Carbon Density And Carbon Flux Pattern Of Age-Sequence Secondary Forests In Eastern Mountainous Area Of Northeast China

As an important factor affecting forest restoration and ecological succession,disturbance affects the carbon dynamics and carbon sequestration potential of forest ecosystem by changing the vegetation community composition and microenvironment.In this study,we selected a chronosequences of secondary forests formed by disturbance in different years in Maoershan,i.e.,the near-mature secondary forest（NS,64 years）formed by natural succession after clear-cutting of the mixed-broadleaved Korean pine forest,the half-mature secondary forest（MS,32 years）formed by natural succession after clear cutting of NS,disturbed young secondary forest（YD,15 years）formed by natural succession after clear cutting of NS and the soil has been disturbed for 15 years,and young secondary forest（YS,9 years）formed by natural succession after clear cutting of NS.The carbon density and carbon flux of each component of ecosystem were determined by forest inventory and allometric growth equation.The aim is to quantify carbon density,distribution pattern and carbon sink effect in temperate forest restoration process.The results show that:Soil disturbance significantly changed the soil organic carbon content.the fluctuation range of soil organic carbon content at each soil layer from 0 to 50 cm in YS,YD,MS and NS was 8.85-64.97 mg/g,7.01-24.74 mg/g,9.02-45.07 mg/g,7.04-86.91 mg/g,respectively;and the soil organic carbon content decreased significantly with the deepening of the soil layer.The organic carbon content of the topsoil（0-20 cm）was significantly different among the treatments（P<0.01）,while the organic carbon content of the subsoil（20-50 cm）was not significantly different.The organic carbon content of the soil was significantly negatively correlated with the soil bulk density.The fluctuation ranges of total carbon density,vegetation carbon density,soil carbon density and detrite carbon density were 75.79-200.69 t/hm²,10.21-109.12 t/hm²,55.91-66.40t/hm² and 0-25.17 t/hm²,respectively.The total carbon density showed a significant increase with stand age,with the main contribution being vegetation carbon density,which increased significantly with stand age（P<0.01）,while the difference of soil carbon density was not significant（P>0.05）.The contribution of tree species in different successional period to vegetation carbon density changed significantly.The carbon density and its contribution of late successional species increasing significantly with stand age,while no significant differences were found for pioneer species.The mean annual increase in vegetation carbon density（MAI）tended to increase,and then decrease with stand age,with YS being the lowest(1.13 t·hm^-2·a^-1)and YD being the highest(3.04 t·hm^-2·a^-1);this trend was mainly attributed to the change of pioneer species in succession.These results suggest that the rapid recovery of pioneer species after logging disturbance enhances carbon fixation in forest vegetation in the short term.Soil surface carbon flux was positively correlated with soil 5 cm temperature.The annual carbon flux and net primary productivity of YS,MS and NS ranged from 664.59 to 865.41g·m^-2·a^-1 and from 828.85 to 978.66 g·m^-2·a^-1.The net ecosystem productivity ranged from270.08 to 398.53 g·m^-2·a^-1.In general,secondary forests of all stand ages showed strong carbon sink,but there was no significant difference among the secondary forests（P>0.05）.These findings indicated that natural succession after clear-cutting had no significant effect on soil organic carbon.The medium-and long-term disturbance to the temperate forest soil changed its microenvironment and bulk density,leading to the release of surface organic carbon.In the process of vegetation restoration,pioneer species promoted short-term carbon sequestration,while later species determined vegetation carbon density and carbon sink potential.This study provides data support and theoretical basis for the evaluation and estimation of carbon sequestration and management of temperate forest ecosystems in Northeast China.