Carbon Density And Carbon Sequestration Capacity Of Six Temperate Forests In Northeast China

Forest ecosystem plays a key role in reducing the ongoing enhanced greenhouse effect and stabilizing the climatic system by sequestering the atmospheric CO2 into vegetation and soils. Quantifying forest carbon (C) storage and flux accurately is prerequisite to assess the contribution of forest ecosystem to the global carbon budget. Exploring forest C distribution and its influencing factors is also enssential to developing terrestrial ecosystem models and predicting responses of forest C cycling to global changes. In this study, we investigated six representative temperate forests with similar stand age (42-59 years old) and under same climate conditions in northeastern China. The forests were aspen-birch forest, hardwood forest, Korean pine plantation, Dahurian larch plantation, mixed deciduous forest, and Mongolian oak forest. The aims of this study were to:(1) examine inter-and intra-specific variations of C concentration ([C]) in biomass tissues for 10 co-occurring temperate tree species in the forests; and (2) measure C density, net primary production (NPP) and their distribution patterns so as to assess C sequestration capacity of the forests with forest inventory and allometry approaches. The main results were as follows:The mean biomass tissue [C] across the ten species varied from 47.1% in fine root to 51.4% in foliage. The mean stem [C] of the species was 49.9±1.3%(mean±SE). The weighted mean C concentration (WMCC) for the species ranked as:Amur cork-tree (Phellodendron amurense Rupr.) (55.1%)> Amur linden (Tilia amurensis Rupr.) (53.9%)> Korean pine (Pinus koraiensis Sieb. et Zucc.) (53.2%)> Manchurian ash (Fraxinus mandshurica Rupr.) (52.9%)> Manchurian walnut (Juglans mandshurica Maxim.) (52.4%)> Mongolian oak (Quercus mongolica Fisch.) (47.6%)> Dahurian larch (Larix gmelinii Rupr.) (46.9%)> Mono maple (Acer mono Maxim.) (46.4%)> white birch (Betula platyphylla Suk.) (46.1%)> aspen (Populous davidiana Dode)(43.7%) (43.7%). Failing to account for the inter-and intra-specific variations in [C] will introduce a relative error of-6.7% to+7.2% in estimates of biomass C stock from inventory data, of which 93% is attributed to ignoring the inter-specific variation in [C]. The WMCC of the dominant trees was negatively correlated to mean annual increment of biomass (MAI), suggesting that planting fast-growing tree species for C sequestration in afforestation and reforestation practices sacrifice some C gain from increasing MAI due to decreasing [C].There were no significant differences in the C densities of ecosystem components (except for detritus) although the six forests had various vegetation compositions under divergent site conditions. The differences, however, were significant when the C pools were normalized with stand basal area. The total ecosystem C density varied from 186.9 tC hm-2to 349.1 tC hm-2 across the forests. The C densities of vegetation, detritus, and soil ranged 86.3-122.7 tC hm-2, 6.5-10.5 tC hm-2, and 93.7-220.1 tC hm-2, respectively, which accounted for 39.7%±7.1% (mean±SD),3.3%±1.1%, and 57.0%±7.9% of the total C density, respectively. The overstory C pool accounted for>99% of the vegetation C pool. The foliage biomass, small root (diameter<5mm) biomass, root-shoot ratio, and small root to foliage biomass ratio varied from 2.08-4.72 tC hm-2,0.95-3.24 tC hm-2,22.0-28.3%, and 34.5-122.2%, respectively. The Korean pine plantation had the lowest foliage productive efficiency (total biomass/foliage biomass:22.6 g g-1) among the six forests, while the Dahurian larch plantation had the highest small root production efficiency (total biomass/small root biomass:124.7 g g-1). The small root C density decreased with soil depths for all forests except for the Mongolian oak forest, in which the small roots tended to be vertically distributed downwards. The C density of coarse woody debris was significantly less in the two plantations than in the four naturally regenerated forests. This study illustrates that the variability of C allocation patterns in a specified forest is jointly influenced by vegetation type, management history, local water and nutrient availability; it also provides important data for developing and validating carbon cycling models for temperate forests.The total NPP (TNPP) of the six forests varied from 615.9 to 860.4 gC m-2a-1, averaging 763.2 gC m-2a-1.There were no significant differences in the aboveground NPP (ANPP) among the six forests, although the forests had various vegetation compositions under divergent site conditions. However, the TNPP differed significantly among the forests, mainly attributed to the difference in the belowground NPP (BNPP). Additionally, the NPP of short-living tissues (NPPSL) (i.e., the tissues that assimilated resource) differed significantly. The allocation patterns of TNPP to BNPP or NPPSL were similar across the six forests except for the Dahurian larch plantation. More than one half and two thirds of TNPP was allocated to ANPP and NPPSL, respectively. The six forests had strongly carbon sequestration potential, The net ecosystem production (NEP) varied from 301.9 to 729.2 gC m-2a-1 across the six forests, suggesting the forests as strong C sinks.This comprehensive investigation on forest C density and sink strength provides important data for developing and validating C cycling models for the temperate forests, and scientific basis for forest C sequestration management in this region.