| In Hanshan secondary forest area in Inner Mongolia is an important ecotone of grassland,deciduous broad-leaved forest and coniferous forest.It plays a very important role in maintaining regional ecological security.However,in recent years,a large area of forest mortality has begun to appear in the secondary forest of Hanshan.Therefore,The vegetation change after forest death and the supplement of forest regeneration are very important for maintaining the structure and function of forest ecosystem,In this context,The research on whether the natural regeneration can be realized quickly after the death of large area of forest and its influencing factors has become a research hotspot in this area.In this study,Populus davidiana forest with high mortality in the secondary forest area of Hanshan mountain in the semi-arid area was taken as the research object,In the study area,(Qingshan National Nature Reserve in the east of Hanshan secondary forest?Hanshan National Nature Reserve in the middle and Saihanwula National Nature Reserve in the West)the mild death sample plot,the moderate death sample plot and the severe death sample plot were selected for the experiment,The main factors influencing the distribution of understory vegetation and the regeneration of Populus davidiana in the south section of Daxinganling Mountains after its death were studied by using the methods of sequencing analysis and multiple linear regression equation,The main results are as follows:(1)83 species of plants were recorded in the secondary forest area of Hanshan mountain.They belong to 31 families and 57 genera.27 species of plants are recorded in Qingshan Nature Reserve in the East.They belong to 17 families and 25 genera.Compared with Qingshan nature reserve,Hanshan Nature Reserve in the middle of China has increased 4 kinds of plants.They belong to 18 families and 26 genera.Saihanwula reserve in the West has the largest number of species.Compared with Qingshan nature reserve and Hanshan Nature Reserve.The number of species increased by 36 and 32 respectively.They belong to 30 families and 48 genera.It is obvious that the number of species is increasing from east to West in Hanshan secondary forest area.The number of species is the largest in the lighter mortality sample plots of each region.The number of species in moderate and severe mortality plots is small.(2)In terms of precipitation gradient,the main environmental factors leading to species distribution are altitude,annual accumulated precipitation and annual average temperature,all of which reach significant level(P?0.05).On the mortality gradient of Populus davidiana,The effects of litter thickness,canopy density and stand density on the species composition and distribution under the forest were significant(P?0.05).(3)Populus davidiana in three areas is dominated by sprouting and tillering regeneration.The regeneration density of Populus davidiana in Qingshan nature reserve is the largest(9065.74 plants/hm2).Its regeneration density is 2.57 times that of the Saihanwula(3533.33 plants/hm2)reserve.Its regeneration density is 6.58 times that of Hanshan Nature Reserve(1377.78 plants/hm2).On the mortality gradient,Among the medium mortality plots,the regeneration density of Populus davidiana is the highest in Qingshan Nature Reserve(13675.00 plants/hm2).It was 1.69 times and 2.52 times of severe and mild mortality plots.There was no significant difference in regeneration density of Populus davidiana between different dead plots in Hanshan nature reserve.The regeneration density of Populus davidiana in Saihanwula reserve showed a trend of severe mortality(7308.33 plants/hm2)>moderate mortality(2866.67 plants/hm2)?mild mortality(425.00 plants/hm2).(4)In Qingshan nature reserve,the number of medium mortality plots and light mortality plots was more.They are distributed below grade 4,accounting for 86.13%and 84.73%of the total number of individuals in the sample plot,respectively.In Hanshan Nature Reserve,the regeneration density of medium mortality plots was the highest when the tree height was 2-6.They account for 83.14%of the total number of individuals in the sample plot.The regeneration of Populus davidiana was mainly distributed under the tree height of grade 4 in the light and heavy mortality plots.The regeneration density of Populus davidiana in the moderate and severe mortality sample plots of Saihanwula Nature Reserve is mainly distributed above the fourth grade of tree height.(5)In Qingshan nature reserve,the regeneration density of Populus davidiana in severe mortality,moderate mortality and mild mortality was mostly concentrated in the range of 0-lcm DBH.They accounted for 87.19%,78.92%and 73.13%of the total regeneration density respectively.The regeneration density of Populus davidiana in different DBH of Hanshan nature reserve fluctuates greatly.The moderate mortality plots were concentrated in the range of 1-2cm DBH.They account for 34.86%of the total density of DBH distribution.The renewal density of moderate mortality plot(683.33plants/hm2)and mild mortality plo(491.67plants/hm2)t was the highest within 0-1 cm of DBH.There was a trend of increasing first and then decreasing in different mortality plots of Saihanwula,and severe mortality plot? moderate mortality plot? mild mortality plot.The regeneration density of Populus davidiana in severe mortality sample plots was mostly concentrated in the range of 1-4cm DBH.They account for 66.67%of the total regeneration density.The moderate mortality plots were concentrated in the range of 1-2cm.The regeneration density of the slightly dead sample plot was small and fluctuated little.(6)Hanshan secondary forest region from east to West,The annual average temperature and altitude had significant effect on the regeneration of Populus davidiana(P ?0.05).However,the annual accumulated rainfall does not reach a significant level.On the mortality gradient,Canopy density,stand density and litter thickness had significant effects on Populus davidiana regeneration(P ?0.05).(7)In Qingshan and Hanshan reserves,the rate of seedling withering is higher.Its average is 53.6%and 55.4%respectively.In Saihanwula reserve,the rate of seedling withering was relatively high(26.11%)in the slightly dead sample plot.The mortality rate of seedlings was the highest(63.89%)in severe mortality plots.(8)On the precipitation gradient,There was no significant difference in the average shoot withering rate between Qingshan(53.6%)and Hanshan(55.4%)reserves.They are 5.46 times and 5.65 times of saihanula respectively.The average mortality of seedlings in Qingshan nature reserve(34.11%)is the largest.It is 1.18 times and 9.48 times of Saihanwula reserve and Hanshan reserve,respectively.The average dieback(0.74%)rate and mortality of young trees(4.22%)in Saihanwula reserve are relatively small.On the mortality gradient,The seedling dieback rate and mortality rate in Qingshan Nature Reserve were higher than the other two.In Hanshan nature reserve,seedling withering rate,young tree mortality and young tree withering rate are higher as a whole.In Saihanwula Nature Reserve,the rate of seedling withering in slightly dead sample plot was higher(26.11%).The mortality of seedlings and young trees increased gradually along the mortality gradient. |
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