| Good and continuous natural regeneration is one of the basis for ensuring the sustainable development of a forest.In order to study the optimal canopy conditions which is most suitable for the growth of saplings(h?30cm and DBH<5cm),and the most reasonable number and the composition of saplings,based on the data obtained from 61 10 mx 10 m square plots in region I,two 100 m×100 m large sample plots and a 50 m×50 m plots,at Jingouling Experimental Forest Farm,Wangqing Forestry Buresu,Jilin Province,China,the number and composition,spatial pattern,species biodiversity,ground diameter distribution,growth,mortality and recruiment for saplings in different canopy clousure were studied using the methods of forest management and statistics and quantitative ecology.The main conclusions were as follw:(1)The canopy closure was divided into four grades:0.61-0.70?0.71-0.80?0.81-0.90 and 0.91-1.00.The results showed that the number of saplings increased with the increase of canopy clousure,and then decreased.The highest number of saplings occurred in the canopy closure of 0.71-0.80,which was 6680 stems/ha.The Maple and Fir had the highest important values under the four canopy closure level.The diversity index in canopy closure between 0.71-0.80 were higher than the others both for adult trees and saplings.At the same time,the community stability of the stands with different canopy density levels was calculated by the membership function value.The average membership value of plots with canopy closure between 0.71-0.80 was the largest.(2)The number of saplings decresed with the increase of ground diameter in all four canopy closure levels.The distribution showed the shape of inverse "J".The numbers of saplings for major species,such as Spruce,Fir,Korean pine and Maple,also showed the shape of inverse "J" with the increase of ground diameter.The Weibull function was the optimal equation fitting the distribution of ground diameter grades and height grades,except for Spruce saplings in canopy between 0.91-1.00 and 0.81-0.90,where the optimal equation was the logarithmic function and were the composite function,the growth equation and the exponential function,respectively.(3)The spatial distribution pattern of saplings under different canopy closure was analyzed by using mean uniform angle index,mean neighborhood comparison,mean mingling degree and second order point pattern.The results showed that mean uniform angle index,mean neighborhood comparison,mean mingling degree for plots in different canopy closure had no signicant difference.When the canopy closure was between 0.71 and 0.80,the young trees showed obvious aggregation distribution in the scales of 0-3m,and the random distribution was dominant in other scales.(4)During the two-year survey period,with the increase of canopy closure,the growth and growth rate of saplings increased first and then decreased,and when the canopy closure was between 0.71-0.80,the height growth and its rate of saplings were highest than others.The average growth rate of the ground diameter of saplings decreased first and then increased with the increase of canopy closure.When the canopy closure was between 0.71 and 0.80,the growth of groud diameter was the lowest.But the growth of groud diameter for Lime and Maple saplings in the canopy closure between 0.71-0.80 were the maximum.With the increase of canopy closure,the growth of Spruce saplings was smaller and smaller.With the increase of canopy density,the growth rate of crown area for Maple and Fir increased gradually,and the average crown area growth of young trees reached 12359.7 cm2 when the canopy closure was above 0.90,which was nearly twice than canopy closure less than 0.70.The results of the multiple regression model showed that the growth of saplings was affected by the many factors such as canopy closure,tree density,stand basal area,shrub coverage and size of saplings themselves,and so on.(5)The results showed that the zero-inflated negative binomial model(ZINB)was the best counts model fitting the recruitment and mortality of saplings,which had the smaller VIC value than Poisson modell and Hurdle model.The results form the zero-inflated negative binomial model showed that the stand density,the thickness of the forest layer,the thickness of the humus and the size of the DBH of the saplings themselves had a significant effect on the recruitment and mortality of saplings.(6)The results showed that when the total basal area of adult trees was 20-30 m2·ha-1 and the q value is 1.4,the growth of ground diameter and DBH of saplings was highest,as the number of saplings was 4267 stems/ha.When the total basal area of adult trees was 41.13 m2·ha-1,the q value is 1.4 and the volume was 400 m3·ha-1,the better growth of ground diameter and DBH of saplings was occurred when saplings density was 7100 stems/ha.(7)Three spots representing three succession stages of spruce-fir mixed forest(SF:polar-birch secondary forest,MF:spruce-fir mixed forest and PF:spruce-fir near primary forest)were established.Structure,growth dynamics during two growth seasons for dominant tree species,regeneration were examined,and a univariate O-ring function statistic was used to analyze the spatial patterns of main regeneration tree species.Results showed that,first,composition of tree species,periodic annual increment(PAI)of the diameter at breast height(DBH),basal area for overstory trees and of ground diameter(DGH)for saplings,were significant different with the succession.Second,the current species composition and regeneration dynamics of SF suggested a development towards MF.Pioneer species like Betula platyphyllaa will gradually disappear while climax species,such as Abies nephrolepis,Pinus koraiensis,Picea koraiensis and Tilia amurensis will dominate forest stands.Third,despite the highest volume occurring in PF,and saplings in it grew better than in the others,this forest type is unstable because of its unsustainable structure of DBH class and insufficient regeneration.;and fourth,MF had the most reasonable distribution of DBH class for adult trees and DGH class for saplings(H>30 cm and DBH<5 cm),as well as an optimal volume increment.Limiting canopy opening size can lessen the physiological stress and promote the growth and competitive status of regeneration.Management implications for increasing the gaps and thus creating better growth conditions for understory saplings and facilitating forest succession,were discussed. |
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