| The ecological restoration of degraded forests is an effective measure to improve forest quality and stability,and is also the main content of the implementation of the National Major Ecological System Protection and Restoration Project Master Plan(2021-2035).At present,a large amount of research is mainly focused on the mechanisms of forest degradation and ecological restoration techniques,while there is relatively little research on the evaluation of the effects of ecological restoration of degraded forests.Exploring the forest structure and evolution patterns after restoration of degraded forests can provide theoretical basis and technical guidance for the implementation of degraded forest restoration projects.This study focuses on the restoration areas of degraded forests on both sides of the Shennongjia Expressway and high-speed railway(referred to as the "Two Highways")entering the city.Three restoration plots and one unrepaired control plot were set up in five different types of degraded forest restoration areas,including the degraded Quercus aliena forest,the Pinus massoniana forest,the Quercus variabilis forest,the Platycarya strobilacea forest,and the snow disaster affected forest.The study investigated and analyzed the growth structure,crowding degree,and hierarchical structure of the degraded forest after one year of restoration The changes of forest structure characteristics,such as regeneration succession and species diversity,and the changes of competition intensity of target tree species were studied by using the method of gradual sample circle radius,in order to explore the impact of degraded forest restoration on forest structure and competition intensity of target tree species,and provide technical support for the implementation and effect evaluation of similar regional degraded forest ecological restoration projects.The main research findings are as follows:(1)The five types of degraded forests in the restoration and control plots are mainly in the small diameter class,but the diameter class with the highest diameter distribution of trees in the restoration plot is generally larger than the control plot.Except for the degraded forest of Pinus massoniana,the diameter distribution curves of trees in other types of restoration plots are steeper and more concentrated than those in the control plots.In the degraded forest of Pinus massoniana,the target trees exist in the form of medium to large diameter classes,and the diameter distribution of trees tends to be balanced after restoration.Compared with the control plot,the tree height distribution in the repaired plot is more concentrated and shifts towards higher height levels.It can be seen that the restoration of degraded forests can to some extent improve the average diameter at breast height and tree height level of the forest,promoting the concentration of tree diameter at breast height and tree height distribution towards higher levels.(2)The restoration of degraded forests significantly reduced stand density,with a decrease of 28.4%-57.6%.Except for the snow disaster sample plots,the average crown diameter and crown length ratio of the forest stands in the restoration sample plots of other types of degraded forests are greater than those in the control sample plots.However,the tree canopy in the snow disaster sample plots is greatly affected by ice and snow disasters and has not yet fully recovered after one year of restoration.The height to diameter ratio of the forest stand in the restoration sample plot was significantly lower than that in the control sample plot,with the snow disaster restoration sample plot having the most significant difference.The mortality rate of various types of degraded forest restoration plots is significantly lower than that of the control plot,and the rates of fallen and broken trees are lower than that of the control plot,but the difference is not significant.This indicates that degraded forest restoration can reduce forest crowding,expand the range of crown growth,increase tree sharpness,and improve the ability of the forest to withstand wind and snow disasters.At the same time,it can also reduce the mortality,fallen and broken tree rates,improve forest health,and promote forest growth.(3)After the restoration of the degraded forest with over dense stands,the decrease in canopy density of the tree layer was 0.18-0.22,and the canopy density of the degraded forest after snow disaster restoration increased by 0.15.The coverage of shrub and herbaceous layers in various types of restoration plots was significantly lower than that of the control plot,indicating that the restoration of degraded forests alleviated the competitive pressure on the tree layer of degraded forests with excessive density.At the same time,it repaired the impact of ice and snow disasters on the tree layer,improved the lighting conditions under the forest,but also caused a temporary decrease in the coverage of shrub and herbaceous layers.(4)Among the five types of degraded forest plots,most of the regenerated seedlings in the tree layer are in a state of insufficient or no regeneration,mainly due to the suppression of natural regeneration by excessive forest density.However,there are more types of regenerated seedlings in the repaired plots than in the control plots.Except for the sample plots of Pinus massoniana forest,the dominant species for regeneration in other types of restoration plots are target tree species,while the dominant species for regeneration in the control plot are interference or companion species.The number,seedling height,and diameter of target tree species in the restoration plot are higher than those in the control plot.However,in the positive Pinus massoniana forest sample plot,no updated seedlings of target trees were found due to excessive forest density,indicating that degraded forest restoration can promote natural regeneration and growth of non positive tree species.(5)Except for the sample plots of Pinus massoniana forest,the recent succession trends of other types of restoration sample plots are dominated by the target tree species,while for the target tree species in the same plot,the target tree species will become companion or declining species during the succession process,indicating that restoration of degraded forests is beneficial for promoting the succession of non positive target tree species.Due to the difficulty of natural regeneration under the forest,the Pinus massoniana forest belongs to a declining species in the restoration plot,and becomes a random species in the control plot,showing a trend of succession towards a mixed coniferous and broad-leaved forest.(6)After the restoration of degraded forests,the number of species in the tree layer significantly decreased,but the important values and average diameter at breast height of the target tree species were significantly higher than those of the control plots,and the difference in the concentration of dominant species was not significant.Compared with the control plot,the proportion of vines in the repaired plot was significantly lower due to thinning compared to the control plot.The number of species in the herbaceous layer of the restored sample plot significantly decreased,but the proportion of sunny herbaceous plants in the restored sample plot was significantly higher than that in the control sample plot.However,the herbaceous layer of the control plot was mostly composed of shade tolerant plants such as ferns,Selaginella,and Polypodiaceae.The difference in species composition of shrub and grass layers before and after the restoration of degraded forests indicates that the restoration of degraded forests can effectively improve the growth conditions under the forest,which is conducive to the regeneration of the forest and the growth of sunny shrubs and grass.(7)Gleason index,Margalef index,Shannon Wiene index and Simpson index of arbor layer and understory shrub grass layer in all types of restoration sample plots are lower than those in the control sample plots.It can be seen that the restoration measures of degraded forests will reduce the species diversity of forests in the short term,especially the high-intensity cutting irrigation measures will damage the species diversity of understory shrub grass layer.(8)On the basis of determining the optimal competition sample circle radii of 9m,7 m,9 m,and 10 m for the single tree level of four target tree species: Quercus aliena,Pinus massoniana,Quercus variabilis,and Platycarya strobilacea,CI1 is selected as the competition index for Quercus aliena,and CI5 is selected as the competition index for Pinus massoniana,Quercus variabilis,and Platycarya strobilacea to analyze the competition intensity of the target tree.The results showed that the total competition intensity and interspecific competition intensity of target trees in the degraded Quercus aliena forest,Quercus variabilis forest,and Platycarya strobilacea forest plots were significantly lower than those in the control plot.The competition intensity of target trees in the degraded forest of Pinus massoniana and degraded forest restoration plots was lower than that in the control plot,but the difference was not significant.The main reason is that the density of degraded forests of Pinus massoniana is relatively high,and intra species competition is stronger than inter species competition.The restoration measures of cutting down interfering trees have little impact on their competition intensity;The main measures for repairing degraded forests caused by snow disasters are to harvest and clean up damaged trees,and competition among target tree species remains high after restoration.It is recommended to increase the restoration intensity of Pinus massoniana and snowstorm degraded forests during the later stage of forest restoration. |
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