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°2'N, annual precipitation 2200 mm). We assessed leaf δ15N as a presumptive indicator of symbiotic N2 fixation, and leaf and wood cellulose δ13C as an indicator of leaf intrinsic water-use efficiency (CO2 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 N2 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 δ15N was spatially quite uniform in the forest (range 3-5‰), whereas average leaf δ15N 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 δ15N values than non-nodulated trees, leaf δ15N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ15N values for nodulated and non-nodulated species at the lower end of the δ15N range. Perturbation did not markedly affect the difference δ15Nsoil - δ15Nleaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf δ13C 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 δ13C was positively related to total leaf δ13C, the former values being 2-3‰ higher than the latter ones. Leaf δ13C was not related to leaf δ15N at either intraspecific or interspecific levels. δ13C 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 δ13C 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.

Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana).

Canopy CO2 concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO2]) throughout the canopy (0.02-38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO2] in the upper and middle canopy decreased on average 7-10 µmol mol-1 below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO2] did not differ between the wet and dry seasons. In contrast, [CO2] below 0.7 m were generally higher during the dry season, resulting in larger [CO2] gradients. Supporting this observation, soil CO2 efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO2] immediately above the forest floor of about 50␣µmol mol-1. Temporal and spatial variations in [CO2]canopy were reflected in changes of δ13Ccanopy and δ18Ocanopy values. Tight relationships were observed between δ13C and δ18O of canopy CO2 during both seasons (r 2 > 0.86). The most depleted δ13Ccanopy and δ18Ocanopy values were measured immediately above the forest floor (δ13C = -16.4‰; δ18O = 39.1‰ SMOW). Gradients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ13C, and between 1.0‰ and 3.5‰ for δ18O. The δ13Cleaf and calculated c i/c a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ13Cleaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c i/c a. Plotting 1/[CO2] vs. the corresponding δ13C ratios resulted in very tight, linear relationships (r 2 = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ13C of respired sources were close to the measured δ13C of soil respired CO2 and to the δ13C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δe, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season.