Carbon Balance Of The Rubber Plantation Ecosystem In Hainan Island

Since the1980s, climate warming has become a global environmental problem, and carbon dioxide is the most important factor in global warming. Human activities lead to the global elevated CO2concerntration, thus result in global warming, and so reducing CO2emissions has become one of the world’s most important tasks for governments around the world. As a large developing country, China, the pressure of reducing CO2emissions is becoming more and more larger. For this part, to study the carbon balance of a variety of plantations, as an important bargaining chip of carbon trade negotiations, is particularly important. On the other hand, due to the price of the natural dry rubber increasing, the planting area of rubber trees has been expanded rapidly in tropic areas of China, which leads some negative impacts and produces a lot of controversy on rubber tress planting in tropic areas. In order to evaluate the carbon sink benefits of rubber plantation ecosystems correctly, using the combination of biomass survey method and eddy covariance method, we studied the carbon storage and balance and its environmental control mechanism of rubber plantation ecosystem in Hainan Island in2009-2013. And the results showed as follows:1. Biomass inventory method and model simulations were used to estimate and calculate each sub-carbon pools of the rubber plantation ecosystem in the western of Danzhou, Hainan Island, and characteristics of changes in soil respiration and its influencing factors were explored, too.The average biomass carbon pool of rubber tree vegetation was35.39tC·hm-2, the soil organic carbon pool94.96tC·hm-2, the litter carbon pools tC·hm-2, and the total amount carbon pool was133.21tC·hm-2. The annual growth of rubber tree biomass carbon pool was4.61tC·hm-2·a-1, the amount of understory vegetation was1.11tC·hm-2·a-1, dry rubber containing a total of1.41tC·hm-2·a-1, plus litter carbon increment2.86tC·hm-2·a-1, the net ecosystem exchange (NEE) of rubber plantation ecosystem was-9.99tC·hm-2·a-1.Forest ages of rubber plantation, management level, soil texture, litter accumulation and decomposition as well as tapping and other factors were the leading factors to affect the carbon accumulation of rubber plantation. And then some reasonable measures would be essential to take in order to improve the carbon storage and carbon sequestration of rubber plantation ecosystem.The diurnal changes of soil respiration (Rs) in rubber plantation at different seasons displayed a single peak curve, while the monthly changes showed a double peaks curve. After separating and quantifying each components of soil respiration, carbon emissions of each components showed the order as:Rh (soil heterotrophic respiration)> Rr (root respiration)> Rl (litter respiration)> Rm (mineral soil respiration). Total annual carbon emissions of soil respiration were between10.03t·hm-2·a-1～11.96t·hm-2·a-1. Soil temperature is the most important factor affecting soil respiration. According to the exponential regression equation of soil respiration and soil temperature, Q10values of rubber plantation soil respiration can be calculated out between1.14～2.37. Moisture often affected soil respiration rate by many factors.2. Three parts studies of the turbulence data quality evaluation, the energy balance closure analysis and flux footprint and its source regions analysis have showed that the fluxes observations of the rubber plantation ecosystem are feasible, and data obtained are reliable, which can be used for research requirements.The quality evaluation of3types fluxes data has been done with the combination method of steady-state testing and vertical wind integrated turbulence characteristic testing. The turbulent flux data quality of rubber plantation is relatively high, and52%to63%of the3types fluxes data was high-quality data, only10%to16%low quality data.Diurnal changes of the canopy net radiation, sensible heat flux and latent heat flux in rubber plantation ecosystem all showed a single peak rule. Most part of the net radiation energy was consumed for the latent heat of evaporation, and secondly for the sensible heat flux. Analyzing the energy balance of different time scales, energy balance of rubber plantation ecosystem ratio was about87%, with13%of the energy disappeared. And the reasons of the energy unbalance were analyzed. Based on the FSAM model, footprint and flux source area were calculated on the50m tower in rubber plantation ecosystem. At the unstable stratification, source areas were smaller than those under stable conditions, and source areas in the dormant season were larger than those in the growing season at the same level. In the prevailing wind direction110°-250°, at unstable atmospheric conditions in the growing season,80%information source located upwind in the range between0-758m, and at steady state conditions during the growing season,80%information sources located upwind range0-1858m. All the results showed that the