Seasonal change of CO2 recycling rate by understory vegetation in a cool-temperate forest in Japan

Three main factors govern dynamics of CO2 within a forest canopy: turbulent mixing with the atmosphere above canopy, photosynthesis and respiration. CO2 released by respiration is either lost from the forest through turbulent mixing or refixed by photosynthesis within the canopy (CO2 recycling). CO2 concentration ([CO2]) in a forest generally increases from canopy layer toward the soil surface due to the high [CO2] by plant and soil respiration. Therefore, the understory vegetation is capable of fixing the respired CO2 through photosynthesis, and this process would highly influence the carbon dynamics within a forest. In a temperate deciduous forest, stand structure is seasonally changed by the seasonal change of foliage biomass distribution and changes the environment factors in the forest understory such as light attenuation and soil temperature which affect photosynthetic activity of understory plants and respirations of plants and soil microbes. In this study, we examined how [CO2] and δ13C of canopy profile change daily and seasonally in a cool-temperate deciduous forest at Takayama Experimental Site (36 deg. 8'N, 137 deg 6'E, 1420 m a.s.l.) in Japan. We also estimated the percentage of respired CO2 recycled by understory vegetation using a model developed by Sternberg (1989). At the Takayama Experimental Forest site, the understory is dominated by an evergreen dwarf bamboo grass (Sasa senanensis (Fr. Et Sav.) Rehdar). There were dramatic changes in canopy foliage density and environmental conditions within the forest in only about five months of plants' growing season. There were clear diurnal changes in [CO2] within the forest ([CO2]canopy), especially on the soil surface, in mid-summer (August). [CO2]canopy and δ13C of CO2 (δ13Ccanopy)were vertically stratified in the forest, with maximum [CO2] (ca. 478ppm) and most negative δ13C value (-12.5 per mille) near the soil surface. The diurnal variations of [CO2]canopy and δ13Ccanopy were also clearly observed in the forest; daytime [CO2]canopy was considerably lower than that in night due to photosynthetically CO2 uptake in daytime while respiratory CO2 efflux and its accumulation occurred around the dense understory vegetation in night. The Sternberg's model using, we calculated the fraction of respired CO2 that was refixed photosynthetically by S. senanensis. The percentage of respired CO2 refixed by S. senanensis ranged from 5 to 16% in summer depending on the diurnally fluctuated [CO2]canopy in the understory. [CO2]canopy and δ13Ccanopy showed small diurnal variation and small vertical difference in spring and autumn, reflecting reduced biological activities due to tree leaves fall. The percentage of respired CO2 recycled by S. senanensis ranged from 22 to 26% in spring, and from 16 to 21% in autumn. These results were discussed in this study in terms of environmental and physiological factors as follows; (1) source CO2 effects ([CO2]canopy and δ13Ccanopy) due to the seasonal change of soil respiration rate and turbulent mixing which depends on forest canopy closure and (2) leaf photosynthetic discrimination effects influenced by the photosynthetic and stomatal responses to diurnal light incident patterns that are different between summer and spring (or autumn)