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 (delta(13)C), 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(13)C varied over a 6.0 parts per thousand range within each stand, cornsponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable delta(13)C values, suggesting a close genetic control of species, delta(13)C. Marked differences in species delta(13)C values were found with respect to: (1) the leaf phenology pattern (average delta(13)C=-29.7 parts per thousand and -31.0 parts per thousand 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(13)C values (average delta(13)C=-30.5 parts per thousand;) (i.e. lower WUE) than hemitolerant species (-29.3 parts per thousand). However, tolerant species (-31.4 parts per thousand) displayed even more negative delta(13)C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species delta(13)C and midday leaf water potential (psi(WM)) suggests that low delta(13)C 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(13)C values was similar in the three stands (average Delta(A)=23.1 parts per thousand), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands.