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Soil CO2 efflux is the main source of CO2 from forest ecosystems and it is tightly coupled to the transfer of recent photosynthetic assimilates belowground and their metabolism in roots, mycorrhiza and rhizosphere microorganisms feeding on root-derived exudates. The objective of our study was to assess patterns of belowground carbon allocation among tree species and along seasons. Pure 13CO2 pulse labelling of the entire crown of three different tree species (beech, oak and pine) was carried out at distinct phenological stages. Excess 13C in soil CO2 efflux was tracked using tuneable diode laser absorption spectrometry to determine time lags between the start of the labelling and the appearance of 13C in soil CO2 efflux and the amount of 13C allocated to soil CO2 efflux. Isotope composition (δ13C) of CO2 respired by fine roots and soil microbes was measured at several occasions after labelling, together with δ13C of bulk root tissue and microbial carbon. Time lags ranged from 0.5 to 1.3 days in beech and oak and were longer in pine (1.6-2.7 days during the active growing season, more than 4 days during the resting season), and the transfer of C to the microbial biomass was as fast as to the fine roots. The amount of 13C allocated to soil CO2 efflux was estimated from a compartment model. It varied between 1 and 21% of the amount of 13CO2 taken up by the crown, depending on the species and the season. While rainfall exclusion that moderately decreased soil water content did not affect the pattern of carbon allocation to soil CO2 efflux in beech, seasonal patterns of carbon allocation belowground differed markedly between species, with pronounced seasonal variations in pine and beech. In beech, it may reflect competition with the strength of other sinks (aboveground growth in late spring and storage in late summer) that were not observed in oak. We report a fast transfer of recent photosynthates to the mycorhizosphere and we conclude that the patterns of carbon allocation belowground are species specific and change seasonally according to the phenology of the species. [ABSTRACT FROM AUTHOR]

We separately examined the temporal patterns of root production by Japanese oak (Quercus crispula) and dwarf bamboo (Sasa veitchii), which is a major understory species in cool temperate forests. We grew Japanese oak seedlings and Sasa stocks (i.e., the rhizome and connected culms) in organic-free sand in rhizoboxes and then scanned roots that were visible through the sides of the rhizoboxes to measure the length of each root in images. Japanese oak root production peaked in July, but Sasa root production peaked in both July and October. Soil temperature was highly correlated with root production of Japanese oak, but less so with Sasa root. Leaves of Sasa expanded in late summer, and the photosynthetic rate of Sasa was highest in September, suggesting that the aboveground phenology influences the extensive root production of Sasa in October due to the supply of carbohydrate. These results demonstrate different temporal patterns of root production by Japanese oak seedlings and understory species (Sasa), even under similar environmental conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]