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Spatial pattern of seedling regeneration of Ulmus pumila woodland in the Otindag Sandland

Ulmus pumila woodland widely distributing in the Otindag sandland,is a kind of sand vegetation types and adapted to semi-arid and semi-humid climate.It is a important local species to protect the ecosystem from desertification and defense the sandstorms.However,the seedlings and small trees are found to be few in natural ecosystem because of overgrazing and felling of great trees,which has seriously affected the distribution of U.pumila and regeneration of population.The aims of the study are to investigate the patterns for seedlings regeneration and the relationship between seedlings density and mature trees in two different habitats of fenced sites.With Ripley′s K function statistical method,the seedlings distribution and regeneration spatial pattern of Ulmus pumila were analyzed in fenced fixed sand dunes and lowland between dunes in the Otindag sandland.The results showed that the vegetation was regenerated very well in U.pumila sparse forest after exempt from the damage of animals and human being by fence.The coverage of shrubs and grasses is getting to about 60% of land area and the density of seedlings in the two different habitats was 88 seedlings per hectare and 77 seedlings per hectare.The seedlings with the same ages have a better growth conditions in lowland between dunes than those in fixed sand dunes.So the lowland between dunes is thought to be more suitable for seedlings development than the fixed sand dunes.The patterns for distributions of Ulmus pumila seedlings were aggregated distributions in two habitats in the area with radius being 0 to 50 m,but the aggregation intensity of seedlings was higher in lowland between dunes than the fixed sand dunes.The distribution patterns were affected by seed shadow,aggregation of seedlings,arid environment and sand blown by the wind.The patterns for distribution of mature trees were aggregated distributions in radius being 0 to 30 m,and random distribution with radius 30 m.The soil water content and nutrients were poorer in fixed sand dunes than in lowland between dunes.The mature trees in fixed sand dunes have a large canopy and a compete advantage to absorb soil water and nutrients from soil,which restricts the seedlings to establish and develop under the mature trees.Therefore,there was a negative correlation between seedlings density and mature trees population in a radius being 0 to 2 m on fixed sand dunes.By comparison,the soil water content and nutrients availability were higher in lowland between dunes than that of fixed sand dunes,and the mature trees have lower crown density and less inhibition to seedlings.So a positively correlation was recorded in similar scale in lowland between dunes.But there was no correlation between seedlings density and mature trees in a larger scale in two different habitats.The establishment and develop of seedlings were independent of the mature trees in both fixed sand dunes and lowland between dunes.The results suggested that U.pumila seedlings displayed various adaptive mechanisms in the different habitats.The study revealed the seedlings distribution and regeneration spatial patterns and provided important scientific evidences for regeneration of Ulmus pumila sparsed forest in practice.