Study On The Effects Of Microhabitat Factors On Natural Regeneration Of Pinus Tabuliformis Plantations On The Loess Plateau

Chinese pine（Pinus tabuliformis Carr.）is one of the main tree species for water and soil conservation on the Loess Plateau.However,most of these plantations face a series of ecological problems,such as low diversity of understory plants and difficulty in natural regeneration.Our study takes the mature Chinese pine plantations in Huanglong of Shaanxi Province as the research object.Based on the investigation of the characteristics of understory woody plants and microhabitat factors in three handred 5 m×5 m micro-plots,we analyzed the structural characteristics and dynamic changes of regeneration population of Chinese pine,and determined the main microhabitat factors affecting the density and height growth of regeneration at different developmental stages,so as to provide scientific basis for promoting the sustainable management of Chinese pine plantations.The main results were as follows:（1）The height structure and age structure of regeneration population of Chinese pine were approximately normal distribution.The population height was mainly concentrated in the range of 50-150 cm,and the population age was mainly concentrated in the age of 11-14 years.（2）The survival curve of regeneration population of Chinese pine was single peak,and the peak value was at the age of 13,which shows that the age structure of Chinese pine regneration is unbalanced and the number of seedlings is insufficient.The overall population quantitative dynamic index(V_pi=4.3%)was greater than zero,which showed that the regeneration population of Chinese pine is a growth type.However,combined with the complex dynamic change trend of continuous increasing first and then decreasing presented by the mortality and disappearance rate curve,when V_pivalue tended to 0,it can be seen that the population is unstable and the growth trend is not obvious.The time series predicted that after 2nd,4th,6th,8th and 10th age class in the future,the number of regeneration individuals will decrease before 15th age class and will increase after 15th age class.（3）The microhabitat factors involved in our study jointly explained 37.68%of the variation in the species composition of understory woody plants in Chinese pine plantations,among which soil total phosphorus（2.66%）,available potassium（2.19%）and natural water holding rate of litter（1.26%）were the main factors.The density of Chinese pine regeneration was significantly affected by soil total nitrogen,available potassium and shrub density,while the differences in regeneration height growth mainly resulted from the density of other understory trees and shrubs,soil total phosphorus and diffuse non-interceptance.（4）The density of saplings（50 cm≤Height<200 cm）and juvenile trees（Height≥200 cm and DBH<5 cm）of Chinese pine were significantly affected by 6 and 7microhabitat factors,respectively,which was more than that of seedlings（Height<50 cm）,but the effect of the microenvironment on the former two was less different,indicating that the sapling stage may be the critical period for successful regeneration of the plantations.Therefore,gap creation or thinning is suggested to promote the growth of saplings and juvenile trees.（5）Chinese pine(7261 steams·ha^-1,124.6 cm)and Quercus liaotungensis Koidz.(3114 steams·ha^-1,117.0 cm)had a relatively higher density,height,important value and niche width than other woody plants,which were likely to be potential canopy replacement species.In the future,Chinese pine plantations may be transformed into a more stable forest of coniferous and broad-leaved mixed forest with high stability and species diversity（e.g.,P.tabuliformis×B.platyphylla）.（6）The correlation between species of understory woody plant populations in Chinese pine plantations were generally not close,which indicate that the degree of competition between woody plant species is low.For regeneration of Chinese pine,the seedlings suffered the highest intensity of neighbor competition,while juvenile trees received the lowest competition intensity.The influence of interspecific competition on Chinese pine regeneration was higher than that of intraspecific competition.The higher intensity of intraspecific and interspecific competition mainly occurred on saplings with a height ranging from 100 to140 cm.Juvenile trees with height higher than 300 cm were hardly affected by neighbor competition.Based on these above results,our study suggested that managers can take some measures such as creating forest gaps or thinning,and further explore the interaction relationship of vegetation-soil-microbial system to promote the growth of saplings and juvenile trees.Through the above measures to increase their opportunities to enter the canopy,so as to form a mixed forest with high species diversity and stability,thereby promoting the sustainable development of Chinese pine plantations.