Carbon Storage And Productivity For Typical Forest Ecosystems In Xiaoxing’an Mountains,China

Accurate quantification of forest carbon storage and productivity has great significance for evaluating the role of forest ecosystem in global carbon cycle.In this study,we investigated five typical forest ecosystems in Xiaoxing’an Mountains.The forest ecosystems were primary mixed broadleaf-Korean pine(Pinus koraiensis)old-growth forest,spruce-fir valley forest,secondary birch forest,Korean pine plantation and Dahurian larch plantation.We quantified the effects of forest conversion(primary forest-secondary forest-plantation)on carbon storage,net primary productivity(NPP)and net ecosystem productivity(NEP);examined the carbon sequestration capability of spruce-fir valley forest which is being declining state;and analysed control factors of carbon storage in different carbon pool(vegetation carbon pool,detritus carbon pool,and soil carbon pool)by combined with specific forest ecosystem.Major conclusions were summarized as follows:(1)Total carbon storage was significantly different among the four forest ecosystems(primary mixed broadleaf-Korean pine old-growth forest,secondary birch forest,Korean pine plantation and Dahurian larch plantation),with the highest carbon storage(315.4 t C·hm-2)in the old-growth forest.Soil organic carbon accounted for the highest proportion(55%-70%)of the ecosystem carbon,followed by vegetation carbon storage(28%-43%)and detritus carbon storage(2-6%).Soil organic carbon storage in the two plantations was significantly lower than that in old-growth and secondary birch forests.The allocation(aboveground and belowground)of NPP,but not the total NPP,differed significantly among the four forest ecosystems.Litterfall(44%-60%)and fine root production(43%-47%)contributed the largest proportion of the aboveground and belowground NPP,respectively.The highest NEP was in the Korean pine plantation(328.0 g C·m-2·yr-1),followed by the old-growth(311.9 g C·m-2·yr-1)and secondary birch(231.1 g C·m-2·yr-1)forests,with the lowest NEP in the Dahurian larch plantation(187.9 g C·m-2·yr-1).(2)Total carbon storage of spruce-fir valley forest was 268.14 t C·hm-2 in 2011,and carbon storage of the vegetation,detritus and soil were 74.25,16.86 and 177.03 t C·hm-2,respectively.From 2006 to 2011,tree layer carbon storage decreased from 80.86 t C·hm-2 to 71.73 t C·hm-2.The average decrease proportion per year of carbon storage was 0.5%,2.7%and 3.7%for Abies nephrolepis,Picea spp.,and Larix gmelinii,respectively.Total NPP was 4.69 t C ·h m-2·yr-1.The ratio of belowground NPP and aboveground NPP was 0.56.Litterfall accounted for the largest proportion of total NPP.As the two most important carbon output approaches of forest ecosystems,the fluxes of heterotrophic respiration form soil and coarse woody debris(CWD)were 293.67 and 119.29 g C·m-2·yr-1,respectively.The NEP was 55.90 g C·m-2·yr-1.The results indicated that the spruce-fir valley forest being on declining state still had a certain carbon sink capacity.(3)In secondary birch forest being early successional stage,only stand density had linearly positive relationships with vegetation carbon storage and aboveground NPP(P<0.05).In mixed broadleaf-Korean pine old-growth forest being late successional stage,both vegetation carbon storage and aboveground NPP were significantly affected by stand density and species richness combined.Vegetation carbon storage increased first,and then decreased with increasing of stand density and species richness,i.e.the hump-shaped pattern(P<0.05).In contrast,aboveground NPP increased with increasing stand density(P<0.05),but its relationship to species richness was hump-shaped(P<0.05).The results indicated that the effects of stand density and species richness on carbon storage and aboveground NPP were influenced by forest stand succession.(4)Coarse woody derbis(CWD)was the main component of detritus carbon pool in spruce-fir valley forest,and its carbon storage was 13.25 t C·hm-2,and the carbon storage of CWD for Picea spp.,Abies nephrolepis and Larix gmelinii were 3.59,2.61 and 3.06 t C·hn-2,respectively.The carbon storage of CWD under different decay classes followed normal distribution,more concentrated in the classes II and III,which accounted for 42.7%and 35.4%of the total carbon storage of CWD,respectively.The carbon storage of CWD with different DBH classes also followed normal distribution,and focused on the classes 30-40 cm and 40-50 cm.The existing forms of the carbon storage of CWD were mainly composed of breakage at trunk,uprooted blow-down,standing die and breakage at rootstock.The carbon storage of CWD showed a high spatial heterogeneity,and it decreased with increased of stand mean diameter at breast height(DBH),maximum DBH and basal area(P<0.05).However,there were no significant relationships between the carbon storage of CWD and stand density,diversity index and evenness index.(5)The surface soil organic carbon concentration(SOC)in mixed broadleaf-Korean pine forest was 70.13 g·kg-1,and the coefficient of variation for SOC was 54.93%,suggesting a moderate variability.The SOC had a strong spatial autocorrelation,and the range of spatial dependence was 18.0 m.As the sampling density decreases,the spatial autocorrelation of SOC became weaker,and the spatial distribution of SOC tends to homogenization.Soil water content was the most important factor in explaining the spatial variability of surface SOC,followed by leaf litter mass,weighted decay class of fallen wood and soil pH,these factors can explain 51.9%of the spatial variation of SOC.The SOC was not significant related to the volume of fallen wood,but positive related to weighted decay class,suggesting that fallen wood quality(i.e.,decay stage)more than quantity,may be important contributing to SOC.No significant relationship was found between surface SOC and both stand structure and topographic factor.