The storage and utilization of carbohydrates in response to elevation mediated by tree organs in subtropical evergreen broad-leaved forests

Global climate change can affect tree growth and carbon sink function by influencing plant carbohydrate synthesis and utilization,while elevation can be used as an ideal setting under natural conditions to simulate climate change effects.The effect of elevation on tree growth may depend on organ type.However,the allocation patterns of nonstructural and structural carbohydrates(NSCs and SCs,respectively)in different tree organs and their response to elevation remain unclear.We selected four dominant tree species,Schima superba,Castanopsis eyrei,Castanopsis fargesii and Michelia maudiae,along an elevation gradient from 609 to 1,207 m in subtropical evergreen broad-leaved forests and analyzed leaf,trunk,and fine root NSCs,carbon(C),nitrogen(N)and phosphorus(P)concentrations and the relative abundance of SCs.Leaf NSCs increased initially and then decreased,and trunk NSCs increased with increasing elevation.However,root NSCs decreased with increasing elevation.The relative abundance of SCs in leaves and trunks decreased,while the relative abundance of root SCs increased with increasing elevation.No significant correlations between SCs and NSCs in leaves were detected,while there were negative correlations between SCs and NSCs in trunks,roots,and all organs.Hierarchical partitioning analysis indicated that plant C/N and C/P were the main predictors of changes in SCs and NSCs.Our results suggest that tree organs have divergent responses to elevation and that increasing elevation will inhibit the aboveground part growth and enhance the root growth of trees.A tradeoff between the C distribution used for growth and storage was confirmed along the elevation gradient,which is mainly manifested in the"sink"organs of NSCs.Our results provide insight into tree growth in the context of global climate change scenarios in subtropical forest ecosystems.

Size- and age-dependent increases in tree stem carbon concentration: implications for forest carbon stock estimations

Aims Forest biomass carbon(C)stocks are usually estimated by multiplying biomass by a C conversion factor,i.e.C concentration.Thus,tree C concentration is crucial to the assessments of forest C cycles.As stems contribute to the large fraction of tree biomass,the canonical value of 50%or other simplified values of stem C concentration are widely used to represent the values of tree C concentration in the estimations of forest C stocks at different scales.However,C concentration variations between tree organs and within tree size and their impacts on forest C stocks are still unclear.Methods We conducted a global analysis of organ C concentration in age-specific trees based on 576 records of tree age,size(diameter at breast height and biomass)and C concentration data to evaluate the relationships between organ C concentrations and the changes of stem C concentration with tree age and size.Important Findings Tree C concentration varied significantly with organs.Stem C concentration of trees was significantly correlated with that of other tree organs,except for barks and reproductive organs.The stem C concentration increased significantly with tree size and age,which contributed to the increases in C contents of stems and trees.Using the C concentration in stems to represent the C concentrations of other organs and the whole tree could produce considerable errors in the estimations of forest C stocks(−8.6%to 25.6%and−2.5%to 5.9%,respectively).Our findings suggest that tree C accumulation in forests is related to the size-and age-dependent increases in stem C concentration and using specific C concentration values of tree organs can improve the estimations of forest C stocks.