To understand climate change's effect on understory CO2 flux components, we established automated chambers in a 56-year-old Japanese larch (La be kaempferi Sarg.) forest in central Japan for long-term continuous measurements. Between 2006 and 2013, annual CO2 fluxes ranged from 7.0 to 8.4 tC ha(-1) yr(-1) for soil respiration (Rs), 5.7-6.8 tC ha(-1) yr(-1) for heterotrophic respiration (R-h), 9.3-10.7 tC ha(-1) yr(-1) for total understory respiration (Re), 2.6-3.5 tC ha(-1) yr(-1) for understory gross primary production (GPP(u)), and 6.1-7.6 tC ha(-1) yr(-1) for net understory CO2 exchange (NUE). Mean annual soil temperature (MAT(s)), especially in spring, was positively related to annual R-s, R-h, R-u, and NUE. Based on the inter-annual relationship between MAT(s) and annual understory CO2 effluxes, a 1 C MAT, increase was estimated to increase annual effluxes by 25.1% for R-s and R-h, 14.4% for R-u, and 23.9% for NUE. The growing season CO2 flux components were weakly associated with soil moisture. However, during a short dry period in the summer of 2013, we observed a strong relationship between soil moisture and understory R-s, R-h, and R-u. GPP(u) was primarily controlled by the understory light intensity; GPP during growing season increased where the canopy was disturbed by typhoonsin the early growing seasons in 2007 (+9.8%) and 2012 (+24.4%) as well as during the 2013 growing season (+12.2%) due to a short drought. Understory CO2 effluxes in this larch forest will likely increase under global warming.
