| As the main part of terrestrial ecosystem,forest ecosystems play a decisive role in improving the ecological environment and maintaining ecological balance.Forest structure is an important property of forest ecosystems and closely related to forest functions.Rational structure is the basis for playing multiple functions.Therefore,the study of forest structure and its impact factors have important theoretical and practical significance for forest management and optimization decision-making.In this study,three typical coniferous and broad-leaved mixed forests of Pine-broadleaved(PYI),Pine-oak(PYII)and Pine-cunninghamia-oak(PYIII)were selected as research objects in Guangxi Yachang Orchidaceae national nature reserve in Nanpan river basin of southwest China,which were disturbed by human beings and recovered naturally for a long time.Sample plots were set up by typical sample plot method to carry out systematic sampling and investigation.Tree species composition,species diversity and structural characteristics such as diameter,tree height,canopy width,uniform angel index,neighborhood comparison,species mingling,open degree and competition index were analyzed in three kinds of coniferous and broadleaved mixed forest(PYI,PYII and PYIII).Environmental factors of topographic factors,soil physical and chemical properties,soil microbial biomass,microbial activity,structure and functional diversity were measured and analyzed,and the relationships between stand structure characteristics and environmental factors were also discussed.The study is intended to provide a theoretical basis for the restoration of degraded forests and the conservation of biodiversity in the region.Our results indicate that:1.The main species and plants diversity of three kinds of coniferous and broadleaved mixed forest are different.The main dominant species of PYI were Pinus yunnanensis var.Tenuifolia,lyonia ovalifolia(wall.)drude var.elliptica and Quercus variabilis Bl.The main dominant tree species of PYII were Pinus yunnanensis,Quercus variabilis and Cyclobalanopsis glauca(Thunb.)oerst.),while the main dominant tree species of PYIII were keteleeria Davidiana(c.e.Bertrand)beissn.),Quercus variabilis and Pinus yunnanensis,the similarity coefficients of tree layer composition of the three natural coniferous-broad-leaved mixed secondary forests were lower due to their large different types of associated trees.The species diversity index of the three mixed coniferous and broadleaved forests was mainly shown as shrub layer > tree layer > herb layer.PYIII had higher species richness in both tree layer and shrub layer,while its Shannon-Wiener index,Simpson index and Evenness index were the lowest among the three mixed coniferous and broad-leaved forests.2.The three mixed forests showed typical uneven aged stand diameter structure characteristics,but the distribution structures of tree height and canopy width were quitedifferent.The diameter distribution curves of PYI,PYII and PYIII were inverse "J" shaped,the average diameter at breast height of PYIII was the largest while PYI was the smallest.The tree height distribution curves showed that PYI is mono-modal shaped,while PYII is mul-modal distribution and bimodal shaped curve was fitted for PYIII,Tree height structure distributions were all left skewed,and the average tree height was also shown as PYIII was the largest and PYI was the smallest.Tree canopy width of the three forest types were mainly distributed between(1.0-3.0 m),and differences in average canopy width showed PYII > PYIII > PYI.3.The mixed degree,differentiation degree of tree size and growth space of the three mixed coniferous and broad-leaved forests were relatively consistent,but the spatial distribution pattern of trees had certain differences and their competitiveness is also unequal.PYI,PYII and PYIII were all mixed in moderate degree or above,but PYII was the most mixed.PYI,PYII and PYIII were all in a state of moderation in terms of DBH or tree height.The growth space of the three stands was insufficient,and PYI was seriously insufficient.The spatial distribution pattern of forest stands PYI was generally clustered,while PYII and PYIII were generally randomly distributed.The average competition of PYIII among the three forest stands is PYIII,and the average competitiveness of PYI is the smallest.Through correlation analysis,it was found that the angle scale,opening degree and competition index of the spatial structure factors of the stand were closely related to the non-spatial structure factors such as tree number density,average diameter at breast height,average tree height,average crown width and cross-sectional area of breast height.At the same time,these spatial and non-spatial structure factors had significant influence on plant species diversity.4.The three mixed coniferous and broad-leaved forests are located in the middle and low altitude areas with different slopes and aspect,and the soil is weakly acidic.Soil physical and chemical factors such as soil water content,p H value,organic matter,total nitrogen,total phosphorus,total potassium,available phosphorus and available potassium content in different stands show significant differences,and