Temporal variation of δ13C of larch leaves from a montane boreal forest in Mongolia

This paper reports the temporal variation (2002-2004) in foliar delta C-13 values, which are indicative of long-term integrated photosynthetic and water use characteristics, of Siberian larch (Larix sibirica Ledeb.) trees in a montane forest at Mongonmorit, NE Mongolia. At the stand, the delta C-13 value for understory shaded leaves was more negative by 2 parts per thousand on average than that for sunlit leaves sampled concurrently from open and sun-exposed environments in a forest gap. The delta C-13 value of both sunlit and shaded leaves showed pronounced intra- but relatively small inter-seasonal variations. The delta C-13 value was more positive for juvenile than mature leaves. We conjecture that juvenile leaves may derive carbon reserves in woody tissues (e.g., stems). Regardless of leaf habitats, the delta C-13 value was also affected by insect herbivores occurred in mid summer of 2003, being more negative in newly emerging leaves from the twigs after defoliation than in non-defoliated mature leaves. This pattern seems to contrast with that for the juvenile leaves in the early growing season. We surmise that the newly emerging leaves used stored organic carbon that was depleted due to fractionation during remobilization and translocation for leaf regrowth. There was also intra- and inter-seasonal variation in the foliar N concentrations and C: N ratios. A good positive (negative) correlation between the foliar delta C-13 values and N concentrations (C:N ratios) was also observed for both sunlit and shaded leaves, suggesting that the relationship between water and nitrogen use is a crucial factor affecting the plant carbon-water relationship in this mid latitude forest with a cold semiarid climate. Our isotopic data demonstrate that the larches in NE Mongolia exhibits relatively higher water use efficiency with a distinct within-season variability.