4 NAOKI MAKITA, 1,2 YASUHIRO HIRANO,3 MASAKO DANNOURA,1,* 5 YUJI KOMINAMI,

11 Summary Fine root respiration is a significant compo-12 nent of carbon cycling in forest ecosystems. Although fine 13 roots differ functionally from coarse roots, these root 14 types have been distinguished based on arbitrary diameter 15 cut-offs (e.g., 2 or 5 mm). Fine root morphology is 16 directly related to physiological function, but few 17 attempts have been made to understand the relationships 18 between morphology and respiration of fine roots. To 19 examine relationships between respiration rates and 20 morphological traits of fine roots (0.15–1.4 mm in diam-21 eter) of mature Quercus serrata Murr., we measured 22 respiration of small fine root segments in the field with a 23 portable closed static chamber system. We found a 24 significant power relationship between mean root diam-25 eter and respiration rate. Respiration rates of roots 26 < 0.4 mm in mean diameter were high and variable, 27 ranging from 3.8 to 11.3 nmol CO2 g 1 s1, compared 28 with those of larger diameter roots (0.4–1.4 mm), which 29 ranged from 1.8 to 3.0 nmol CO2 g 1 s1. Fine root 30 respiration rate was positively correlated with specific 31 root length (SRL) as well as with root nitrogen (N) 32 concentration. For roots < 0.4 mm in diameter, SRL had 33 a wider range (11.3–80.4 m g1) and was more strongly 34 correlated with respiration rate than diameter. Our results 35 indicate that a more detailed classification of fine roots 36 < 2.0 mm is needed to represent the heterogeneity of 37 root respiration and to evaluate root biomass and root 38 morphological traits. 39 Keywords: closed static chamber system, immediate 40 field measurements, mean root diameter, root CO2 efflux