Seasonal variation of carbon exchange of typical forest ecosystems along the eastern forest transect in China

The long-term and continuous carbon fluxes of Changbaishan temperate mixed forest (CBS), Qianyanzhou subtropical evergreen coniferous forest (QYZ), Dinghushan subtropical evergreen mixed forest (DHS) and Xishuangbana tropical rainforest (XSBN) have been measured with eddy covariance techniques. In 2003, different responses of carbon exchange to the environment appeared across the four ecosystems. At CBS, the carbon exchange was mainly determined by radiation and temperature. 0℃and 10℃were two important temperature thresholds; the former determined the length of the growing season and the latter affected the magnitude of carbon exchange. The maximum net ecosystem exchange (NEE) of CBS occurred in early summer because maximum ecosystem photosynthesis (GPP) occurred earlier than maximum ecosystem respiration (Rθ). During summer, QYZ experienced severe drought and NEE decreased significantly mainly as a result of the depression of GPP. At DHS and XSBN, NEE was higher in the drought season than the wet season, especially the conversion between carbon sink and source occurring during the transition season at XSBN. During the wet season, increased fog and humid weather resulted from the plentiful rainfall, the ecosystem GPP was dispressed. The Q10 and annual respiration of XSBN were the highest among the four ecosystems, while the average daily respiration of CBS during the growing season was the highest. Annual NEE of CBS, QYZ, DHS and XSBN were 181.5, 360.9, 536.2 and -320.0 g·C·m-2·a-1, respectively. From CBS to DHS, the temperature and precipitation increased with the decrease in latitude. The ratio of WEE/Rθincreased with latitude, while Rθ/Gpp, ecosystem light use efficiency (LUE), precipitation use efficiency and average daily GPP decreased gradually. However, XSBN usually escaped such latitude trend probably because of the influence of the south-west monsoon climate which does not affect the other ecosystems. Long-term measurement and more research were necessary to understand the adaptation of forest ecosystems to climate change and to evaluate the ecosystem carbon balance due to the complexity of structure and function of forest ecosystems.

Characterization of CO_(2) flux in three Kobresia meadows differing in dominant species

Aims Kobresia meadows,the dominant species of which differ in different habitats,cover a large area of alpine grassland on the QinghaiTibetan Plateau and act as potential CO_(2) sinks.Kobresia meadows with different dominant species may differ in carbon sink strength.We aimed to test the hypothesis and to clarify the differences in CO_(2) sink strength among three major Kobresia meadows on the plateau and the mechanisms underlying these differences.Methods We measured the net ecosystem exchange flux(NEE),ecosystem respiration flux(ER),aboveground biomass(AGB)and environmental variables in three Kobresia meadows,dominated by K.pygmaea,K.humilis,or K.tibetica,respectively,in Haibei,Qinghai.NEE and ER were measured by a closed-chamber method.Environmental variables,including photosynthetic photon flux density(PPFD),air and soil temperature and air and soil moisture,were monitored during the above flux measurements.Important findings The measured peak AGB increased with soil water content and was 365,402 and 434 g dry weight m
2 for K.pygmaea,K.humilis and K.tibetica meadow,respectively.From the maximum ecosystem photosynthetic rate in relation to PPFD measured during the growing season,we estimated gross ecosystem photosynthetic potential(GEPmax)as 22.2,29.9 and 37.8 lmol CO_(2) m
2 s
1 for K.pygmaea,K.humilis and K.tibetica meadow,respectively.We estimated the respective gross primary production(GPP)values as 799,1063 and 1158 g C m^(-2) year^(-1) and ER as 722,914 and 1011 g C m^(-2) year^(-1).Average net ecosystem production(NEP)was estimated to be 76.9,149.4 and 147.6 g C m^(-2) year^(-1) in K.pygmaea,K.humilis and K.tibetica meadows,respectively.The results indicate that(i)the three meadows were CO_(2) sinks during the study period and(ii)Kobresia meadows dominated by different species can differ considerably in carbon sink strength even under the same climatic conditions,which suggests the importance of characterizing spatial heterogeneity of carbon dynamics in the future.