Interspecific variability of delta 13C among trees in rain forests of French Guiana: functional groups and canopy integration

The interspecific variability of sunlit leaf carbon isotope composition (delta 13C), 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 rainforest 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 delta 13C 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 delta 13C values, suggesting a close genetic control of species delta 13C. Marked differences in species delta 13C values were found with respect to: (1) the leaf phenology pattern (average delta 13C=-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. Heliophilic species exhibited more negative delta 13C values (average delta 13C=-30.5 °/°°) (i.e. lower WUE) than hemitolerant species (-29.3 °/°°). However, tolerant species (-31.4 °/°°) displayed even more negative delta 13C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species delta 13C and midday leaf water potential suggests that low delta 13C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination calculated from the basal area-weighed integral of the species delta 13C values was similar in the three stands (average =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. (Résumé d'auteur