community coverage of plantations decreased from 10 yr to 70 yr. The moss coverage of canopy closed plantations were very low, but that of secondary forests were relatively high, and that of natural old fir forest was very high, and the layer depth was more than 10 cm thick.The change of the volume content of soil NCV-N, and the mass and volume content of soil available nitrogen, total nitrogen among the different age stages of plantations were significant (p<0.05) or very significant (p<0.01). The content of the soil NCV-N was increased slightly from lOyr to 20 yr, and decreased to the lowest point to 40yr, and then increase to 70yr. The change of the content of soil available nitrogen, total nitrogen was the inverse of that of the soil NCV-N. The mass content of available nitrogen and the mass and volume content of total nitrogen among the different stages of natural restoration were significant or very significant. The content of soil nitrogen of natural series decreased from 1 Or to 40 yr, due to the slowly release of the nitrogen from the remainder after clear-cut But the contend of soil NCV-N of plantations of 20yr and 30yr was high due to not sufficient use the nitrogen released from the remainder after clear-cut maybe because of lowly biodiversity. The content of available nitrogen were significantly less than naturally restored forests. So the phenomenon of possible nitrogen loss and not enough available nitrogen might make the degradation of the soil in the plantations. The mass and volume contents of soil NCV-N, available nitrogen and total nitrogen of oak forest were almost the lowest among the soil of all vegetation types. Sub-alpine meadow had relatively lower content of soil NCV-N, but relative higher available nitrogen and the highest total nitrogen. So sub-alpine meadow had the largest bank of nitrogen in sub-alpine ecosystem. Comparing to other types forest in China, thesub-alpine forests had higher nitrogen bank due to relatively higher content of total nitrogen, and the content of soil available nitrogen was also relatively high. However, the content of NO3-N was relatively low, maybe due to low temperature, lower soil PH and relatively higher precipitation. The factors that affecting the soil nitrogen content need more research.The basic idea on restoration and reconstruction of degraded forest was put foreword that by conforming to the law of natural succession, imitating the composition and construction, making the best use of the power of spontaneous succession, we can take rational measures timely on the key species and some important stages of the succession process to turn the degrading trend, span or shorten some stages of succession to accelerate the process of restoration the composition, construction of the natural forest and rehabilitation the functions, especially. It is mainly by using the spontaneous power of succession to restoration and reconstruction of degraded natural forest and the best way is "closed mountains for forest conservation" scientifically. The concept of "closed mountains for forest conservation" was expanded that protection by close mountains was only the precondition and basis for degraded natural forest restoration and reconstruction, the "conservation" —assistant by manmade measures was more important; not only by the natural spontaneous succession power, but also it was most important by analyzing the history and status of degradation, considering the object for restoration, the extend of degradation, stage of succession, ability of natural regeneration, characteristics of spatial distribution, landscape heterogeneity, etc., and then taking different assistant measures to accelerate the process of restoration, to restore the composition, structure, and ecological functions of the degraded natural forest as soon as possible.After analyzing the history and status and disturbance regime of the sub-alpine degraded forest, the standard, indices, and class system for the technical system of "closing mountains for forest restoration and reconstruction" were established; and four ecological strategies were put forward as following: a) for the old growth & primitive natural forest and light garaged natural forest, must take strictly protect for conservation of gene bank and biodiversity; b) for badly degraded site measures must take to restore the habitat and reconstruct population; c) for moderate degraded secondary natural forest measures must take to restore the structure and control the process of restoration; d) for low effect in ecological function of pure plantations, must adjust the structure and manage the succession direction. It is needed to restoration and reconstruction the degraded natural forest at landscape scale.
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