Changes In Growth Characteristics Of Seedlings In Evergreen Broad-leaved Forest Along Altitudinal Gradients In Dabie Mountain,Anhui Province

Temperature is an important factor determining the distribution of evergreen broad-leaved forest(EBF).At the north edge of the subtropical zone,the temperature changes significantly on the altitude gradient and the low temperature is considered to be a limit factor on the upward of EBF.The seedlings are the most sensitive stage of the life history of the woody plants and its recruitment directly determines the development trend of EBF at the upper limit.In this study,we selected Dabie Mountain in the northern margin of EBF in eastern China as search region.We measured the shoot length and leaf traits of seedlings(H<2m)for the 61 common woody plants at an altitude of 592 to 1200 m,to compare the seedlings growth rate among altitude gradients.The relationship between growth rate and individual size,leaf traits and environment and the difference between interannual shoot growth in different altitude were analyed,to reveal the growth adaptation strategy in low temperature environment.The results will help to understand the mechanism of the pattern of seedling recruitment of the EBF at its upper limit in eastern China.The main results are as follows:1)The seedlings growth rates of Cyclobalanopsis gracilis and Litsea elongate,the dominant trees of EBF,showed a decreasing trend with increasing altitude.The growth rate in low altitude was the largest.The seedlings growth rates of the Pittosporum illicioides,Pterostyrax corymbosus and Lindera glauca,the common associated species,did not have a significant difference between altitudes,and the growth rate of seedling of Lindera praecox at high altitude was significantly greater than that at middle and low altitude.2)Correlation analysis showed that: in general,the seedlings growth rates were significantly positively correlated with plant height and base diameter,and were significantly negatively correlated with canopy closure.That is,the larger the plant and the lower canopy closure,the faster the growth rate.According to the generalized linear model analysis for the six main tree species,the sedling growth rates of the three evergreen broad-leaved trees were significantly negatively correlated with canopy closure.Two of them,C.gracilis and L.elongate were significantly positively correlated with plant height,and significant difference of growth rate among altitudes only existed between low and middle altitude for in C.gracilis.For deciduous broad-leaved trees,there was no significant correlation between growth rate and plant size,canopy closure and altitude for P.corymbosus and L.glauca,while the larger the individual,the smaller the canopy closure and the higher altitude,the faster growth rate for L.praecox.3)Comparing the correlation of interannual growth of shoots,the results showed that whether at the lifeform level or at the species level,there was a positive correlation between the interannual shoot growth,and the correlation coefficient of evergreen trees was larger than that of evergreen shrubs.Standardized major axis analysis showed that the shoot of C.gracilis and L.elongate kept isometric growth at all altitudes,and there was no significant difference in slope and intercept.this means that there was no significant changes in the interannual shoot growth at different altitudes and the growth strategy did not change without active carbohydrates storing at their upper limit.4)In general,the length/mass of the current shoot was significant negative correlated with SLA and was positive correlated with leaf number and total leaf area;But for either evergreen or deciduous broad-leaved species,the length/mass of the current shoot was only significant positive correlated with total leaf area.There was no significant difference in SLA,single leaf area,the leaf number per twig mass/volume and the leaf area per twig volume at all altitude gradients for C.gracilis,L.elongate and P.illicioides.At high altitude,the leaf area per twig mass of C.gracilis was significantly lower than the middle and lower altitudes,while there was no significant difference for L.elongate and P.illicioides.In conclusion,the seedlings growth rate of dominate species of EBF in Dabie Mountain did not change with the increase of altitude with stabe interannual growth and leaf-twig relationships.This may indicate that the change of temperature on the altitude gradient did not affect the seedling growth of evergreen broad-leaved trees,and it is speculated that the low temperature limit on the recuitment of evergreen broadleaved tree did not casued by its effect on seedling growth at the upper limit of EBF.