A Study On The Greenhouse Gas Dynamics And Carbon Balance In Tropical Forests Of Hainan And Xishuangbanna,China

Tropical forest ecosystem is an ideal place for ecological research because of its rich species diversity,huge biomass,complex ecosystem processes and high-efficient material circulation and energy flow.Tropical forests play an irreplaceable role in the global carbon cycle and global changes.However,it is not clear what impact human interference and climate change affect the carbon balance of tropical forests.China's tropical forests are mainly distributed in the southernmost of Yunnan and south of Hainan island.Compared with the forests near the equator,the forests in this two site are affected by the typical monsoon climate and have obvious seasonality,so it is an ideal land to study the forest carbon storage and carbon balance.In this study,the main research results are as follows:1.Through the measurement of methane flux in tropical forest soil over a period of two years,we found that the tropical forest soil in this region is a large and persistent methane sink,and showing obvious seasonal regularity.The methane sink of tropical lowland rainforest and tropical mountain rainforest were estimated to be 2.00 kg CH₄-C ha^-1 yr^-1and 1.15 kg CH₄-C ha^-1 yr^-1,respectively.The tropical lowland rainforest showed strong methane uptake in the dry season(-1.00 nmol m^-2 s^-1)and weak soil methane uptake in the wet season(-0.24 nmol m^-2 s^-1).The peak soil methane uptake rate was observed as-1.43 nmol m^-2 s^-1 in February,2018,the driest and coolest month during the past 24 months.Methane uptake was relatively weak in tropical montane rain forests(-0.30 nmol m^-2 s^-1).Soil temperature could not enhance the explanation of seasonal variation of soil methane fluxes on the top of soil moisture.Soil moisture was the dominant controller of methane fluxes,and could explain 94%and 45%seasonal variation of soil methane fluxes.A positive relationship between soil methane uptake and soil respiration was also detected,which might indicate co-variation in activities of methanotroph and roots and/or microbes for soil heterotrophic respiration.2.The diurnal variation of the two tropical forests at different elevations was not obvious,but there were strong seasonal dynamics.Soil moisture is the main controlling factor of soil respiration in diurnal pattern.The soil respiration rate of tropical lowland rain forest (2.40±0.85?mol m^-2 s^-1)was higher than that of tropical mountain rain forest(1.70±0.82?mol m^-2 s^-1).Soil moisture was the dominant controller of soil respiration,and could explain 67%and 71%seasonal variation of soil respiration fluxes in tropical lowland rain forest and tropical montane rain forest,respectively.Soil respiration was significantly different under different microhabitats.The soil respiration Q₁₀ values of tropical montane rain forest and lowland rain forest are 2.24 and 2.67 respectively, showing high temperature sensitivity,which is related to special geographical location and climatic conditions.3.Soil respiration was separated into two components using the trenching method.Each component was measured at multiple temporal scales and in different microhabitats.Trenching has a limited effect on soil temperature but considerably affects soil water content due to the exclusion of water loss via tree transpiration.Soil autotrophic respiration and heterotrophic respiration have different diurnal dynamic patterns and totally opposite responses to temperature,which is an unexpected diurnal variation rule and may be related to soil moisture diurnal dynamic.In different microhabitats, autotrophic respiration and heterotrophic respiration have significant differences and obvious seasonal rules.Soil temperature is the main controlling factor of soil respiration. The soil respiration Q₁₀ value derived by the Q₁₀ model is 2.54,which shows high temperature sensitivity.Heterotrophic respiration accounted for 57.01%of total soil respiration,and its contribution was slightly higher than that of autotrophic respiration. Soil respiration plays an important role in the evaluation of the carbon balance of forest ecosystem.4.The forest biomass and carbon storage of tropical forests in this region were estimated by using 38 field plots in hainan island.It was found that these secondary forests were moderate carbon sinks with an average size of 1.96-2.17 t C ha^-1 yr^-1.The increase in biomass was mainly contributed by medium-sized(10cm?DBH?35cm)trees,unlike those selectively harvested in the amazon.Tree mortality accounts for almost 30%of the biomass and plays a important role in biomass accumulation estimates.Tree mortality is highly correlated with tree size.The high mortality of young trees and seedlings may be related to competition caused by increased light exposure after logging.Regarding prospective biomass and rates of accumulation,recovery is not as rapid as in secondary forests of cleared land.Therefore,tropical forests are susceptible to logging operations and need careful forest management.5.The estimation method of biomass minimum sampling area based on species area curve is feasible,and the estimation and verification of Xishuangbanna tropical monsoon rain forest,Dinghu mountain subtropical evergreen broad-leaved forest and Changbai mountain cold temperate coniferous forest are respectively conducted.The biomass minimum sampling area at the acceptable error of 5%of the three forest types is 9.9,6.0 and 2.1 hectare,respectively.