The Study On Dynamice Succession Of Community In Degraded Steppe Of Leymus Chinensis After Different Omprovement Measures

Objectives:Grazing-induced grassland degradation in Inner Mongolia is the primary impediment to the sustainable socioeconomic development of the region. Understanding the natural succession processes of the degraded grasslands is necessary for managing these prestigious natural resources.Practical techniques are urgently needed to add or accelerate the restoration of the degraded grasslands. Effects of two mechanical intervention techniques,shallow plowing and harrowing,in facilitating restoration of a degraded Leymus chinensis grassland were assessed,in comparison with grassland natural recovery,based on a long-term motoring research.Methods:The research was conducted in the Inner Mongolia Grassland Ecosystem Research Station of the Chinese Academy of Sciences,which is located in the Xilingol League of Inner Mongolia(43°20′N-44°00′N,116°06′E-117°05′E).The degraded L chinensis grassland was fenced in 1983(foe excluding grazing animals), when grassland community was dominated by Artemisia frigida,Cleistogenes squarrosa and Agropyron michnoi,etc.Shallow plowing and harrowing were applied on the grassland in designated areas in the same year.The plant species composition and biomass production of these treated grasslands and that in natural recovery were monitored following the treatments at plant growing season.The integrative analysis of the long-term data set resulted in following main findings:Results:1.Shallow plowing." The grasses were dominant in the whole restoration period, having higher herbage production and population density.Rhizomateous L.chinensis had extremely high biomass,especially in the early stage.Grassland production was not significantly related with rainfall in the season,but related with restoration years. This indicated that the changes in plant community structures over the period were related with restoration following the treatment.The species diversity(DI) and evenness indices of plant community(in terms of species biomass percentage) decreased first then increased during the observation period.DI had following relation with restoration years(x):DI=0.0004x~2 +0.0113x+0.228;r=0.7777.Four restoration stages can be distinguished over the restoration period in grassland community succession:(1) L.chinensis + annual or biennial forbs community(1983-85);(2) L.chinensis predominant community(1986-91);(3) L. chinensis + Agropyron michnoi + perennial forbs community(1992-96);and(4) L. chinensis + bunchgrasses + perennial forbs community(1997-07),which were very similar to the climax community in the region.2.Harrowing:Grasses restored following the harrowing,gradually becoming dominant in plant communities.Rhizomateous L.chinensis was facilitated by the treatment,which increased quickly at the early stage,but decreased later due to the increase of other species.Semi-shrub and bunchgrasses were inhibited by the treatment at the early stage.Species diversity(DI) and evenness indices also decreased in the early stage,but increased later.DI had following relation with restoration years(x):DI= 0.001x~2-0.042x + 0.896;r = 0.7169 The four stages following the treatment are(1) L.chinensis + annual and biennual forbs + Artemisia frigida community(1983-86);(2) L.chinensis + short bunchgrasses + forbs community(1987-96);(3) L.chinensis + Agropyron michnoi + bunchgrasses + forbs community(1997-2000);and(4) L.chinensis + Stipa grandis + short bunchgrasses community,in which L.chinensis had the same important value as it was in climax community,and community structure was very similar to natural climax community.3.Natural recovery:The change in plant community was gradual.The biomass and population density of grasses increased while that of Artemisia frigida (semi-shrub) deceased in the early recovery stage,but the composition of species biomass maintained a relatively stable proportion in the later stage.The recovery of grassland biomass was slower than that of the shallow-plowed and harrowed grasslands.The species diversity and evenness indices had no significant change trend during the period.The four recovery stages were(1) Artemisia frigida + short bunchgrasses + L.chinensis community(1983-85);(2) Transitional stage from A. frigid to L chinensis dominant communities(1986-88);(3) L.chinensis + Agropyron michnoi + short bunchgrasses community(1989-91);and(4) L.chinensis + Stipa grandis + short bunchgrasses communities(1991-2007).4.Environmental changes:Soil physical and chemical properties improved during the restoration for all the treatments,but the differences among the treatments last for the whole period.Soil bulk density of the plowed,harrowed grassland and in that of natural recovery in the exclosure(animal grazing excluded) were 22.3%,21.6%and 11.5%lower than that of public open grazing grassland at the end of observation period.Soil moisture content increased by 93.71%,91.36%and 86.39%respectively when compared with the soil moisture in grazed grassland.The soil bacteria number in soil layer L1 also increased by 91.56%,89.66%and 73.56%,and Actinomyces increased by 41.72%,38.89%and 22.63%,in comparison with that in the soil of grazed grassland.These results indicated that shallow plowing and harrowing benefited soil micro-organisms.5.Natural recovery was superior to the two mechanical interventions for restoring the widely distributed A.frigida grassland,derived from L chinensis or other native grasslands.Human intervention might be justifiable for the severely-degraded or desertified grassland beyond the threshold of natural regeneration.