Functional diversity in an Amazonian rainforest of French Guyana:: a dual isotope approach (δ15N and δ13C)

Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5 degrees 2'N, annual precipitation 2200 mm). We assessed leaf delta(15)N as a presumptive indicator of symbiotic N-2 fixation, and leaf and wood cellulose delta(13)C as an indicator of leaf intrinsic water-use efficiency (CO assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential ability for N-2 fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation (clear cutting, mechanical soil disturbance). Bulk soil delta(15)N was spatially quite uniform in the forest (range 3-5%), whereas average leaf delta(15)N ranged from -0.3% to 3.5% in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g(-1). Although nodulated trees displayed significantly lower leaf delta(15)N values than non-nodulated trees. leaf delta(15)N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of delta(15)N values for nodulated and non-nodulated species at the lower end of the delta(15)N range. Perturbation did not markedly affect the difference delta(15)N(soil) - delta(15)N(leaf), and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf delta(13)C were observed in the forest (average values from -31.4 to -26.7%), corresponding to intrinsic water-use efficiencies (ratio CO2 assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose delta(13)C was positively related to total leaf delta(13)C. the former values being 2-3% higher than the latter ones. Leaf delta(13)C was not related to leaf delta(15)N at either intraspecific or interspecific levels. delta(13)C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant species. For a given species, leaf delta(13)C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation; rather, they support the hypothesis that each trait leads to a separate grouping of species.