Roles of soil chemistry and water availability in site-related 13C variations in French beech forests

Abstract

The carbon isotopic composition (δ¹³C) 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 δ¹³C reflect the spatial distribution of soil water availability, and we assessed whether δ¹³C could be used as a bioindicator of soil water availability. Patterns of variation in δ¹³C were compared with estimates of monthly water balance and with other soil characteristics. Between-site variability in δ¹³C was high (2.9‰ range in wood cellulose, 2.1‰ in leaf cellulose), but variation in water availability appeared to be only a minor factor contributing to this variation in δ¹³C. Unexpectedly, spatial variations in wood and leaf cellulose δ¹³C 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‰ lower δ¹³C in both wood and leaf material than trees growing on neutral or calcareous soils. Our initial hypothesis of a strong negative relationship between δ¹³C and site water availability was not confirmed. In the study zone, neither wood nor leaf δ¹³C 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.