Measuring And Modeling Seasonal Variations Of Carbon Dioxide And Water Vapor Exchange Of A Subtropical Man-planted Forest

Water, heat and carbon dioxide transfer processes in the forest ecosystem arefocused intensively by the international researches. Understanding the mechanism ofwater, heat and carbon dioxide transfer in the forest ecosystem is important forassessing the role of the forest ecosystem in the water and carbon cycle of theterrestrial ecosystem and learning the ecosystem process and function of forest inresponse to the drought and climate change. EALCO model is a process-based modelused to simulate carbon, water and energy exchange between the atmosphere and landsurface. In this study, EALCO model is parameterized to simulate the ecosystemcarbon, water and energy exchange process in the man-planted evergreen forest.Modeling result was tested by use of the flux tower measurements. The mainobjective of the study was to (1) investigate the driving factors of the carbon andwater fluxes of the ecosystem (2) study the effect of seasonal drought on carbonassimilation and the coupling of carbon with water (3) simulate and evaluate thecarbon and water exchange conditions under the scenarioes of climate change.Main results of the study were as follows:(1) EALCO model is parameterized and initialized to simulate the ecosystem carbon,water and energy exchange of the man-planted forest from 2003 to 2005. Simulatedecosystem water and energy exchanges are analyzed on half-hourly, daily, and annualtime scales and compared with tower eddy correlation flux measurements.The results show that the simulated two critical variables, T_c andψ_c, werereasonable compare to the observations. Comparing the canopy conductance with thecalculation from the Penman-Monteith equation we found that the modeling results of2003 and 2004 are just satisfied while the results of 2005 are overestimated. Thecorrelation between the simulated and observed results of net radiation for the 3 yearsis more than 0.98. Modeling result shows that the ratio between net radiationabsorption by the forest floor and by the canopies showed an obvious seasonal pattern,with monthly mean values ranging from 0.35 in May to 0.05 in December. The modelreproduced well the annual course of daily latent heat (LE) and sensible heat flux (H)above the forest canopy as measured by EC method at 2003 and 2004. Thecorrelations between the simulated and observed results of H and LE for the 3 yearsare 0.82 and 0.62, respectively. The computed daily-averaged forest floor latent heatdensity (LE_s) ranged between 5 and 40 W m-2 throughout the three years and theseasonal sum of LEs accounted for 8, 11 and 12% of that of LE_e in 2003, 2004 and 2005, respectively. The latent heat fluxes of the whole ecosystem accounted low valueof Rn at the beginning of the year (from 30 to 50%) and increase with the growing ofthe forest (from 50 to 80%). The daily-averaged sensible heat flux did not show adiscernible seasonality. Peak-averaged values often accrued in the early spring.(2) Simulated plant, soil and ecosystem CO₂ exchanges are analyzed on half-hourly,daily, and annual time scales and compared with tower eddy correlation fluxmeasurements and estimates from various authors.At half-hourly timescale, the simulated plant CO₂ exchange explained 67% of thevariance of the measured CO₂ fluxes derived from eddy correlation measurements.The simulated soil respiration was close to or higher than the soil chambermeasurements in the three years. At a coarser timescale, the model was verysuccessful in simulating variations of GPP, Re and NEP of the three years except forthe overestimation of GPP and Re in July and October of 2003 when during thedrought period. The simulated daily soil respiration was in consistent with thechamber measurements except for the year of 2005. Modeling results of thecomponent of ecosystem respiration show that the contribution of auto-respiration toecosystem respiration was 88% for the three years. As for autotrophic respiration,maintenance respiration was morn than growth respiration through the whole year andthe contributions of maintenance respiration to auto-respiration of 2003, 2004 and2005 were 77%, 72% and 76%, respectively. Modeling results of soil respirationcomponent show that root autotrophic respiration account for 70% of the soilrespiration.(3) The impact of seasonal drought on carbon and water exchanges of man-plantedforest was analyzed.Results show that deep soil water content decided the canopy conductancegenerally and GPP depended on canopy conductance to a large extent. Drought havemore intense impact on GPP than on Re, which lead to the decrease of net carbonexchange during the water stress period. Further analysis suggests that deep soil watercontent controls the canopy photosynthesis dramatically in sunny day before noontime during soil water stress. While after noon time both high temperature and deepsoil water content eliminate the GPP and their elimination percents are equal. Duringthe drought and high temperature period of 2003, ecosystem respiration decreased33%, which was caused by the decrease of plant autotrophic respiration and soilheterotrophic respiration. Among them the decrease of autotrophic respiration wasgenerally caused by the reducing of plant growth respiration. The net carbon exchange of 2003 was decreased due to the combination effect of summer drought and heatwave while in October of 2004 the water stress has more influence on ecosystemrespiration so the net carbon exchange was not influenced largely, from which we cansee the net effect of ecosystem carbon balance depends on how these two quantifiesare affected relatively to each other. In addition, drought also has impact on thecoupling of carbon and water fluxes.(4) Climate change effects on the carbon and water fluxes of the man-planted forest.The ecosystem model EALCO was used to evaluate possible changes inecosystem carbon and water exchange and accumulation under changes inatmospheric CO₂ concentration and accompanying changes in air temperature andprecipitation. Model results indicated that warming will cause increased ET andtherefore reduced soil water flux. Combination impact of warming and increased CO₂concentration will lead to reduced ET, which attributed to the reduced stomatalconductance under the high CO₂ concentration. Warming will cause increased growth.Compared to increased precipitation, the increase of CO₂ concentration improved theecosystem production effectively. Future CO₂ induced enhancements of grossphotosynthesis would not offset by temperature-induced increases in respiration, thecombined influences of the scenario resulted in a 23% increase in NEP.