Characteristics And Mechanism Of Autotrophic Respiration In Rubber Plantation In Hainan Island

The pattern,differentiation and driving forces of terrestrial ecosystem carbon balance are the core issues in the study of global change.Forest ecosystem is one of the most important parts of terrestrial ecosystem,and its carbon storage accounts for 40%.Forest effectively maintains the global carbon balance.The forest vegetation is a large and heterogeneous system.Each individual and its organs are different at respiration rates which shows spatial heterogeneity.To clarify the characteristics of forest ecosystem respiration and its regulatory mechanism are the one of basic problems in the study of carbon balance.During the observation period from 2019 to 2020,the respiration volume of each component and population of rubber tree was measured by logging in vitro（VM）,and compared with the area expansion method（AE）and eddy covariance method（EC）at the same time.The variation characteristics of each component in rubber plantation in Hainan island were estimated by area expansion method.The influencing factors and mechanism were explored.The eddy covariance method was used to measure the respiration of rubber plantation ecosystem（R_e）.We also discussed the dynamic relationship and mechanism between autotrophic respiration of each component in rubber plantation and R_e.The conclusions are as follows:1.The VM method and EC method will bring uncertainty and deviation to the data due to objective factors,so the AE method is highly feasible.On the experimental day,the VM method was 47.97%higher than EC,and 43.08%higher than AE method.The sample trees was cut down and the water loss taked away the CO₂ stored in the tree.Thus,the measured value was larger than AE and EC measurement.In addition,VM method due to human interference,environment,instruments and other objective factors also bring data uncertainty.VM method requires destructive sampling,which is almost impossible to carry out in the primary forest of nature reserves.EC method has the advantages of fast response without destroying plant communities and continuous measurement.However,EC method requires sufficient underlying vegetation surface and suitable turbulent exchange.Due to the limitation of geographical and climatic conditions,EC method lacks of atmospheric turbulence exchange caused by topography at night.In the AE scheme,the chamber method is used to measure the respiration flux per unit area of different components in the forest ecosystem（roots,stems and leaves）.Then,the respiration of stand level and whole community level are calculated by the tree shape parameters.This method does not require destructive sampling and highly feasible.However,the premise needs to clarify the spatial and temporal characteristics of autotrophic respiration in each organ and its influencing mechanism.In this experiment,the rubber plantation composed of a single species which the planting varieties and cultivation management were unified.So,the daily monitoring and expansion are simplified.This area expansion method was feasible.2.The respiration of canopy,stem and root in rubber plantation was measured and estimated by in-situ monitoring.The factors and mechanisms were studied.The spatial and temporal characteristics of autotrophic respiration in rubber plantations were seasonal,which were the result of the interaction of environmental and physiological factors.Through the analysis of datas,the diurnal and seasonal variation patterns of leaf respiration rate（R_L）were correlated with air temperature.There was an exponential relationship between R_Land air temperature.R_Lwas highly correlated with leaf area index（LAI）,and showed same trend.The tree stem respiration rate（E_s）showed a consistent seasonal trend with single peak at different vertical heights.As the sample position in the tropics,the rain and heat were over the same in summer.The trees grow all the year round,and the maintain respiration are relatively strong.The seasonal variation in stem respiration was related to temperature and sap flow.Removal the resistance of diffusion barriers in bark by manual tapping can increase the rate of CO₂ efflux.The variation characteristics of root respiration rate showed obvious seasonal variation in rubber plantation.Root respiration rate increased exponentially with soil temperature.The soil respiration and soil water content showed highly consistent seasonal changes.3.The seasonal characteristics of R_e was estimated by EC method.The influencing factors were discussed.The differents of the relationship between autotrophic respiration and ecosystem respiration were compared and analysed with AE method.（1）The R_e of rubber plantation showed a consistent seasonal trend with rainfall,photosynthetically active radiation（PAR）,daily mean air temperature,vapor pressure deficit（VDP）and daily mean soil temperature.（2）The rubber plantation ecosystem played the role of carbon sink and had obvious diurnal variation.It presented a"U"shape.The Net ecosystem exchange（NEE）value in the daytime was negative and positive at night,which reached its maximum value from 11:00 to 13:00.（3）By the EC method,the annual total ecosystem respiration of rubber plantation was 16.37 tC·hm^-2·a^-1in 2019,and 14.46tC·hm^-2·a^-1in 2020.By the AE method,the Re was 15.64tC·hm^-2·a^-1in 2019,and 15.73 tC·hm^-2·a^-1in 2020.（4）By the EC method,the annual autotrophic respiration R_aratio R_ein rubber forest was 61.48%in 2019 and 69.94%in 2020.However,the AE method estimated that R_a/R_ein rubber forest was 58.62%in 2019(R_leaf/R_e27.01%,R_branch/R_e6.15%,R_stem/R_e7.47%,R_root/R_e17.99%),and 69.22%in 2020(R_leaf/R_e28.65%,R_branch/R_e8.19%,R_stem/R_e9.75%,R_root/R_e22.63%).The R_a/R_eratio of two methods were more than 50%.Autotrophic respiration is an important part of community respiration in rubber plantations.We found that the total amount of autotrophic respiration between 2019 and 2020 was not significantly different in each month.The main reason may be that the study site is located in tropical region,and rubber trees grow all the year.The total amount of autotrophic respiration of rubber trees was stable.This study contained estimation method,continuous monitoring and mechanism discussion.The purpose was to improve the accuracy estimating of rubber plantation respiration in this region and to reveal the process of mitigating the change of atmospheric CO₂ concentration in the rubber plantation.As the observation period limited,it needs to study other species,sample size,model integration in the future.Exploring the intrinsic mechanism between environmental and physiological factors of autotrophic respiration is an important way to understanding the balance of forest carbon.