Abstract
Evaluation of carbon dioxide (CO2) sinks in forest areas of East and Southeast Asia (especially tropical regions) is important for assessing CO2 budgets at the regional scale. To evaluate the CO2 flux of large forest areas, we collected vertical CO2 profiles over the forest using a CO2 sonde and measured surface CO2 concentrations around the forest using continuous CO2 measurement equipment. These observations were performed over a typical northern forest (Hokkaido) in Japan, a subtropical forest island (Iriomote Island) in Japan, and a tropical forest in Borneo Island. We detected the differences in CO2 vertical profiles between dawn and daytime, and at the upwind and downwind sites of the forests with the observational results from the CO2 sonde. We also clarified that CO2 concentrations during daytime at the downwind sites (affected by the forest) were systematically lower than those at the upwind sites (not affected by the forest). In contrast, CO2 concentrations during dawn at the downwind sites were larger than those at the upwind site. We estimated the CO2 fluxes (μmol m−2 s−1) at dawn and daytime of the forests from these observational results. The CO2 fluxes of Borneo’s forest were very large (16.5 and −37.7 at dawn and daytime, respectively), whereas the CO2 fluxes of the forests in Hokkaido and Iriomote were lower (3.9 to 11.8 at dawn and −11.8 to −15.0 at daytime). These evaluated values were consistent with fluxes measured by the eddy-covariance method in the same region. Thus, use of the CO2 sonde to collect observations of CO2 vertical profiles was considered to be an effective method to verify CO2 absorption and emission in large forest areas. This method can also be used to evaluate dynamic CO2 absorption and emission processes in tropical forests.
Highlights
The estimate of the uptake of atmospheric CO2 by the terrestrial biosphere (2.6 ± 1.2 PgC yr−1 in terms of the global annual average from 2000 to 2009) (IPCC, 2013) is associated with a relatively large uncertainty compared to that by the ocean (2.3 ± 0.7 PgC yr−1), and this uncertainty is partly due to the difficulties associated with estimating the variation of the land flux
We evaluated the height of the stably stratified planetary boundary layer (SBL) at dawn and the convective planetary boundary layer (CBL) in daytime at the time when the sondes were launched from the level of the maximum vertical gradient in potential temperature, which is indicative of the transition from a convectively less stable region below to a more stable region above
The CO2 sondes launched from TSO and TKB in Hokkaido flew towards the east–north-east, and the sonde launched from TSO passed over TKB after 30 min and arrived at the sea after 60 min (Fig. 3a)
Summary
The estimate of the uptake of atmospheric CO2 by the terrestrial biosphere (2.6 ± 1.2 PgC yr−1 in terms of the global annual average from 2000 to 2009) (IPCC, 2013) is associated with a relatively large uncertainty compared to that by the ocean (2.3 ± 0.7 PgC yr−1), and this uncertainty is partly due to the difficulties associated with estimating the variation of the land flux. Variations in the land flux are driven by the effects of disturbances and changes induced by natural and anthropogenic origins, such as changes in temperature and precipitation (Malhi and Wright, 2004), forest fires, logging practices and plantings (Amiro et al, 2006; Ramankutty et al, 2007; Hirata et al, 2014); these changes cannot be monitored at different temporal and spatial scales because of the complicated features of terrestrial surfaces. In Southeast Asia, tropical forests are very important in terms of emissions from frequent large forest fire events and continuous land use change
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