The natural 15N abundance method for estimating symbiotic biological N2-fixation was tested on legume trees from two rain forests on contrasting soils (oxisols and spodosols) in French Guiana. When possible, the significance of N2-fixing species in the plant community was evaluated in terms of density, biomass and contribution of N2-fixation to the building up of the total nitrogen mass in the leaves. Of the two sites, the rain forest on spodosols was the less favourable for application of the δ15N method: the available soil nitrogen was isotopically similar to fixed-N2. Hence, the results showed that a reliable estimate of N2-fixation could not be obtained. A substantial contribution of fixed-N2 to the nitrogen nutrition of legumes was found on oxisols, with an average value of 54 % Ndfa (Nitrogen derived from the atmosphere). The contribution of the N2-fixing legumes to the biomass of the stand was estimated to be 2 t ha−1 for the leaf biomass and 136 t ha−1 for the total above-ground plant biomass. With 7.5 % of trees in the stand able to fix N2 (462 out of 6156), N2-fixation was estimated to be 7 kg ha−1 y−1. These results are the first use of the δ15N method to estimate nitrogen input by N2-fixing legumes to a natural rain forest. The inter-site variability observed in the δ15N of the non-fixing plants suggested different nitrogen-cycling processes in the two soils. The δ15N of the non-N2-fixing plants could be related to the soil nitrogen availability and be used as an indicator of efficient or non-efficient nitrogen-cycling rain forests. The spatial variability of the δ15N in the plant-available soil nitrogen pool and the nitrogen balance in tropical rain forests are discussed.

Recent studies have shown that western lowland gorillas (Gorilla gorilla gorilla Savage & Wyman) are highly frugivorous and expel in their dung viable seeds of many fruit species they consume. As part of a wider study of their role as seed dispersers in tropical rain forest, the importance of gorillas as consumers of three species of tropical tree was investigated. The species studied were Ganophyllum giganteum (Sapindaceae), Cola lizae (Sterculiaceae) and Uapaca guineensis (Euphorbiaceae). Gorillas swallowed large quantites of seeds of these species whenever they were available, and removed a considerable proportion of the crop from trees they visited. The differences in abundance and distribution of these trees, and their varied dispersal strategies influenced the quantity of seeds that was removed by gorillas. Uapaca can be seen as having a generalist fruiting strategy, with a wide range of consumers, whereas the other species are more specialist, gorillas being the principal dispersers. Compared to other consumers, this study showed that gorillas provided potentially high quality dispersal because: (i) they were reliable visitors to trees of these species; (ii) they swallowed and dispersed large numbers of seeds; (iii) they treated seeds benignly and often deposited them in sites favourable for establishment.

Evidence is presented for the importance of western lowland gorillas as seed dispersers in the Lopé Reserve, Gabon. The survival and growth of seedlings of three tree species, growing from dung deposited at gorilla nest sites, were monitored for up to 2 y after deposition. A comparison was made between this and seeds (i) deposited in dung on gorilla feeding trails, (ii) dropped (spat out) under conspecific canopies, and (iii) scatter-dispersed (spat out) by other consumers away from conspecific trees but under intact forest canopy. The highest survival and the best performance of seedlings was always at a gorilla nest site, although not every site was favourable for survival or growth. Seedling performance was related to the surrounding vegetation conditions: better performance was observed in seedling clumps with less vegetation cover. Gorillas at Lopé apparently provide high quality seed dispersal for these species, not just because they consume large quantities of seeds, but because the pattern of seed deposition, especially in nest sites, can result in the enhanced survival and growth of seedlings.

Fruiting phenology, habitat types and proximity to conspecific fruiting adults may influence the degree of escape from pre- and post-dispersal seed predators. Successful predator escape by means of satiation is considered to be especially important for selection of masting and monocarpic reproduction in trees, such as exhibited by Tachigali versicolor, a tropical canopy tree. How pre- and post-dispersal predation rates varied with dispersal timing and among trees in T. versicolor was examined in young and old forests on Barro Colorado Island in Panama during a 4-mo period. Seeds were collected from above-ground traps to assess predispersal predation by bruchid beetles, and from quadrats on the ground to record predation and removal by terrestrial mammals. Proportion of seeds aborted varied greatly among trees (range 6–30 %, mean 16 %), and was especially high for trees on the edge of the island with fruiting conspecifics nearby during the early part of fruiting season. The proportion of seeds killed by bruchid beetles varied less among trees (14–25 %, mean 20 %), and remained constant throughout the fruiting season. Seeds on the ground were attacked mostly by rodents, and possibly by deer (26 % of all seeds and 43 % of intact dispersed seeds). The post-dispersal predation level was higher in the young forest than in the old forest (61 and 26 % of intact dispersed seeds, respectively), and was unaffected by the proximity of fruiting conspecifics. Temporal satiation of seed predators was evident only for post-dispersal mammalian predators in the old forest.

Light fluctuations and crown traits were studied for saplings of four tree species in a Costa Rican rain forest. Light fluctuations (1988–1994) were assessed by annual light estimations above saplings, using a visual crown position index. Crown traits in 1994 and growth between 1994 and 1995 were measured. Crown position values varied between 1.5 and 2.5. Of the 70 saplings only four were found at higher light levels (crown position of 3 or 3.5), but for 1–2 y only. Over the 6 y of investigation, 55–75 % of the saplings experienced no or only one light fluctuation, and 25–45 % two or three fluctuations. Crown traits in 1994 were either most strongly correlated with light levels in 1993 (Lecythis) and with light levels in 1991 and 1992 (Dipteryx and Simarouba), or they were not significantly correlated with light levels (Minquartia). It is hypothesised that: (1) the saplings require 1–3 y to establish a crown trait in response to light levels in the forest; and (2) and species that can economically produce a leaf can adjust crown traits more quickly in an environment dominated by low light levels than species that are less economic. Crown trait responses may track environmental changes in three of the four species, in particular in Lecythis. In this latter species, leaf area had no significant effect on growth and survival, whereas light level had a positive effect. Conversely, in the other three species, light levels had no discernible effect on growth and survival (due in part to low variation in crown position in two of these species), whereas leaf area had a positive effect on both.

The reproductive phenology of a population of 39 adult trees of Couratari multiflora was studied in ‘terra firme' forests located 90 km N of Manaus, Amazonas, Brazil. Trees of this species, found in low densities, were inventoried in a 400-ha plot in which only one tree ≥ 20 cm DBH per 10 ha was encountered. This species flowers from January to June, with a peak in the mid-wet season (March), and fruits during the dry season (May-September). Median distances between synchronized flowering trees were 600 to 1000 m depending on the time of the flowering season. The number of synchronous flowering trees at each observation date in the 400-ha plot was 14 or 17.5 per 1 d, 13 (both) per 15 d, and three or 13 per 30 d, depending upon which of the two methods was used to evaluate synchrony. Fifty percent of the flowering trees overlapped in flowering with six to seven other flowering trees growing within a radius of 1000 m during most of the annual flowering period. Outcrossing rates were 100 % and the relatively long distances between trees in flower at the same time, support the assumption that this species is pollinated by pollinators travelling distances as far as 1000 m.