fluxes observations of the rubber plantation ecosystem are feasible, and data obtained are reliable, which can be used for research requirements.3. Annual variations of environmental factors in rubber plantation were analyzed first and dynamic characteristics of the3fluxes components and their responses of environmental driving factors were studied then.The daily dynamic of air temperature showed a single peak curve in different seasons. The PAR (photosynthetic active radiation) showed a single peak curve whenever in rainy season or dry season. The VPD (vapour pressure defict) show a single peak curve in different seasons, with no obvious vapour defict all the year. The soil temperature showed a single peak curve, too, which was between18.79～27.62℃, and the volume water content of soil showed irregular curve all the year.The daily NEE dynamic of rubber plantation ecosystem in Hainan Island showed a single peak curve in all seasons. The daytime NEE was negative which means absorbing carbon, the nighttime NEE positive releasing carbon. The diurnal NEE variation of the year showed closely U shape curve. The total annual NEE of rubber plantation ecosystem was-11.10tC-hm-2·a-1, and its capacity of carbon fixation is very strong. The driving factors of rubber plantation ecosystem NEE are PAR, VPD and soil temperature under the ground5cm at half an hour scale, and the stepwise regression equation of NEE with factors is:NEE=-0.001PAR+0.096VPD-0.10T_-5-0.243. The NEE of rubber plantation ecosystem showed very notable negative correlation with the amount of precipitation and the average LAI (Leaf Area Index) of the month at month scale. In addition, the environmental CO2concentration and human activities affect the rubber plantation ecosystem NEE, too.The daily ecosystem respiration (Re) dynamic of rubber plantation ecosystem showed a single peak curve in all seasons. The Re of the whole year is positive which means releasing carbon. The Re amount and the change amplitude of daytime were obviously larger than nighttime. The diurnal Re changes of the year showed irregular curve. Monthly Re amounts of rainy season were larger than those of dry season. The strongest month of respiration in rubber plantation ecosystem was August, and the weakest December. And the total Re of the year was20.98tC·hm-2·a-1. At half an hour scale, the Re showed an exponental relation with the average air temperature (Ta), which can be calculated Q10as1.96. And the Re showed weake negative correlation with soil water content, weak positive correlation with VPD. At month scale, the Re had a notable positive correlation with the precipitation, a weak positive correlation with LAI. The ratio of soil respiration to ecosystme respiration (Re) was very low, only37.1%, and the vegetative respiration took proportion62.9%. Factors affecting vegetative respiration will show more impact to ecosystme respiration than those affecting soil respiration.The daily dynamics of gross ecosystem productivity (GEP) in rubber plantation was a single peak curve all seasons. Daytime GEP is positive, solar fixiation and CO2sequestratation, the night zero. There is no obvious midday depression of photosynthesis of GEP. The diurnal variation of GEP is irregular unimodal curve. The GEP of each month in rainy season is larger than it during the dry season. The strongest GEP month is August, the weakest February of the year. And the average total gross ecosystem productivity of rubber plantation ecosystem in Hainan Island is32.09tC·hm-2·a-1. There are positive correlations between the GEP with PAR, all the temperature factors and VPD at half an hour scale. And at month scale, there are positive correlations between the GEP with LAI and the amount of precipitation in rubber plantation ecosystem. The LUE (light use efficiency) of each month showed U shape, which is opposite with PAR.4. Study on carbon balance mechanisms of rubber plantation ecosystemRubber plantation ecosystem in Hainan is a large carbon sink which obtained with biomass inventory method and eddy covariance method, and the average annual carbon uptake is9.99～11.10tC·hm-2·a-1. The carbon balance of rubber plantation ecosystem is mainly affected by rubber trees’ growth characteristics and driving factors of external environment. Environmental factors include photosynthetic active radiation, temperature, moisture etc.. Compared with other forests, the photosynthetic capability of rubber plantation ecosystem is very strong (GEP is huge); and at the same time, the total respiration of the system (Re) is very strong, too; the net ecosystem exchange (NEE) is large. The yearly average Z value of the rubber plantation ecosystem is about1.53, the average value of CUE (carbon use efficiency) is0.35. That is indicated that Re takes a large proportion in GEP, but NEE a small proportion. In rubber plantation ecosystem,65.4%GEP is used for Re,34.6%for NEE (i.e. carbon sequestratation). In the65.4%ecosystem respiration,37.1%is used for soil respiration,62.9%for vegetation respiration.