Simulation Of The Effects Of Forest Succession And Climate Change On Forest Carbon Cycling

Forest ecosystem is the most complex part of terrestrial ecosystem.It plays an important role in regional and terrestrial carbon cycle.The natural secondary forest in Northeast of China was formed after the broad-leaved Korean pine forest,a zonal climax community,which was damaged by different degrees of cutting,burning and other disturbances.Forest in Northeast of China is mostly still in the stage of secondary succession recovery.However,it is difficult for the broad-leaved Korean pine forest recovered from secondary forest through naturally succession at present,due to a lack of Korean pine seedlings for succession.It will be difficult for this region to recover to the naturally broad-leaved Korean pine forest without artificial regeneration.Otherwise,it will stay at the sub-climax stage such as hard broad-leaved forest dominated by Fraxinus mandshurica,Juglans mandshurica,and Juglans mandshurica.The mechanism in carbon cycle changing with the vegetation successional dynamics is a hot issue in ecological study.Therefore,it is necessary to study the impact of forest succession on carbon cycle in order to predict the terrestrial carbon sink function under the background of global climate change.Study on global terrestrial ecosystem carbon cycle simulation has been carried out for a long time.However,forest succession is insufficient among most global vegetation dynamic models.In order to make IBIS model better reflect forest vegetation dynamics and adapt to forest succession of broad-leaved red pine forest in Northeast China,Visual Fox Pro and Visual C++modeling techniques were integrated in this study.A Vegetation Succession Dynamic Model(VSDM)applicable to IBIS model was constructed,and the model relationship between VSDM and IBIS was established by vegetation dynamic parameters.The VSDM-IBIS can simulate the real forest succession process in the VSDM model,and simulate the impact of the real forest succession on the carbon cycle in the IBIS.The established VSDM-IBIS was used to simulate the vegetation dynamic composition and carbon cycle characteristics at different forest succession stages in the study area,and the simulation results of VSDM and IBIS models were verified by the measured data.According to the current secondary succession status of broad-leaved pine forest,three succession hypotheses were made:(1)Forest succession with stable climatic conditions and no sufficient Korean pine seedlings;(2)The future climate condition is stable,there are enough Korean pine seedlings,and the initial forest is aspen-birch forest;(3)The future climate conditions is stable,there are enough Korean pine seedlings,and the initial forest is hardwood forest.Firstly,the carbon cycle dynamics of three forest succession hypotheses were simulated under the condition of constant climate,and the influence of forest succession itself on the carbon cycle was discussed.Then,the effects of climate change on carbon cycle at each forest succession stage were simulated and discussed under the condition of no forest succession according to IPCC-AR4 climate change scenario.Finally,the carbon cycle response of natural secondary forest under the combined action of forest succession and climate change in the future study area was simulated.Main conclusions:(1)The VSDM model can simulate the real dynamic process of forest vegetation according to the climate,tree species and site parameters of simulated plots.Individual tree is the minimum simulation unit with specific spatial coordinates.The dynamic process of regeneration,growth and death of all individual trees in the region is simulated annually.Shading and crowding effect is based on the relative spatial positions between central trees and neighbors,reflecting the competition between individual trees for light and nutrition.The VSDM model can simulate forest succession from bare plot or existing stands.The annual simulation results of VSDM,converted to vegetation dynamic parameters,are directly read by IBIS.IBIS simulates the carbon cycle dynamics according to the annual vegetation dynamic parameters from VSDM.(2)IBIS achieved a good agreement on the daily soil respiration in the growing season.The fitting effect between the simulated value and the observed value passed the significance test of0.001,and the R~2 of the simulated value and the measured value of soil respiration daily flux was0.4352.The R values of simulated and measured daily fluxes of soil respiration at different forest succession stages were 0.711 for aspen-birch forest,0.648 for hardwood forest,0.662 for mixed forest and 0.712 for oak forest(P<0.001),respectively.The difference between simulated and observed values was less than 20%,and the simulation results of soil respiration and NPP fluxes at different forest succession stages were basically consistent with the measurement.The VSDM model was verified by a large number of sample plot survey data,and the R value between the simulated stand break area and the average stand break area was 0.934(P<0.001)with a mean simulation deviation of 1.7%;The simulation of tree species coexistence relationship by VSDM model is consistent with the real law,which indicates that the simulation of forest spatial pattern by VSDM is consistent with the real forest spatial pattern.(3)The impact of forest succession on NPP is nonlinear.At the same forest succession stage,NPP increases with the increase of forest age.When NPP increase to peak,it will be affected by succession progress.Broad-leaved Korean pine forest has the potential to increase NPP limit.Under the SERS A2a climate change scenario,the NPP of poplar birch forest,hardwood broad-leaved forest,mixed wood forest,and Broad-leaved Korean pine forest will increase by-1.8%,0.4%,-3.2%,and 1.7%,respectively.Under the SERS B2a climate change scenario,NPP will increase by 25.5%,31.1%,25.7%,and 22.2%,respectively.Forest succession has the ability to mitigate the negative impact of climate change on NPP.(4)The influence of forest succession on soil respiration is reflected in the time of forest development and the stage of forest succession.With the forest age increasing,the annual average value of soil respiration showed a significant increasing trend(P<0.05).The proportion of autotrophic respiration and heterotrophic respiration is adjusted by forest types in different succession type.Under the SERS A2a scenario,the Rs of poplar birch forest,hardwood broad-leaved forest,mixed wood forest,and broad-leaved Korean pine forest will increase by 13.1%,16.9%,20.8%,and 8.9%(P<0.05),respectively;In the SERS B2a scenario,Rs will increase by13.1%,17.4%,20.1%,and 13.1%(P<0.05),respectively.Under the SERS A2a and SERS B2a climate scenarios,the increase in Rs affected by forest succession was 22.2%and 33.1%(P<0.05),respectively.(5)Broad-leaved pine forest succession is an important factor in ensuring the net carbon sink of forests.Under the SERS A2a climate scenario,the NEP of poplar birch forest,hardwood broad-leaved forest,mixed wood forest,and broad-leaved pine forest increased by 3.4%,4.5%,-0.1%,and 0.1%(P<0.05),respectively,compared to the baseline period.Under the SERS B2a climate scenario,the NEP of poplar birch forest,hardwood broad-leaved forest,mixed wood forest,and broad-leaved pine forest increased by 115.3%,60.6%,46.5%,and 40.1%(P<0.05),compared to the baseline period,respectively.In conclusion,forest succession can change stand structure,promote NPP,and regulate the ratio of Ra and Rh.Therefore,Rs decreases with succession and NEP increases with succession.Under the background of climate change,broad-leaved pine forest is the key to the continuous increase of carbon sink in this region.The soil respiration of broad-leaved red pine forest in the late succession period is less affected by climate than that in the early succession period,so that the NEP of broad-leaved pine forest maintain considerably positive value in the late succession.The results of this study are helpful to further understand the characteristics of carbon and nitrogen cycling in temperate forest soil,and provide scientific support for climate change assessment and prediction.