Long-term chamber measurements reveal strong impacts of soil temperature on seasonal and inter-annual variation in understory CO2 fluxes in a Japanese larch (Larix kaempferi Sarg.) forest

- Soil temperature strongly affected understory CO2 effluxes.
- Spring soil temperature strongly affected inter-annual variations of the effluxes.
- The influence of soil moisture on the fluxes was minor, except during a dry summer.
- Understory photosynthesis increased after canopy disturbance by a typhoon.
- Understory CO2 efflux is likely to increase under a future warmer environment.

To understand climate change's effect on understory CO2 flux components, we established automated chambers in a 56-year-old Japanese larch (Larix 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 (Rh), 9.3-10.7 tC ha−1 yr−1 for total understory respiration (Ru), 2.6-3.5 tC ha−1 yr−1 for understory gross primary production (GPPu), and 6.1-7.6 tC ha−1 yr−1 for net understory CO2 exchange (NUE). Mean annual soil temperature (MATs), especially in spring, was positively related to annual Rs, Rh, Ru, and NUE. Based on the inter-annual relationship between MATs and annual understory CO2 effluxes, a 1 °C MATs increase was estimated to increase annual effluxes by 25.1% for Rs and Rh, 14.4% for Ru, 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 Rs, Rh, and Ru. GPPu was primarily controlled by the understory light intensity; GPPu during growing season increased where the canopy was disturbed by typhoons in 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.