| Pinus tabulaeformis is a unique tree species in China.It has stronger resistance for drought and cold,as well as its widely adaptability,Pinus tabulaeformis have been one of the main afforestation species in Huabei mountain region.Four different stand densities including 900,1260,1460 and 1660 trees·hm-2plantation with 60 years sited in Beijing Badaling forest farm were selected,the difference of stand structure,undergrowth species diversity,the variation of physical,chemical and biological properties and hydrological effects of soil physical,chemical and biological properties for litter and soil layer were compared,the density effect of Pinus tabulaeformis plantation was comprehensively evaluated based on the plant-litter-soil complex using the principal component analysis,and the reasonable operating density of Pinus tabulaeformis plantation was obtained.The results could provide scientific basis for knowing the circle process of organic carbon,nitrogen and phosphorus under density adjusting measurement.On the other hand,the results could provide practice basis for the management of Pinus tabulaeformis plantation in Beijing and even in north China.The results were as follows.(1)Stand density significantly influenced the diversity of undergrowth vegetation.With the increasing of the stand density of Pinus tabulaeformis plantation,the competition index among the stand individuals was increased,and a right-skewed division was showed,the neighborhood comparison of forest stand with the density of 1260 trees·hm-2was the largest,and was greater than 0.5000.The average diameter at breast height(DBH),trees height and trees crown of with the density of 900 trees·hm-2was the largest,and the stock volume was the smallest at 900 trees·hm-2.The dominant species at the shrub and grass horizon under the different stand densities were similar,whereas the different species were presented.The richness index of shrub layer decreased with the increase of stand density,and the diversity index at the stand with 900 trees·hm-2was the highest.The richness index of grass increased at first and then decreased with the increase of stand density.The biomass of shrub layer and herbaceous layer decreased with the increase of stand density,and the highest biomass both of them were presented at the density with 900 trees·hm-2,and were obviously higher than that at the other stand density.Principal component analysis found that the stand density of 1260 trees·hm-2promoted the increase of species diversity of understory vegetation.(2)Different stand densities significantly affected the physical,chemical and biological characteristics of litters.The thickness and accumulation of litters increased firstly and then decreased with the increase of stand density,and the density with highest value were 33.37 mm and 25.74 t·hm-2was 1260 trees·hm-2.The contents of NO3–-N and total nitrogen of litters were the highest when the density was 1260 trees·hm-2,whereas the contents of water-soluble organic carbon and p H were the highest when the density was1460 trees·hm-2,and these were significantly higher than other stands.The content of gram-negative bacteria and bacteria was the highest when the density was 1260 trees·hm-2,while the content of arbuscular mycorrhizal fungi was the highest when the density was1460 trees·hm-2,they were obviously higher than other stands.The content of gram-positive bacteria and actinomycetes when the density was 1260 trees·hm-2were lower than that when then density was 1660 trees·hm-2and 900 trees·hm-2.For the enzyme activities of litter,the activity of?-glucosidase,leucine aminopeptidase,chitinase,acid phosphatase,phenol oxidase and dehydrogenase of the forest stand with 1260 teees·hm-2significantly increased by 31.68%,39.42%,9.00%,24.24%,8.67%and 27.20%,respectively when compared with the density 900 trees·hm-2.Principal component analysis showed that the characteristics of litters in Pinus tabulaeformis plantation forest were best in 1260 plant·hm-2.In short,it was found that the water holding capacity,the diversity of microbial communities'structures and enzyme activities were the highest when the stand density was 1260 trees·hm-2.(3)Stand density significantly influenced the change of soil chemical and biological properties.The soil p H value and ammonium nitrogen content were the highest when the stand density was 1260 trees·hm-2,and the contents of soil water-soluble organic carbon and nitrate nitrogen of the forest stand with the density of 900 and 1660 trees·hm-2were higher than that of other stands.The contents of total phospholipid fatty acids and arbuscular mycorrhizal fungi at the density of 1260 trees·hm-2were significantly higher than other stands.However,the contents of bacteria and fungi at the density of 1460trees·hm-2were significantly lower than that of other stands.Except for soil phenol oxidase,the activities of soil enzymes were increased firstly and then were decreased with the increase of stand,the enzyme activities were highest when the density was 1260 trees·hm-2,and the lowest when the density was 900 trees·hm-2.Compared with the density at 900trees·hm-2,soil enzyme activities of?-glucosidase,leucine aminopeptidase,chitinase,acid phosphatase and dehydrogenase at the density of 1260 trees·hm-2were significantly increased by 48.67%,28.10%,58.28%,33.87%and 37.50%,respectively.Principal component analysis showed that the soil enzyme activity and the diversity of microbial community structure were the highest when the stand density was 1260 trees·hm-2.(4)Through principal component analysis,the results of the synergistic density effect of integrated plant-litter-soil showed that the comprehensive evaluation index of stand density was as follows:1260>1460>1660?900 trees·hm-2.Among different stand densities,the comprehensive evaluation index of 1260 trees·hm-2was the highest,while that of 1660 plants and 900 plants was lower.To sum up,our study found that the vegetation species diversity of four plantations of Pinus tabulaeformis with different stand densities decreased as the density increased,and the litters'thickness and water-holding performance increased first and then decreased as the stand density increased,with the maximum value of 1260 trees·hm-2.The microbial community structure diversity and enzyme activity of litters and soil increased first and then decreased with the increase of stand density of Pinus tabulaeformis plantation.Based on litter layer of microbial community structure and enzyme activity of organic matter decomposition and nutrient cycling,and comprehensive plant-litter-soil layer characteristics of short-term density effect study of 60 years Pinus tabulaeformis artificial forest in Beijing badaling forest farm,1260 trees·hm-2is reasonable stand density. |
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