To estimate the aboveground biomass of tropical secondary forests dominated by pioneer species, allometric equations to relate diameter at breast height with the dry mass of the aboveground organs of several pioneer species were developed. The aboveground biomass of secondary forests was estimated using four methods based on the allometric equations. Biomass estimated with an allometric equation for all species combined was equivalent to that estimated with species-specific allometric equations. However, the estimated biomass based on a general-purpose allometric equation was substantially higher than that using other allometric equations. The allometric equation for all species combined is suitable for estimating the biomass of a secondary forest from the view points of accuracy and labor.

It is acknowledged that exogenous nutrient addition often stimulates early-stage litter decomposition in forests and late-stage decomposition is generally suppressed by nitrogen addition, whereas the interactive effects of nutrient addition and abiotic environmental factors, such as climate, on decomposition remain unclear. The tea bag method, which was developed to provide the decomposition rate constant k of early-stage decomposition and stabilization factor S of labile materials in the late stage, is a potentially useful tool for examining the impacts of nutrient addition on both early- and late-stage litter decomposition and their interactions with climate. At a long-term (38-year) continuous fertilization experimental site (an Abies sachalinensis Fr. Schmidt stand) in Hokkaido, Japan, we examined whether a standard tea bag method protocol was sufficiently sensitive to reveal any impacts of nutrient addition on early- and late-stage decomposition. In addition, we tested the interactive effects of nutrient addition and climate on litter decomposition. The short incubation period of the tea bag method (ca. 90 days) enabled us to obtain decomposition data from the same location at three different times in a year, i.e., early summer, midsummer, and winter, providing an opportunity to test interactive effects. We demonstrated that the decomposition rate of rooibos tea and the decomposition rate constant k of early-stage decomposition were clearly stimulated by fertilization in midsummer, but no impacts were detected in other seasons, probably because the relative importance of nutrient availability was elevated in midsummer, during which decomposition rates were less constrained by temperature and moisture. The green tea decomposition rate and stabilization factor S, an index related to late-stage decomposition, were unaffected by fertilization. This was probably because the tea bag method does not take into account lignin degradation, which is considered a key factor controlling late-stage litter decomposition. Overall, the present study (i) successfully determined the interactive effects of nutrient addition and climate factors on litter decomposition by making full use of the tea bag method, and (ii) the results suggest that the tea bag method can be a suitable tool for examining the direct effects of nutrient addition and their interactions with environmental factors on early-stage litter decomposition, but not those on late-stage decomposition.

Forest soil is a huge reserve of carbon in the biosphere. Therefore to understand the carbon cycle in forest ecosystems, it is important to determine the dynamics of soil CO 2 efflux. This study was conducted to describe temporal variations in soil CO 2 efflux and identify the environmental factors that affect it. We measured soil CO 2 efflux continuously in a beech secondary forest in the Appi Highlands in Iwate Prefecture for two years (except when there was snow cover) using four dynamic closed chambers that automatically open after taking measurements. Temporal changes in soil temperature and volumetric soil water content were also measured at a depth of 5 cm. The soil CO 2 efflux ranged from 14 mg CO 2 m -2 h -1 to 2,329 mg CO 2 m -2 h -1 , the peak occurring at the beginning of August. The relationship between soil temperature and soil CO 2 efflux was well represented by an exponential function. Most of temporal variation in soil CO 2 efflux was explained by soil temperature rather than volumetric soil water content. The Q 10 values were 3.7 ± 0.8 and estimated annual carbon emissions were 837 ± 210 g C m -2 year -1 . These results provide a foundation for further development of models for prediction of soil CO 2 efflux driven by environmental factors.

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