Carbon budget and its response to environmental factors in young and mature poplar plantations along the middle and lower reaches of the Yangtze River, China

Although poplar forest is the dominant plantation type in China, there is uncertainty about the carbon budget of these forests across the country. The observations, performed in 2006, of two eddy covariance flux towers on a young poplar plantation (Yueyang, Hunan province) and a mature poplar plantation (Huaining, Anhui province) provide an opportunity to understand poplar CO 2 exchange at diurnal, seasonal and annual scales. The levels of net ecosystem carbon exchange (NEE) at these two sites are similar, but both ecosystem respiration (Reco) and gross ecosystem productivity (GEP) have significant differences, reflecting differences in climate and land management. Although more rain and lower photosynthetically active radiation (PAR) were recorded in the summer, the young plantation sequesters -476 g C m yr, compared to -465 g C m yr in the mature plantation. Daily maximum NEE values are also similar (9.98 g C m d vs. 9.75 g C m d). Differences in GEP and Reco between the two sites are greater. The young plantation assimilates 1629 g C m yr and releases 1153 g C m yr through respiration, while the corresponding values in the mature plantation are 1439 g C m yr and 976 g C m yr. Daily maximum GEP and daily maximum Reco of young plantation are 14.56 g C m d and 8.04 g C m d, which are 11% and 35% higher than the corresponding values of the mature plantation (13.10 g C m d, 5.97 g C m d). These two sites also had different parameters describing the responses of NEE on a half-hour time scale to PAR, but the responses of NEE, GEP and Reco to PAR and air temperature at daily and monthly scales do not differ among sites. These results suggest that a general parameterization could be sufficient at coarse time resolutions to model the response of carbon exchange to environmental factors in poplar forests from different climatic regions in China.