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

The interspecific variability of sunlit leaf carbon isotope composition (de lta(13)C), an indicator of leaf intrinsic water-use efficiency (WUE, CO2 as similation rate/leaf conductance for water vapour), was investigated in can opy 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 tre es 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 stabl e delta(13)C values, suggesting a close genetic control of species, delta(1 3)C. Marked differences in species delta(13)C values were found with respec t to: (1) the leaf phenology pattern (average delta(13)C=-29.7 parts per th ousand and -31.0 parts per thousand in deciduous-leaved and evergreen-leave d species, respectively), and (2) different types of shade tolerance define d by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative d elta(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, tolera nt species (-31.4 parts per thousand) displayed even more negative delta(13 )C values than heliophilic ones. We could not provide a straightforward eco physiological interpretation of this result. The negative relationship foun d between species delta(13)C and midday leaf water potential (psi(WM)) sugg ests that low delta(13)C is associated with high whole tree leaf specific h ydraulic conductance. Canopy carbon isotope discrimination (Delta(A)) calcu lated 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 t ypes among stands.