To determine the means and variations in CH4 uptake and N2O emission in the dominant soil and vegetation types to enable estimation of annual gases fluxes in the forest land of Japan, we measured monthly fluxes of both gases using a closed‐chamber technique at 26 sites throughout Japan over 2 years. No clear seasonal changes in CH4 uptake rates were observed at most sites. N2O emission was mostly low throughout the year, but was higher in summer at most sites. The annual mean rates of CH4 uptake and N2O emission (all sites combined) were 66 (2.9–175) µg CH4‐C m−2 h−1 and 1.88 (0.17–12.5) µg N2O‐N m−2 h−1, respectively. Annual changes in these fluxes over the 2 years were small. Significant differences in CH4 uptake were found among soil types (P < 0.05). The mean CH4 uptake rates (µg CH4‐C m−2 h−1) were as follows: Black soil (95 ± 39, mean ± standard deviation [SD]) > Brown forest soil (60 ± 27) ≥ other soils (20 ± 24). N2O emission rates differed significantly among vegetation types (P < 0.05). The mean N2O emission rates (µg N2O‐N m−2 h−1) were as follows: Japanese cedar (4.0 ± 2.3) ≥ Japanese cypress (2.6 ± 3.4) > hardwoods (0.8 ± 2.2) = other conifers (0.7 ± 1.4). The CH4 uptake rates in Japanese temperate forests were relatively higher than those in Europe and the USA (11–43 µg CH4‐C m−2 h−1), and the N2O emission rates in Japan were lower than those reported for temperate forests (0.23–252 µg N2O‐N m−2 h−1). Using land area data of vegetation cover and soil distribution, the amount of annual CH4 uptake and N2O emission in the Japanese forest land was estimated to be 124 Gg CH4‐C year−1 with 39% uncertainty and 3.3 Gg N2O‐N year−1 with 76% uncertainty, respectively.

To clarify the reason for the higher CH 4 uptake rate in Japanese forest soils, twenty-seven sites were established for CH 4 flux measurement. The first order rate constant for CH 4 uptake was also determined using soil core incubation at 14 sites. The CH 4 uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH 4 uptake rates ranged from 2.7 to 24.8 kg CH 4 ha -1 y -1 (the average of these rates was 9.7 or 10.9 kg CH 4 ha -1 y -1 , depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH 4 uptake rate correlated closely with the CH 4 oxidation rate of the topsoil (0-5 cm) in the study sites. The CH 4 oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio (p \ 0.05, R 2 = 0.684). This result and comparison with literature data suggest that the high CH 4 uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH 4 uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH 4 uptake rate in temperate forests was estimated to be 5.4 Tg y -1 (1 SE is 1.1 Tg y -1 ) on a continental basis.

The individual growth of tree diameter at breast height (dbh) is analyzed in an even-aged plantation of Cryptomeria japonica from stand age of 45 to 94 years, to examine how the growth of individual trees has been affected by the changes in spacing resulting from thinning operations. At any age, a significant proportion (0.37-0.46) of the variation in dbh growth during a 5-11-year period was explained by dbh at the beginning of the period, probably due to greater leaf mass of larger trees. Next, either one-sided or two-sided competition was added to the model, by calculating the basal area (BA) of neighboring trees around each tree within a given radius or BA for trees having larger dbh than the focal tree within the radius. After preliminary analyses, a radius of 8 m was selected as the critical range for tree competition. Although both types of competition explained a significant proportion (0.09-0.43) of growth variation, one-sided competition was not significant at ages greater than 54 years. Based on the model at 45 years of age, the initial deviation of growth rate for each tree from the predicted rate was calculated and added to the models as a third variable. This raised the coefficient of determination up to 0.50-0.74. These findings have practical significance for forest plantation management, particularly for controlling the growth of standing trees via thinning, to produce high-quality timber in the future.

-The CO 2 fluxes from the forest floor were measured using a closed chamber method at 26 sites from 26 • N Lat. to 44 • N Lat. in Japan. Seasonal fluctuation in CO 2 flux was found to correlate exponentially with seasonal fluctuation in soil temperature at each site. Estimate of annual carbon emission from the forest floor ranged from 3.1 to 10.6 Mg C ha −1 . The emission rate of soil-organic-carbon-derived CO 2 , obtained by incubation of intact soil samples, correlated closely with the carboxymethylcellulase (CMCase) activity in the soil. The sum of cool-water soluble polysaccharides, hot-water soluble polysaccharides, hemicellulose, and cellulose content in the soil was greater at the sites with low CMCase activity than that at the sites with high CMCase activity. Because the sites in cooler-climate sites had a high content of easily decomposable soil organic carbon and organic litter, the potential increase in CO 2 efflux from forest floor with increasing soil temperature would be greater in cooler-climate sites.cellulose / Japanese forest / soil organic carbon / soil respiration Résumé -Fort potentiel d'accroissement de flux de CO 2 issu de la surface du sol forestier en relation avec le réchauffement global dans les régions fraîches du Japon. Le flux de CO 2 issu du sol forestier a été mesuré dans 26 sites, allant du 26 • au 44 • de latitude Nord dans l'archipel japonais, en utilisant la méthode des chambres fermées. Il a pu être mis en évidence que la fluctuation saisonnière du flux de CO 2 était corrélée de façon exponentielle avec celle de la température du sol de chacun des sites étudiés. L'estimation annelle de l'émission de carbone venant du sol variait de 3,1 à 10,6 Mg C ha −1 . Le taux d'émission de CO 2 obtenu par incubation d'échantillons intacts de sol est corrélé positivement avec l'activité de la carboxyméthacellulase (CMCase), dans le sol. La somme totale des polysaccharides solubles dans l'eau froide, des polysaccharides solubles dans l'eau chaude, des hémicelluloses et de la cellulose contenus dans le sol était plus grande dans les sites caractérisés par une faible activité CMCase que dans les sites avec une forte activité CMCase. Du fait que les sites en climat frais ont un contenu élevé en carbone organique du sol facilement décomposable et une litière organique, le potentiel d'accroissement du flux de CO 2 avec l'accroissement de la température du sol devrait être plus grand dans les sites à climat frais.cellulose / forêt japonaise / carbone organique du sol / respiration du sol

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