*4.2. Seasonal Variations in the* δ*13CR and its Linkage to Environmental Factors*

The lack of replication of in situ online δ13CR measurements using non-automated systems is a common problem in previous studies [27,56], especially for studies measuring the δ13CR of different C pools simultaneously [12,17], including the present study. Nonetheless, the continuous δ13CR measurements in the present study accurately captured the seasonal variation of δ13CR in the target components. No consistent pattern was found in the δ13CR between the needles of *P. koraiensis* and the leaves of *A. mono* throughout the growing season (Figure 4). One possible reason is that the δ13C in the leaf respiratory substrates of *P. koraiensis* and *A. mono* experienced relatively weak seasonal variations (Figure 3), because leaf δ13CR found to be linked with the δ13C of leaf metabolites [5,57]. By contrast, both of the δ13CR values in the trunks of *P. koraiensis* and *F. mandshurica* showed a decreasing trend over the growing season (Figure 4). This result is consistent with a previous study of *Quercus petraea* in a temperate forest [8]; they ascribed that to the use of 13C-enriched respiratory substrates (i.e., starch) in trunks at the beginning of the growth period. The seasonal changes in leaf δ13CR differed from the δ13C of leaf organic matter (Figures 3 and 4), which may indicate changes in the proportion of different respiratory substrates used in leaf respiration during the growing season [42]. However, trunk δ13CR shared a similar seasonal variation pattern to the δ13C in leaf organic matter (Figures 3 and 4). This may be because a large amount of assimilated photosynthate was transported through the phloem to the trunk, and these were subsequently used as substrates for trunk respiration over the entire growing season.

Furthermore, due to the weak seasonal variation of leaf δ13CR in *P. koraiensis* and *A. mono*, we found no significant correlation between leaf δ13CR and the environmental variables (Table 1). It is still unclear how the leaf δ13CR of other dominant species in our forest are linked to environmental variables due to our limited chamber measurements. However, as the δ13C in the leaf organic matter of other dominant species showed larger seasonal variations (Figure 3), it is expected that the leaf δ13CR of other dominant species could have more significant seasonal variations and could be more sensitive to environmental conditions. The trunk δ13CR in our forest was found to be negatively correlated with air temperature (Table 1), which is consistent with the results of Maunoury et al. [8] on sessile oak in France. Interestingly, our study found that the δ13C in trunk respiration was more sensitive to environmental changes at a seasonal scale than that in other flux components, regardless of species. Comparably, the successful utilization of δ13C in tree rings to reconstruct climate demonstrates that δ13C in the structural carbohydrates in wood is closely related to environmental changes [58]. This suggests that the specific compounds responsible for tree ring δ13C may also be responsible for the δ13C in trunk respiration, resulting in the sensitivity of trunk δ13CR to environmental factors.

Soil δ13CR showed a relatively stable variation over the growing season (Figure 4). Variability in soil δ13CR has been linked to environmental factors such as air temperature, soil moisture, VPD, and photosynthetically active radiation [6,13,59,60]. Our results found that soil δ13CR had no significant correlation with environmental factors except for global radiation (Figure 6 and Table 1). This result is partly supported by Bowling et al. [56], who measured soil δ13CR in a US subalpine forest using

chambers. The authors' results showed that environmental forcing does not induce temporal variation in soil δ13CR. The lack of a linkage between soil δ13CR and environmental factors at our site could be due to the lack of in soil δ13CR variation during the growing season. The large range in potential soil respiratory substrates [3] and the correspondingly large range of δ13C values could also have dampened the isotopic linkage between soil δ13CR and current environmental conditions. Moreover, a recent study suggested that variations in soil δ13CR are mainly derived from diffusive fractionation rather than biological causes [32], which provides another possible explanation for the weak relationship between soil δ13CR and the environmental factors we found.
