Roles of soil chemistry and water availability in site-related δ13C variations in French beech forests

The carbon isotopic composition (delta C-13) of wood and leaf cellulose of beech trees (Fagus sylvatica L.) was studied at 80 sites in northeastern France. We sampled sites with contrasting water balance, depending on soil type and precipitation. We tested the hypothesis that inter-site variations in plant delta C-13 reflect the spatial distribution of soil water availability, and we assessed whether delta C-13 could be used as a bioindicator of soil water availability. Patterns of variation in delta C-13 were compared with estimates of monthly water balance and with other soil characteristics. Between-site variability in delta C-13 was high (2.9 parts per thousand range in wood cellulose, 2.1 parts per thousand in leaf cellulose), but variation in water availability appeared to be only a minor factor contributing to this variation in delta C-13. Unexpectedly, spatial variations in wood and leaf cellulose VC were significantly and positively related to soil fertility expressed by soil pH (r = 0.42 and 0.43, respectively) and cation content. On average, trees growing on acidic soils displayed 0.5 parts per thousand lower delta C-13 in both wood and leaf material than trees growing on neutral or calcareous soils. Our initial hypothesis of a strong negative relationship between delta C-13 and site water availability was not confirmed. In the study zone, neither wood nor leaf delta C-13 appeared to be a reliable bioindicator of spatial variations in water availability. Possible causes for the lack of a relationship are discussed. Our findings confirm, under natural conditions, the strong effect of soil fertility on water-use efficiency previously observed in experiments. This effect needs to be considered in isotopic studies involving different sites.