Study On Obstacle Mechanism Of Early Natural Regeneration Of Pinus Koraiensis In Secondary Forest

Broad leaved Korean pine（Pinus koraiensis）forest is a zonal climax vegetation community in Northeast China.The natural regeneration of Pinus koraiensis is the core problem in the process of secondary forest restoration to broad-leaved Pinus koraiensis forest.In this experiment,natural regeneration Pinus koraiensis seedlings in the eastern mountainous area of Liaoning Province were taken as the research object.Fish eye photos were used to monitor the typical secondary canopy structure and the dynamics of understory light environment in the secondary forest,and they were divided into three different light gradients according to the light transmittance of the forest when the canopy was closed.The photosynthetic physiological parameters of 1-8-year-old natural regeneration Pinus koraiensis seedlings with different light gradients were measured in the growing season and at the end of the growing season,The biomass,total carbon,total nitrogen and total phosphorus contents in various organs of natural regeneration Pinus koraiensis seedlings were measured by the periodic harvest method.The growth and survival strategies of natural regeneration Pinus koraiensis seedlings were explored by analyzing the characteristics of material distribution and nutrient storage in various organs of 1-8-year-old natural regeneration Pinus koraiensis seedlings under different light environments.The main conclusions are as follows:1.Under different light gradients,the scattered light under the forest is significantly higher than the direct light under the forest.From the time change of the light radiation under the forest,it can be seen that with the arrival of the non growing season,the three kinds of light radiation show a trend of first decreasing and then increasing,with obvious seasonal dynamic changes,which is due to the change of the canopy structure,resulting in the change of the light environment under the forest.2.With the increase of age,the average height growth and diameter growth of natural regeneration Pinus koraiensis seedlings gradually increased.The height and diameter growth of natural regeneration Pinus koraiensis seedlings were positively correlated with the light index.The height and diameter growth of natural regeneration Pinus koraiensis seedlings in higher light gradient could be earlier,while the height and diameter growth of natural regeneration Pinus koraiensis seedlings in lower light gradient were slower and slower.Natural regeneration Pinus koraiensis seedlings are more suitable to have a certain degree of shade at the young stage.With the growth of age,the shade tolerance gradually weakens and the light demand gradually increases.3.The growth time node of the biomass of each organ of natural regeneration Pinus koraiensis seedlings with relatively good light conditions under the forest is earlier,the growth rate is faster,and the growth is relatively rapid,while the growth time node of the biomass of each organ of natural regeneration Pinus koraiensis seedlings with relatively poor light conditions under the forest is later,the growth rate is slower,and the growth is slow.The growth rates of root,stem and leaf biomass of natural regeneration Pinus koraiensis seedlings are different.The growth rate of leaf biomass is the fastest,followed by the stem,and the root is the slowest,indicating that Pinus koraiensis seedlings are more preferred to allocate biomass to leaves in the understory environment.This light adaptation strategy is conducive to improving the utilization of light resources.4.Light,age and month had significant effects on Photosynthetic Physiological Indexes of natural regeneration Pinus koraiensis seedlings.Different light intensities had significant effects on transpiration rate,net photosynthetic rate and intercellular CO₂ concentration of natural regeneration Pinus koraiensis seedlings（P<0.05）.During the growing season,with the decline of light gradient,the net photosynthetic rate of natural regeneration Pinus koraiensis seedlings decreased significantly,and the intercellular CO₂ concentration of natural regeneration Pinus koraiensis seedlings with lower light gradient was the highest.5.The total carbon content of leaves,stems,roots and whole plants of natural regeneration Pinus koraiensis seedlings under different light gradients increased gradually with age,the total nitrogen content decreased year by year,and the total phosphorus content decreased year by year;The C/N ratio of each organ increased year by year with age,and the N/P ratio had no obvious trend with age,but the C/P ratio increased year by year with age.In conclusion,Pinus koraiensis seedlings have certain shade tolerance,and the light demand increases with age;Natural regeneration Pinus koraiensis maintained a high proportion of leaf biomass and a low proportion of root biomass under different light gradients,but the proportion of root biomass increased year by year under low light conditions;Under different light gradients,the nutrient content of natural regeneration Pinus koraiensis leaves was higher,and that of roots and leaves was the lowest.Under low light conditions,the content of N and P in roots decreased year by year;At the same time,with the decrease of light gradient,the photosynthetic capacity of natural regeneration Pinus koraiensis seedlings decreased gradually.The comprehensive effects of the above factors lead to the obstacles of early natural regeneration of Pinus koraiensis in the secondary forest.Light is still the key factor affecting the growth and survival of natural regeneration of Pinus koraiensis seedlings.