Photosynthetic Carbon Assimilation And Carbon Use Efficiency Of Mixed-broadleaved Korean Pine Forest In Changbai Mountain

Photosynthesis,one of the essential fundamental processes in many ecophysiological models,plays a vital role in forest ecosystems’ material cycle and energy flow.The mixedbroadleaved Korean pine forest is a climax community in Northeast China,and its productivity and carbon storage are important for the national carbon sink and the global carbon cycle.Exploring forests’ carbon storage and interannual carbon sink capacity can effectively and accurately assess the relative importance of mixed-broadleaved Korean pine forests in Changbai Mountain in regional and national carbon sequestration.This study took the mixedbroadleaved Korean pine forest on the northern slope of Changbai Mountain as the research object.We first established a leaf phenology model for fifteen tree species in the forest to predict the dynamic change of leaf area during the growing season.Second,we used the biomass survey data to construct an additive aboveground biomass model of fifteen species divided into the stem,bark,branch,and leaf components and combined the plot survey and growth core data to simulate the annual biomass growth of fifteen species.We determined the extinction coefficient according to the Lambert-Beer equation,and simulated the light environment of the forests combined with the distribution and seasonal dynamics of leaves in the canopy.Photosynthetic models coupled with light and temperature were constructed to estimate the gross primary productivity and carbon use efficiency for each species and the whole stand.The main findings are as follows:(1)The leaf-spreading phenology of the fifteen species in the mixed-broadleaved Korean pine forest in Changbai Mountain was mainly concentrated in May-June,and the deciduous phenology was mainly concentrated in September-October.The active accumulation temperature and cold temperature days were the driving factors of the leaf-spreading phenology and the deciduous phenology,respectively.We developed species-specific phenology models for the fifteen species to predict leaf area based on the driving climate factors.(2)The allometric growth equations of fifteen species in the mixed-broadleaved Korean pine forest were established,and the models had an excellent fitting effect.It can be seen from the model evaluation that DBH was the best indicator for predicting shoot biomass and leaf biomass.For the simulation of standing wood volume,stem biomass,and bark biomass,the combined variables of DBH and height were used for better prediction.(3)The stem was the main aboveground biomass reservoir,accounting for 60-70% of the aboveground biomass.There were significant differences in the distribution ratio of aboveground biomass of trees with different diameter classes.The stem biomass ratio increased with increasing DBH until it stabilized.The percentage of bark biomass in the aboveground biomass of small-DBH trees was higher than that of large-DBH trees.The relative contribution of branch biomass to aboveground biomass increased with tree growth,whereas the opposite was true for leaf biomass.The leaf biomass ratio of the Pinus koraiensis was about 2-3 times broad-leaved species of the same DBH.(4)The leaf area distribution of different canopy heights in mixed-broadleaved Korean pine forest was quite different.Most of the leaves were attached to the middle and upper part of the canopy,mainly contributed by species such as P.koraiensis,Tilia amurensis and Quercus mongolica.The height with the largest leaf area was about 60% of the average height of stand or 50% of the average top height.Only a small amount of leaf was distributed below 10 m.In addition,the leaf area index of the mixed-broadleaved Korean pine forest was 12.1 after leaf spreading.(5)The extinction coefficient of mixed-broadleaved Korean pine forest was 0.3212.There were significant differences in the vertical distribution of light,and the transmittance decreased with the decline of canopy height.In the growing season,the transmittance at 20 m of the canopy was about 0.2,while the transmittance at 10 m and 1 m was close,only 0.02.(6)In this study,light-temperature coupled photosynthetic models were established for fifteen species in the mixed-broadleaved Korean pine forest,all of which had an excellent fitting effect and predictive ability.It can be seen from the model that the light compensation point and light saturation point of the upper canopy species such as P.koraiensis,Q.mongolica,Fraxinus mandshurica and Betula platyphylla were higher,while the light compensation point and light saturation point of the lower canopy species such as A.pseudosieboldianum and A.tegmentosum was lower.Lower light compensation points and light saturation points can enhance the ability of trees to survive in low-light environments.(7)The dynamic trends of gross primary productivity,photosynthetic yield,and aboveground carbon increment of fifteen species in mixed-broadleaved Korean pine forest were different in 2017～2020.No significant inter-annual differences were found in aboveground carbon increment,while inter-annual variations were observed in gross primary productivity and photosynthetic yield.The percentage of aboveground carbon increment in gross primary productivity was between 10.3% and 52.9%,and the average of the whole sample plot was18.6%.The aboveground carbon use efficiency of the tree species in the upper canopy was significantly lower than that of the tree species in the middle and lower canopy.The ratio of photosynthetic yield to gross primary productivity was between 59.0% and 85.8%,with an average sample plot of 81.2%.The ratio of tree species in the upper canopy was significantly higher than that in the middle and lower canopy.The gross primary productivity per unit leaf area of different species varies significantly in the growing season.The highest carbon assimilation of P.koraiensis was about five times that of the lowest carbon assimilation of A.tegmentosum.In addition,the gross primary productivity and photosynthetic yield of the mixed-broadleaved Korean pine canopy increased first and then decreased with the decrease of canopy height.The canopy height of the maximum photosynthetic yield was higher than that when the gross primary productivity reached the maximum.The canopy height of the peak gross primary productivity is about 90% of the average height of stand or 78% of the average top height.The canopy height of the peak photosynthetic yield is about 87% of the average height of stand or 75% of the average top height.