To evaluate the effects on CO2 exchange of clearcutting a mixed forest and replacing it with a plantation, 4.5 years of continuous eddy covariance measurements of CO2 fluxes and soil respiration measurements were conducted in a conifer‐broadleaf mixed forest in Hokkaido, Japan. The mixed forest was a weak carbon sink (net ecosystem exchange, −44 g C m−2 yr−1), and it became a large carbon source (569 g C m−2 yr−1) after clearcutting. However, the large emission in the harvest year rapidly decreased in the following 2 years (495 and 153 g C m−2 yr−1, respectively) as the gross primary production (GPP) increased, while the total ecosystem respiration (RE) remained relatively stable. The rapid increase in GPP was attributed to an increase in biomass and photosynthetic activity of Sasa dwarf bamboo, an understory species. Soil respiration increased in the 3 years following clearcutting, in the first year mainly owing to the change in the gap ratio of the forest, and in the following years because of increased root respiration by the bamboo. The ratio of soil respiration to RE increased from 44% in the forest to nearly 100% after clearcutting, and aboveground parts of the vegetation contributed little to the RE although the respiration chamber measurements showed heterogeneous soil condition after clearcutting.

[1] CO 2 efflux in the period of snow cover can be a large carbon source in the yearly carbon budget of snowy ecosystems. However, the behavior of CO 2 in snowpacks and the mechanisms of the snow surface efflux are still unclear. We performed continuous (half-hourly) midwinter measurements of CO 2 concentrations in a conifer-broadleaf mixed forest snowpack, and found that concentrations in the snowpack fluctuated significantly as wind speeds varied. The snow surface efflux was evaluated as the sum of the CO 2 storage change in the snowpack and the CO 2 input from the soil to the snowpack, taking into account the mixing due to airflow. The median value over 52 days (49 mmol m À2 d À1 ) was almost the same as the daily net ecosystem exchange rate in this forest (50 mmol m À2 d À1 ) estimated by the eddy covariance technique and the storage-change flux in the air column. These values are clearly larger than the value we estimated using Fick's law of diffusion. These results show that airflow can be a dominant cause of mixing within snowpacks in midwinter. In addition, in the soil pores under the snowpack, the CO 2 concentration was primarily related to air temperature, implying that soil respiration responds directly to air temperature, not to soil temperature, even beneath a 1-m-thick snowpack. We infer that the air temperature affected the root activity of trees through their trunks and that the variation in root respiration strongly affected the CO 2 concentration fluctuation in soil under the snowpack.

Single tree selection has been employed widely in northern Japanese mixed forests, but management-induced changes in forests are not been well understood. This study examined demographic parameters of major tree species during 20-years in a 68-ha study stand, in which single tree selection has been conducted since 1971. Results showed that conifers (mostly Abies sachalinensis) gained the strongest positive effects from the treatment on growth and survival.Nevertheless, recruitment of conifers was not sufficiently improved, suggesting their decreased dominance over a longer-term. Instead, shade-intolerant broadleaved (mainly Betula ermanii) will gradually increase because of their higher recruitment rates after the treatment. Shade-tolerant broadleaved (mainly Acer mono and Tilia japonica) appeared to experience the most distinct negative effects, especially on survival. These trends differed markedly from those reported in previous papers concerning partial harvesting system, which predicted increase of shade-tolerant species. The results shown here should be generalized carefully because we have investigated only one stand without repetition of the control area. Nevertheless, trends described in this large-scale and long-term study could provide a basis for simulating stand dynamics. We discussed possible reasons for the observed patterns, and provided implications for sustainable management in the region.

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