We evaluated the carbon budget of coarse woody debris (CWD) in a temperate broad‐leaved secondary forest. On the basis of a field survey conducted in 2003, the mass of CWD was estimated at 9.30 tC ha−1, with snags amounting to 60% of the total mass. Mean annual CWD input mass was estimated to be 0.61 tC ha−1 yr−1 by monitoring tree mortality in the forest from 1999 to 2004. We evaluated the CWD decomposition rate as the CO2 evolution rate from CWD by measuring CO2 emissions from 91 CWD samples (RCWD) with a closed dynamic chamber and infrared gas analysis system. The relationships between RCWD and temperature in the chamber, water content of the CWD, and other CWD characteristics were determined. By scaling the measured RCWD to the ecosystem, we estimated that the annual RCWD in the forest in 2003 was 0.50 tC ha−1 yr−1 or 10%–16% of the total heterotrophic respiration. Therefore, 0.11 tC ha−1 yr−1 or 7% of the forest net ecosystem production was sequestered by CWD. In a young forest, in which CWD input and decomposition are not balanced, the CWD carbon budget needs to be quantified for accurate evaluation of the forest carbon cycle and NEP.

No abstract

No abstract

A B S T R A C TTo separate CO 2 efflux from roots (R r ) and soil (R s ), we developed a system to measure R r continuously. Using this system, seasonal variation in R r was obtained in a temperate forest in Japan. We measured R s , CO 2 efflux from mineral soil (R m ) and environmental factors simultaneously, and the characteristic and seasonality of R r were analysed in comparison with R s . R r and R s showed different responses to soil water content: R s decreased with decreasing soil water content, whereas R r peaked at relatively low soil water content. R r /R s decreased from 64.8% to 27.3% as soil water content increased from 0.075 to 0.225 cm cm −3 . The relationship between respiration and temperature appears to change seasonally in response to phenological and biological factors. R r showed clear seasonal variation as a function of soil temperature. During the growing period, R r exhibited a higher rate at the same soil temperature than during other periods, which may be due to phenological influences such as fine root dynamics. R s decreased during the summer despite high soil temperatures. The seasonal peak for R s occurred earlier than that for soil temperature. R r /R s ranged between 25% and 60% over the course of the year.

We measured the sap flux densities of 12 deciduous trees in a tropical dry deciduous forest with high seasonality of available water located in Cambodia and evaluated the seasonal trends in transpiration and leaf phenology. For all trees, the minimum transpiration was recorded in the middle of the dry season, and almost all trees restarted transpiration before the first monsoon rain. The occurrence of the 'paradox' of leaf phenology was confirmed. During the dry season, transpiration was controlled by leaf phenology and decreased with an increase in the duration of the leafless period. In contrast, during the wet season, daily changes in transpiration were determined by changes in evaporative demand. Transpiration during the dry season accounted for more than 30% of the annual total among trees, and at the stand scale, the dry season contribution was 38%. The dry season transpiration was not negligible for the water balance in this ecosystem. The soil water condition in the shallow layer, where the main root system is located, was not the source of transpiration during the dry season. This implied that the root probably extended to a deep layer and absorbed water. The relationships between the mean canopy stomatal conductance and vapour pressure deficit revealed that most trees were isohydric. Isohydric behaviour controlling stomatal openness to avoid xylem hydraulic failure was also confirmed at the stand scale and was advantageous for these trees, allowing them to continue transpiring under the high evaporative demand during the dry season.