Study On The Differences Of Understory Woody Plants And Impact Factors Between Plantation And Secondary Forest Of Pinus Tabuliformis On The Loess Plateau

Chinese pine（Pinus tabuliformis Carr.）is one of the major soil and water conservation tree species on the Loess Plateau.However,most of the Chinese pine plantations are currently facing a series of ecological problems such as low understory plant diversity and difficulties in natural regeneration.This study mainly focuses on the mature plantations and natural secondary forests of Chinese pine in the Ziwuling Forest Region（Zhengning County）and Huanglong Mountain Forest Region（Huanglong County）,where forest vegetation is well preserved.In this study,we separately investigated the species composition and natural regeneration of understory woody plants in plantations and secondary forests located under similar stand conditions.Meanwhile,a series of environmental factors such as stand site conditions,canopy characteristics,the density and growth of the regenerated seedlings of Chinese pine,the characteristics of other understory trees and shrubs,the thickness and water-holding characteristics of the litter and the physicochemical properties of the soil were measured.Through pairwise comparisons of the two types of stands,our study mainly investigated the differences on the species composition of understory woody plants,species diversity and key influencing factors between plantations and secondary forests;elucidated the differences in natural regeneration growth,population dynamics and key factors between plantations and secondary forests;and revealed the differences in interspecific relationships between understory woody plants in plantation and secondary forests.Our findings aim to provide scientific basis for promoting the sustainable management of Chinese pine plantations.The main results are as follows:（1）Generally,19 trees species and 19 shrubs species were found in secondary forests,and 29 trees species and 21 shrubs species were recorded in plantations of Chinese pine.These results showed that the composition of woody plant communities under plantations and secondary forest had a high similarity,and there were no significant differences in diversity indexes and diversity patterns between these two forests.（2）The redundancy analysis showed that environmental factors explained 35.02%and 28.13%of the variations on the species composition of understory woody plants in plantations and secondary forests,respectively.Among them,elevation（3.76%）,understory tree density（3.14%）,slope（3.04%）,litter thickness（2.38%）,the effective retention rate of litter（1.71%）,soil p H（1.18%）and natural water-holding rate of litter（0.97%）significantly influenced the species composition of understory woody plants in plantations,while the species composition of understory woody plants in secondary forests was significantly affected by slope（1.51%）,understory tree density（1.41%）,litter thickness（1.28%）and total soil phosphorus（1.13%）.（3）The density of regenerated seedlings in secondary forests(1.91 stems·m^-2)was obviously greater than that in plantations(0.74 stems·m^-2),which was 2.6 times greater than that of plantations.However,the growth indexes such as height and ground diameter of regenerated seedlings in plantations were significantly greater than those in secondary forests.The results of the random forest showed that the effective retention rate of litter（8.60%）,the mean crown width of canopy trees（8.50%）,the mean diameter at breast height of canopy trees（8.10%）,the shrub density（7.00%）and the soil bulk density（6.90%）had a great influence on the density of regenerated seedlings,while the altitude（10.40%）,the soil alkaline hydrolysis nitrogen（9.70%）,the effective retention rate of litter（7.70%）,canopy closure（7.70%）and soil available phosphorus（6.90%）had a significant influence on the height of regenerated seedlings.Therefore,it can be inferred that the density of regenerated seedlings was mainly influenced by canopy trees,while the height of regenerated seedlings was mainly influenced by soil characteristics.（4）The frequency distribution of the height structure of regenerated seedlings in secondary forests and plantations both showed an inverted“J”shape,meaning that the number of individuals gradually decreases with the increase of height,and these two types of forests reached their peaks at 50-75 cm and 50-125 cm,respectively.The age structure of regenerated seedling population was similar to normal distribution,mainly concentrated in the age range of 11-19 years in secondary forests and 8-17 years in plantations.（5）In addition to the Chinese pine regeneration,Quercus liaotungensis Koidz.was an understory tree species with a greater importance value,wider ecological niche width and higher height in the secondary forests and plantations,and has a high degree of overlap with the Chinese pine regeneration,which indicated that it may be a potential canopy replacement species.Therefore,we speculate that a more stable mixed conifer forest（e.g.,P.tabuliformis×Q.liaotungensis）may be formed in the future based on the process of forest succession.In summary,based on the differences in the density and growth of regenerated seedlings,and the key influencing factors in plantations and secondary forests,we suggest that different measures can be taken to promote natural regeneration depending on the state of regeneration in the forest stands.For example,in some stands with sparse regenerated seedlings,the space within the stand can be released through moderate removal of understory trees and shrubs to provide some ecological space for the establishment of regenerated seedlings.In areas where regeneration is well established,measures such as loosening soil can be taken to accelerate the decomposition of the fallen material or increase the soil nutrient content through fertilization,promoting the high growth of Chinese pine regenerating seedlings.