Refixation of respired CO 2 by understory vegetation in a cool-temperate deciduous forest in Japan

Abstract

CO 2 released by respiration is either lost from the forest through turbulent mixing or refixed by photosynthesis within the canopy (CO 2 recycling). CO 2 concentration ([CO 2]) in a forest generally increases from canopy layer toward the soil surface due to the high [CO 2] by plant and soil respiration. Therefore, the understory vegetation is capable of fixing the respired CO 2 through photosynthesis, and this process would influence the carbon dynamics within a forest. In this study, in order to evaluate the magnitude of CO 2 recycling by understory vegetation under different environmental conditions and co-occurring photosynthetic/respiratory activity in understory between summer and autumn (after defoliation of forest canopy), we measured [CO 2] and carbon isotope ratios ( δ 13C) of CO 2 and plant organic matter in a cool-temperate deciduous broad-leaved forest at Takayama Experimental Site from the top of the forest canopy to understory layer. The forest canopy is dominated by birch ( Betula ermanii) and oak ( Quercus crispula), and the understory is dominated by an evergreen dwarf bamboo grass ( Sasa senanensis). [CO 2] and δ 13C of CO 2 were vertically stratified in the forest in mid-summer (August), with maximum [CO 2] (ca. 478 ppm) and most negative δ 13C value (−12.5‰) near the soil surface. The diurnal variations of [CO 2] and δ 13C were also clearly observed in the forest; daytime [CO 2] was considerably lower than that in night due to photosynthetic CO 2 uptake in the daytime while respiratory CO 2 efflux and its accumulation occurred around the dense understory vegetation in night. Based on the Sternberg's model [Sternberg, L., 1989. A model to estimate carbon dioxide recycling in forests using 13C/ 12C ratios and concentrations of ambient carbon dioxide. Agric. Forest Meteorol. 48, 163–173; Sternberg, L., Moreia, M.Z., Martinelli, L.A., Victoria, R.L., Barbsoa, E.M., Bonates, L.C.M., Nepstad D.C., 1997. Carbon dioxide recycling in two Amazonian tropical forests. Agric. Forest Meteorol. 88, 259–268] using the data of [CO 2] and δ 13C values of CO 2 and leaves in the forest, we calculated the fraction of respired CO 2 that was refixed photosynthetically by S. senanensis. The percentage of respired CO 2 refixed by S. senanensis was 15% in summer. After the forest canopy defoliation in autumn (October), [CO 2] and δ 13C showed small diurnal variation and small vertical gradients. Although the photosynthetic activity of S. senanensis is enhanced due to sunlight readily reaching the ground surface, the percentage of refixed CO 2 recycled by S. senanensis was 1%. Our result suggests that the dense understory vegetation have an important role in refixing the respired CO 2 in a cool-temperate deciduous forest.