Interspecific Variability of δ13C among Trees in Rainforests of French Guiana: Functional Groups and Canopy Integration

The interspecific variability of sunlit leaf carbon isotope composition (δ13C), an indicator of leaf intrinsic water-use efficiency (WUE, CO₂ assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainfoest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ13C varied over a 6.0% range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ13C values, suggesting a close genetic control of species δ13C. Marked differences in species δ13C values were found with respect to: (1) the leaf phenology pattern (average δ13C=-29.7‰ and -31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Helophilic species exhibited more negative δ13C values (average δ13C=-30.5‰) (i.e. lower WUE) than hemitolerant species (-29.3‰). However, tolerant species (-31.4‰) displayed even more negative δ13C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ13C and midday leaf water potential ($\Psi _{\text{wm}}$) suggests that low δ13C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination ($\Delta _{A}$) calculated from the basal area-weighed integral of the species$\Delta _{A}$values was similar in the three stands (average δ13C=23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands.