Seasonality of net ecosystem carbon exchange in two major plant functional types in Korea

The processes in terrestrial ecosystems form a dynamic boundary interface within the earth climate system. The ecosystems in Asia, which are characterized by their diverse plant functional types, are currently undergoing the most rapid land-use changes, likely changing the feedback mechanism in land-atmosphere interaction. In this paper, we report seasonal variability of carbon balance over two major plant functional types in Korea: (1) a deciduous forest in a complex terrain (GDK site) and (2) a farmland with heterogeneous mosaic patches in a relatively flat terrain (HFK site). Our analysis is based on the year 2006, which was considered near normal in the sense of climate change and was of high quality data. Annually integrated values of gross primary productivity (GPP), ecosystem respiration (RE), and net ecosystem CO₂ exchange (NEE) at the GDK site were 835, 746, and -87 g C m^-2 whereas GPP, RE and NEE at the HFK site were 1003, 993, and -10 g C m^-2, respectively. Despite its greater GPP, the HFK site was a weaker carbon sink than the GDK site because the RE was as large as the GPP. The annual patterns of each carbon budget component had a mid-season depression, showing two distinctive peaks with different timing and magnitude for the two sites. Such a different bimodal seasonality was associated with different timing and intensity of the disturbances such as summer monsoon and the subsequent passing of typhoons later in the season. As far as we know, this is the first observation of the occurrence of mid-season depression of NEE, which is likely due to the reduced GPP rather than the enhanced RE. Considering the current changes in hydrological cycles in the Asian region, further investigations along with flux measurements in the rain are needed to clarify the causal relationship of ecosystem properties (e.g., NEE) with disturbances (e.g., summer monsoon, typhoons) and management (e.g., crop rotation).