Carbon Density And Distribution In Broad-leaved Mixed Forest Of Different Age Classes In Heilongjiang Province, Northeast China

Stand age is a powerful predictor of forest ecosystem structure and function. Literatures published indicated that individual C pools displayed various relationships with stand age in different forest types. The forest in Northeastern China plays an important role in balancing the national and regional C, especially that in Heilongjiang Province. There is the largest vegetation C storage and density in Heilongjiang Province. Second to Quercus spp. forests and Larix spp. forests, mixed broad-leaved forests is one of the largest vegetation C storage in China. At present, we have plenty of data on the northeastern forest C density. There are few studies on C density of two broad-leaved mixed forest types (soft broad-leaved mixed forest and hard broad-leaved mixed forest), especially the comprehensive temporal and spatial study on the C density and distribution for ecosystem components varying with different stand age classes. In this study, forest inventory and allometric approaches were used to measure C density and allocation in two broad-leaved mixed forests (soft broad-leaved mixed forest and hard broad-leaved mixed forest) of different stand age classes in Heilongjiang Province of Northeast China. The main results are as follows:(1) In soft broad-leaved mixed forest, the differences in C densities were significant among the different stand age classes (young, half-mature, near-mature, mature and over-mature forest) in terms of the forest ecosystem, vegetation, detritus, and soil C pool, and the C densities ranged from229.18to337.00,50.71to75.12,3.22to6.17and175.25to256.96t C-hm"2, respectively. However, there were no significant differences in distribution proportions of C densities among different stand age classes in terms of the vegetation, detritus, and soil C pool, which accounted for(mean±SD)24.20±4.93%,1.69±0.66%, and74.11±5.21%of the total C densities, respectively.(2) In hard broad-leaved mixed forest, the differences in C densities were significant among the different stand age classes (young, half-mature, near-mature forest) in terms of the forest ecosystem, detritus, and soil C pool. However, there were no significant differences in C densities among different stand age classes in the vegetation C pool. The C densities of the forest ecosystem, vegetation, detritus, and soil C pool ranged from224.18to345.32,56.29to85.93,3.10to7.20and164.79to252.19t C-hm"2, respectively. There were no significant differences in distribution proportions of C densities among different stand age classes in terms of the vegetation, detritus, and soil C pool, which accounted for25.58±5.49%,1.92±0.87%, and72.50±5.57%of the total C densities, respectively.(3) The C densities of the ecosystem C pools increased with stand age class in soft broad-leaved mixed forest. The C densities of the overstory were ordered:young forest<half-mature forest<near-mature forest<over-mature forest<mature forest. There were significant differences in the C densities of the overstory components (except branches) among different stand age classes. The change of the leaves and stems C densities was consistent with the overstory. The C densities of the roots increased with stand age class. There were no significant differences in the distribution proportions of C densities in the overstory components among different stand age classes. The distribution proportions of C densities in stems and roots accounted for>62%and19%of the overstory, respectively. No obvious patterns were observed in C densities of the tree(DBH<2cm)+shrub layer and herb layer among different stand age classes in vegetation C pool. The C density of litter was significantly different among stand age classes, and the C densities increased with stand age class in the detritus C pool. No significant differences were found for woody debris C density among stand age classes.(4) The C densities of the ecosystem components (except herb layer) were ordered:young forest<half-mature forest<near-mature forest in hard broad-leaved mixed forest. There were no significant differences in C densities of the vegetation C pool components among different stand age classes. There were no significant differences in the C densities of the overstory components (except roots) among different stand age classes. The C densities of the stems, branches and roots increased with stand age class. The change of the leaves C densities was little. There were significant differences in the distribution proportions of C densities in the overstory components among different stand age classes. The distribution proportions of C densities in the stems and roots accounted for>57%and19%of the overstory, respectively. The C density of litter was significantly different among stand age classes.No significant differences were found for woody debris C density among stand age classes.(5) For the same stand age class, woodland characteristic differences in different areas to a certain degree have influence on C densities and the distribution proportion of ecosystem C pools in soft broad-leaved mixed forest in Heilongjiang Province, and have a certain degree influence on C densities of the soil C pool and the forest ecosystem C pool in hard broad-leaved mixed forest in Heilongjiang Province. However, there were no significant differences in C densities of the vegetation and detritus C pools and distribution proportion of carbon density for ecosystem C pools in hard broad-leaved mixed forest between different areas.(6) The differences were significant in the C densities of forest ecosystem and soil C pools in near-mature forest between two broad-leaved mixed forest types (soft broad-leaved mixed forest and hard broad-leaved mixed forest). There were significant differences in the C densities of the leaves in young forest and the C densities of leaves and branches in half-mature forest between two broad-leaved mixed forest types. No significant differences were found for the C densities of other components.For the same stand age class, there were no significant differences in distribution proportions of ecosystem components C densities. There were more differences in distribution proportion of overstory components C densities.(7) Compared with different temperate forest types in Northeastern China, there are the larger C density and stronger C sink function for broad-leaved mixed forest in Heilongjiang Province. Basal area can accurately reflect the dynamic variations of forest C density with the forest age.In conclusion, our study shows that C density and distribution patterns in the broad-leaved mixed forest expressed comprehensive temporal and spatial dynamic variations among the different stand age classes. We expect that our in-depth analysis can provide scientific reference value for developing C density in an age-sequence models of broad-leaved mixed forest in Heilongjiang Province, provide important data for developing and validating C cycling models of temperate and boreal forest ecosystems, and provide basic data for accurate estimation of regional and national forest carbon storage.