Carbon Balance Of A Poplar Plantation Ecosystem In Daxing, Beijing

Carbon balance of terrestrial ecosystem has become an important issue in ecological research concerning about the severe climate change and global warming. Relative to other ecosystem, forests store vast amounts of C and play a dominant role in the carbon cycle, and therefore, many researchers are currently focusing on carbon sequestration and mechanisms of environmental control to this process. China's plantation area is 5.4×10⁷hm² in 2006 and forest ecosystem has turned into a carbon sink because of the national afforestation and reforestation activities these years. Poplar plantation distributes widely in China because of its fast growth and strongly adaptive ability to ecological environment. The poplar plantation occupies 7×10⁶ hm² which is 13% of the total plantation area, so the carbon immobility of the poplar plantation is very important for forest carbon in China. However, little work has been done on the dynamics and mechanism of carbon balance in plantations, especially on poplar forest. Based on biomass inventory and eddy covariance system measurement, we examined the carbon storage and carbon balance and its environmental response in a poplar (Populus× euramericana cv."74/76") plantation in Daxing, Beijing. The purposes of the study were â‘  to get the amount of carbon storage and carbon exchange between the atmosphere and poplar plantation ecosystem; â‘¡ to investigate the influence of environmental factors on poplar plantation carbon sink; and â‘¢ to find ways to get more confident results on net ecosystem change in forest ecosystem by comparing the results of the two methods. The main results were as follows:1) The statistical models of tree biomass of the forest ecosystem were developed based on the data from classical biomass inventory. Model results show that the biomass values were 25.2 Mg·hm^-2, 42.4 Mg·hm^-2, and 53.1 Mg·hm^-2 in 2005, 2006 and 2007, respectively. We found that there are significant differences in carbon content among the poplar tree organs, and the results were very similar to those by the coefficient 0.5 on the whole tree. The soil carbon accounts for more than 50% of the total forest ecosystem. The carbon storage slowly increased which indicates that the reforestation in the in lean soil area will not result in long-term carbon source. The carbon in storage is 55.11 Mg·hm^-2 in 2006, and the average of the carbon immobilization is 6.9 Mg·hm^-2 a^-1 for the young poplar plantation. The net ecosystem productivity is 9.38±2.62 Mg·hm^-2and 7.02±2.59 Mg·hm^-2 in 2005 and 2006 respectively, and rainfall distribution is the dominating factors of the tree biomass and carbon enrichment.2) We use the ordinary linear regression (OLS), energy balance ratio (EBR) and the distribution of frequency (8) statistic methods to examine the energy balance in the poplar plantation. The EBR is 89% in 2006 which shows that the measurement of eddy covariance system is reliable. The energy balance closure is better in the day than at night time, in the growing season than in non-growing season. It is probably the missing measure of the equipment and the low turbulence that cause the low EBR at night time. The slope of the OLS increased 7.6% when taking account for canopy heat storage(S), the S is an important available energy and should not be neglected in the poplar plantation. The carbon fluxes will be underestimated at both "HF" and "LF" conditions.3) Canopy photosynthesis and soil respiration (Rs) were measured with the Licor6400 Portable photosynthesis system. The daily change and seasonal variation is dominated by environmental and physiological variables in the poplar plantation. The soil respiration efflux shows positive correlation with the soil temperature and soil water content, and the Q10 is 2.01 between the soil respiration efflux and the temperature at 5cm depth.4) The threshold of friction velocity is 0.1m·s^-1, which is determined by the AVT (average value test) method. Ecosystem respiration (Re) shows high correlation with the air temperatureand the regression equation is Re = 0.021e^0.070Ta. The Rs rises from the least in January, up to 7.0g·m^-2d^-1 in the rapidly growing season. The poplar plantation ecosystem released 10.63 Mg·hm^-2 and the Rs/Re dropped from 98% to 50% from February to May.5) The poplar plantation ecosystem is a carbon sink in the day and a carbon source at night time, a weak source in the non-growing season and a strong sink in growing season. The NEE reaches its maximum in June, July and August, sequestered 88% of the total carbon in this period. NEE shows negative correlation with the net radiation (Rn), air temperature (Ta), soil temperature (ts), soil volume water content (vwc), leaf area index (LAI) and CO₂ concentration (gashound), and positive to vapor pressure deficit (vpd).We make the multiple regression analysis to these variables with the SAS program and find that the most significant variables are Ra, Ta and vpd at 0.01 significant level in the growing season, and the regression modelis NEE =-0.049Rn - 0.391Ta + 2.95vpd + 3.784.6) Determined by the water and energy variables, carbon balance of the poplar ecosystem could be viewed as three stages: single respiration, respiration rate above photosynthesis rate, respiration rate below photosynthesis rate. The average value of index Z is 1.34, which indicates that the ecosystem is a carbon sink. Although the Re lagged behind the GPP, there lies a good correlation with each other. GPP is 15.25 Mg·hm^-2, Re is 10.63 Mg·hm^-2, and NEE is 4.62 Mg·hm^-2 in the year of 2006.7) NEE measured by EC method is 4.62 Mg·hm^-2 which is similar with NEP calculatedfrom the biomass inventory to some extent. The method of biomass inventory is feasible in carbon budget and carbon exchange on large temporal scale, while the EC method is predominant in the research on the process and mechanism of carbon balance. The combination and tests of the two methods will contribute to the improvement of precision on carbon storage and carbon balance.The results provide not only the basic data of carbon storage and carbon balance, but the process and mechanism of carbon exchange as well, and this will helpful for estimating carbon sink of the poplar plantation in China. The study provides a reference on the management of plantation from the aspects of the carbon enrichment.