Distributional Patterns And Physiological Mechanisms Of An Evergreen Broadleaved Tree Species(cyclobalanopsis Gracilis) Along An Elevational Gradient In The Dabie Mountain

Plants inhabit species-specific distribution range.In general,low temperature is a major limit of the leading edge(i.e.upper elevational limit or northern horizontal limit)of species distribution.Although the physiological mechanisms underlying the formation of vegetation boundaries,such as treelines and timberlines,under the harsh conditions at treeline ecotones have been studied well,the leading edges of tree taxa below the treeline ecotone are largely unexplained.Previous studies mainly focused on the deciduous broadleaved tree species and conifers,while few studies are on the evergreen broadleaved tree species.The studies on the physiological mechanisms underlying the low-temperature limitation on the leading edges of tree taxa will help understand the relationships between the plant distributions and climates,and predict the responses of species distribution to the future climate under global climate change.Evergreen broadleaved forest(EBLF)in China is the main body of the global EBLF.Its leading edge is primarily determined by low temperature.This study chose an evergreen broadleaved tree species(Cyclobalanopsis gracilis)which reached the upper elevational limit of EBLF as a canopy tree species in Anhui Tianma National Nature Reserve.The distribution patterns of the population along an elevational gradient and their environmental drivers were investigated with the population survey and the application of dendrochronology.And the corresponding environmental factors of each plot were monitored.Further analysis of variations in concentration of non-structural carbohydrate(NSC)in adults along a temperature gradient and the responses of NSC concentration and growth of C.gracilis seedlings to defoliation and debudding treatments was made to reveal whether the physiological pathway underlying lowtemperature limitation at the upper limit of C.gracilis was carbon source limitation or carbon sink limitation.Meanwhile,the variations in structural traits of photosynthetic organs(leaves)and storage organs of photosynthate(parenchyma in secondary xylem)along a temperature gradient were ananlyzed to elucidate the adaptive strategies of C.gracilis at the upper limit.The main conclusions are as follows:1)The structures of DBH(diameter at breast height)classes and height classes of C.gracilis population in this study showed that regenetation of population at the upper limit were limited.Seedlings were decreasd from 1175 to 0 ind/ha along increasing elevations.Population density,as well as seedling density and density of individuals that are taller than 1.5 m,were all positively correlated with temperature.And the correlations between seedlings and temperature were more prominent compared with the correlations between individuals that are taller than 1.5 m and temperature.To sum up,the descending patterns of C.gracilis along the elevational gradient were closely associated with decreasing temperature,indicating low temperature at high elevation is the main environmental driver for the leading edge of C.gracilis.2)The variations in tree ring width of adult C.gracilis at the upper limit were significantly positively correlated with mean temperature in January(the coldest month of a year)of current year and mean precipitation in June of previous year,as well as significantly negatively correlated with mean precipitation in August of previous year.However,there was only marginally significant relationship between the tree ring width of the adults and mean precipitation in October of current year at the low elevation.The results suggested that the effects of temperature limitation,especially the winter low temperature,on growth of C.gracilis were increased with the increasing elevation.The overmuch precipitation in August exerted negative effect on the growth of C.gracilis at the upper limit.3)NSC concentrations of C.gracilis at upper elevational limit were not lower than that at low elevation in each season,and NSC concentrations in old-branches increased significantly with increasing elevation at the stages of early growing season(spring)and the fastest growing period(summer),indicating the carbon supply of C.gracilis at upper elevational limit in each season was adequate.The results preferred to support the carbon sink limitation hypothesis instead of carbon source limitaion hypothesis.4)There was no significant difference in seedling growth indicators(current-year shoot length,increment of height,increment of basal diameter)between the defoliation,debudding and control treatments.While concentrations of NSC in trunks of seedlings showed no significant difference between different treatments,NSC concentration in twigs in the defoliation group was significantly lower than that in the control and debudding group.The results suggested the status of carbon supply had little influence on growth of seedlings,which supported the carbon sink limitation hypothesis instead of carbon source limitation hypothesis.5)The seasonal variations in average mass based leaf nitrogen(N)and leaf phosphorus(P)concentration across elevations were 1.23%-2.31% and 0.08%-0.24% respectively,with the highest level in spring.The seasonal variations in average carbon(C)concentration across elevations were 48.27%-51.73%,with the highest level in summer.The seasonal variations in average NSC concentration across elevations were 10.41%-19.71%,with the highest levels in winter.The seasonal variations in average stoichiometric nutrient ratios(N/P,C/N and C/P)across elevations were 8.98-19.08,21.23-41.37 and 208.22-645.18,respectively,with the minimum levels in spring.Temperature had significant effects on leaf N,P,C/N,C/P and leaf mass per area(LMA)of mature leaves.Leaf N and P concentration decreased with decreasing coldest month mean temperature,while C/N,C/P and LMA showed opposite trends.Leaf N/P and NSC concentration showed no clear trends along the gradient of coldest month mean temperature.The results suggested the individuals which own high LMA and nutrient use efficiency were more adaptive at high elevations and increasing leaf NSC concentrations in winter was also one of the adaptive strategies to cope with the low temperature.6)Total proportions of parenchyma(RAP)in the secondary xylem were 20.64%-42.99%,showing increasing trend with decreasing temperature.The amount of axial parenchyma(AP)(4.67%-11.67%)was positively correlated with temperature,while the amount of ray parenchyma(RP)(13.02%-37.84%)was negatively correlated with temperature.The linear relationship between NSC concentration and RAP content was not significant.The results indicated the space of carbon storage(i.e.RAP concent)of C.gracilis at the upper elevational limit was adequate and there were little effects of total space of carbon storage on carbon supply.In summary,the physiological pathway underlying low temperature limitation on C.gracilis at the upper elevational limit is more likely to be carbon sink limitation.That means that low temperature restricts growth(i.e.cell proliferation)directly instead of limiting carbon supply to restrict growth indirectly.The structural traits of photosynthetic organs(leaves)and storage organs of photosynthate(parenchyma in secondary xylem)of C.gracilis at the upper elevational limit consistently showed some variations to adapt to low temperature.This study will help understand the causes of forming lead edges of the trees that not reach treelines and the responses to future climate changes.This study will also be of great significance in theory and practice for conservation and restoration of EBLF in northern subtropical zones.