these factors are the lowest in PYI with higher stand density and higher density of Pinus yunnanensis with fine leaves.Seasons also have significant effects on soil physical and chemical factors.Except for p H,soil water content and soil C/N ratio,other factors are higher in dry season than in wet season.5.The various biological index factors reflecting soil microbial activity,biomass and function were different in the three different coniferous and broadleaved mixed forests.The differences of catalase activity,phosphatase activity,urease activity,invertase activity,soil microbial biomass,total PLFAs amount,bacterial PLFAs amount,fungal PLFAs amount,arbuscular mycorrhizal fungi PLFAs amount,AWCD,functional Shannon-Wiener index and functional Simpson index all reached significant levels,and their change trends are related to soil water content,ph value,soil organic matter content,total nitrogen content and total phosphorus content.The soil microorganisms of the three coniferous and broad-leaved mixed forests had higher utilization of 6 types of carbon sources,all of which were carbohydrates and amino acids.Biolog carbon source principal component analysis showed that there were significant differences in the utilization characteristics of soil microbial carbon sources amongthe three coniferous and broad-leaved mixed forests,with amino acid carbon sources playing a major role in the differentiation.Principal component analysis of phospholipid fatty acids showed that the soil microbial community structures of the three stands were significantly different.The difference of soil G-/G+,Mono/sat among the three stands also reached significant level,while Bact/Fungi had no significant difference,indicating that although the soil aerobic environment of different mixed stands changed the structure and function of soil microbial community,these differences would not change the stability of soil ecosystem.6.The redundancy analysis method was a good explanation of the relationship between the stand structure and site environmental factors of the coniferous and broadleaved mixed forest.Monte Carlo permutation test showed that the soil water content,p H value,organic matter content,total nitrogen content,total phosphorus content,slope and aspect had significant effects on the stand structure in the non-biological environment.Among the biological environmental factors,urease activity,catalase activity,microbial biomass carbon,microbial biomass nitrogen,total PLFAs amount,bacterial PLFAs amount,arbuscular mycorrhizal fungi PLFAs amount,microbial utilization carbon source number,microbial function Shannon-Wiener index,microbial function Simpson index,and microbial utilization of carbohydrate,amino acid carbon source,carboxylic acid carbon source and phenolic acid carbon source had significant effects on the stand structure.The order of correlation based on redundancy analysis and differences in the relationships between site environment index factors and individual stand structure factors showed that tree layer species diversity index,stand height and shrub density were closely related to the slope.The degree of mixture and differentiation of stand size were closely related to the aspect of slope.Forest structure factors such as shrub species richness,average DBH,basal area,open degree,canopy density,tree density and competition index were also closely related to soil nutrient factors such as soil moisture content,p H value,organic matter content,total nitrogen content and total phosphorus content,and soil microbial community biomass,activity,structure and functional diversity,etc.Based on the main research results above,it was found that the stand structures of the three typical coniferous and broadleaved mixed secondary forests were different after long-term natural restoration,and the abiotic environment and soil biological environment were also significantly different,and there was a significant correlation between the stand structure and environmental factors,especially when the soil nutrient content of the stand was higher,the microbial activity was stronger,and it was more conducive to increasing the DBH growth of the forest,at the same time,the stand with higher soil fertility had more sufficient growth space and stronger forest competitiveness.Since vegetation,soil and microorganism interacted with each other,the reverse was also true.When the overall growth of the stand is good and the growth space is sufficient,the soil nutrient status and microbial activity and fecundity can be better improved.The superior hydrothermal conditions and abundant tree species resources abundant tree species resources in the Nanpanjiang River Basin provided the material basis and environment for natural vegetation restoration.However,invasive tree species are disorderly and it took long time to gradual elimination in the competition of the tree species during thenature vegetation restoration.Therefore,artificial promotion measures were still needed to accelerate the restoration of the degraded forests,which included regulating the species composition and density appropriately to improve the stand structure and site environment. |
PDF Full Text Request