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This paper presents results of 1 year (from March 25, 2003 to March 24, 2004 of continuous measurements of net ecosystem CO 2 exchange (NEE) above a steppe in Mongolia using the eddy covariance technique. The steppe, typical of central Mongolia, is dominated by C 3 plants adapted to the continental climate. The following two questions are addressed: (1) how do NEE and its components: gross ecosystem production (GEP) and total ecosystem respiration (R eco ) vary seasonally? (2) how do NEE, GEP, and R eco respond to biotic and abiotic factors? The hourly minimal NEE and the hourly maximal R eco were À3.6 and 1.2 lmol m À2 s À1 , respectively (negative values denoting net carbon uptake by the canopy from the atmosphere). Peak daily sums of NEE, GEP, and R eco were À2.3, 3.5, and 1.5 g C m À2 day À1 , respectively. The annual sums of GEP, R eco , and NEE were 179, 138, and À41 g C m À2 , respectively. The carbon removal by sheep was estimated to range between 10 and 82 g C m À2 yr À1 using four different approaches. Including these estimates in the overall carbon budget yielded net ecosystem productivity of À23 to 1 20 g C m À2 yr À1 . Thus, within the remaining experimental uncertainty the carbon budget at this steppe site can be considered to be balanced. For the growing period (from April 23 to October 21, 2003), 26% and 53% of the variation in daily NEE and GEP, respectively, could be explained by the changes in leaf area index. Seasonality of GEP, R eco , and NEE was closely associated with precipitation, especially in the peak growing season when GEP and R eco were largest. Water stress was observed in late July to early August, which switched the steppe from a carbon sink to a carbon source. For the entire growing period, the light response curves of daytime NEE showed a rather low apparent quantum yield (a 5À0.0047 lmol CO 2 lmol À1 photons of photosynthetically active radiation). However, the a values varied with air temperature (T a ), vapor pressure deficit, and soil water content.

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Forest ecosystems in the Northern Hemisphere function as carbon (C) sinks for atmospheric carbon dioxide; however, the magnitude, location, and cause of the sinks remain uncertain. A number of field measurements of forest biomass and systematic national forest inventories in Japan make it possible to quantify the C sinks and their distribution. Allometric relationships between forest biomass and stem volume were obtained for the major forest types in Japan from 945 sets of direct field measurements across the country. These relationships were used to estimate the changes in C accumulations of aboveground biomass and total living biomass from 1947 to 1995 from the national forest inventories of 1947, 1956, 1961, 1965, 1975, 1980, 1985, 1990, and 1995. The results showed that the C accumulations have significantly increased during the last 50 years. The C density (C stock per hectare) and total C stock of aboveground biomass increased from 27.6 Mg C/ha and 611.7 Tg C in 1947 to 43.2 Mg C/ha and 1027.7 Tg C in 1995, respectively, and those of total living biomass increased from 33.9 Mg C/ha and 751.8 Tg C in 1947 to 53.6 Mg C/ha and 1274.8 Tg C in 1995. These increases were remarkable during 1976–1995, with a net increase of 5.6 Mg C/ha and 369 Tg C for the C density and total living biomass. These results suggest that Japan's forest vegetation is a significant C sink. In the past 20 years, living vegetation has sequestered 18.5 Tg C annually, 14.6 Tg C of which was accumulated in aboveground biomass. The total C sink for the whole forest sector (including nonliving biomass) of Japan was estimated as 36 Tg C/yr if using the net change ratio of nonliving biomass C to living biomass C derived from the United States and Europe. On the basis of average C sink per hectare, Japan's forests have a higher sequestration rate (0.77 Mg C ha−1 yr−1) than the average of the other northern countries (0.14–0.19 Mg C ha−1 yr−1). The expansion and regrowth of planted forests are two major causes for this increased C uptake; planted forests contribute ∼80% of the total C sink in Japan. The suitable oceanic climate for fast forest growth and effective forest management practice may be the principal factors for such a large sink.

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The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster in the past half-century in Japan, was reconstructed based on observations of the proximal pyroclastic density current (PDC) and fallout deposits. Witness observations were also used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which are characterized by massive, extremely poorly sorted, and multimodal grain-size distribution with 30-50 wt% of fine ash (silt-clay component). The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in the relatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As observed in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect of water in the plume on the aggregation increased with time and distance. Based on the deposit thickness, duration, and grain-size data, and by applying a scaling analysis using a depth-averaged model of turbulent gravity currents, the particle concentration and initial flow speed of the PDC at the summit area were estimated as 2 × 10 −4-2 × 10 −3 and 24-28 m/s, respectively. The tephra thinning trend in the proximal area shows a steeper slope than in similar-sized magmatic eruptions, indicating a large tephra volume deposited over a short distance owing to the wet dispersal conditions. The Ontake eruption provided an opportunity to examine the deposits from a phreatic eruption with a complex eruption sequence that reflects the effect of external water on the eruption dynamics.

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[1] We measured the net ecosystem CO 2 exchange (NEE) in an alpine meadow ecosystem (latitude 37°29 0 -45 0 N, longitude 101°12 0 -23 0 E, 3250 m above sea level) on the QinghaiTibetan Plateau throughout 2002 by the eddy covariance method to examine the carbon dynamics and budget on this unique plateau. Diurnal changes in gross primary production (GPP) and ecosystem respiration (R e ) showed that an afternoon increase of NEE was highly associated with an increase of R e . Seasonal changes in GPP corresponded well to changes in the leaf area index and daily photosynthetic photon flux density. The ratio of GPP/R e was high and reached about 2.0 during the peak growing season, which indicates that mainly autotrophic respiration controlled the carbon dynamics of the ecosystem. Seasonal changes in mean GPP and R e showed compensatory behavior as reported for temperate and Mediterranean ecosystems, but those of GPP max and R emax were poorly synchronized. The alpine ecosystem exhibited lower GPP (575 g C m À2 y À1 ) than, but net ecosystem production (78.5 g C m À2 y À1 ) similar to, that of subalpine forest ecosystems. The results suggest that the alpine meadow behaved as a CO 2 sink during the 1-year measurement period but apparently sequestered a rather small amount of C in comparison with similar alpine ecosystems.