Effects of mammalian herbivory and seasonal drought were studied for Virola surinamensis (Myristicaceae) juveniles on Barro Colorado Island, Panama. Seedlings were planted at three months of age and the juveniles were monitored for two years; Treatments included: intact plants protected from mammals by cages, defoliated plants similarly protected, and unprotected plants, each planted in treefall gaps, on gap edges, and in the shaded understorey.

Juveniles planted in treefall gaps survived seasonal drought far better than those planted on gap edges or in shaded understorey. Two years after establishment, juveniles protected from mammalian herbivory showed a 78% survival in gaps (mean 6.8% skylight), 50% survival on gap edges (mean 3.0% skylight), and 33% survival in shaded understorey (1.4% skylight). This advantage was due to accelerated growth in gaps. Juveniles in gaps increased 616% in height, 1075% in leaf number, and 1800% in total leaf area. Comparable numbers in edges were 247%, 378% and 690%; in understorey 33%, 222% and 289%. Accelerated growth in gaps permitted yearlings to survive drought that killed suppressed yearlings in understorey. Mean light differentials as small as 0.6% and 0.3% skylight significantly influenced survival on edges and in shaded understorey, respectively.

Mammalian herbivory killed juveniles directly, and defoliation by mammals strongly accentuated drought mortality by suppressing root development. Natural defoliation was not attributable to gap conditions. Demographic projections from experimental data suggest that mammalian herbivory kills at least 48% of the juveniles of this species over two years, and contributes to the death of 32% more that actually die of drought stress. These projections suggest that 14% of the juveniles of this species die of drought mortality, independent of herbivory, during the first two years. Herbivory most strongly affects plants < 0.5 m in height, and is a continuing source of mortality among suppressed juveniles in the understorey. Steep slopes and large seed size each enhanced juvenile growth and survival in the intermediate conditions of gap edges, but not under the extreme conditions of gaps or shaded understorey.

The context of establishment determines the ‘shade tolerance’ of this conspicuous canopy tree. Without serious mammalian herbivory or extreme dry seasons, V. surinamensis can easily recruit as a shade tolerant plant in the understorey. Under present conditions on Barro Colorado Island, it cannot. Persistence involves both the chances of arrival in different microhabitats, and survival therein. Projections that include both the forest area represented by gaps, gap edges, and understorey and the experimental results from this study indicate that juvenile V. surinamensis can survive for two years in gaps, edges, and understorey, but that the higher proportions of vigorous individuals survive in edges, gaps and understorey, respectively.

Nitrogenase activity in the surface soil and litter layers from a tropical lowland rain forest and an adjacent fernland in Sri Lanka was measured by the acetylene reduction method. The total amount of acetylene reduced, a measure of biological nitrogen fixation, was greater in the fernland (38.4 µmol m−2 h−1) than in the forest (15.3–29.2 µmol m−2 h−1). In both ecosystems the nitrogenase activity was greater (59–85%) in the soil than in the litter layer. The rates of acetylene reduction in soil and litter showed an inverse relationship with the decay constants of mixed litter and a positive relationship was found between the rate of acetylene reduction and the C:extractable nitrogen ratio of the soil. The relatively greater rate of nitro genase activity coupled with the slower rate of litter decomposition in the fernland indicates that accumulation of nitrogen may result in the soil and litter layers of the deforested areas during their aggrading phases.

Termites, particularly the mound building, fungus growing Macrotermitinae, reach densities of up to 400 termites m−2 in soils of dry tropical Africa. The influence of Macrotermi tinae in increasing certain soil nutrients in mounds compared to adjacent soils has been documented, but the links between litter harvesting by termites, soil fertility, and global C cycling have not been explored. This study reviews the evidence from soil science, ecology and atmos pheric chemistry and generates hypotheses to explain the role of termites in dry tropical eco systems. It is suggested that termite activity exhaustively partitions litterfall among adjacent com peting colonies, where it is so thoroughly decomposed that little or no organic C is incorporated into the soils. Associated N, P, and cations build up in the mounds, but C apparently is emitted as CO2 and CH4 from the mounds. While not adequate to calculate nutrient fluxes through termites, the data available support the argument that termites contribute significantly to atmospheric fluxes of CO2 and CH4. Moreover, they suggest a coupling of regional soil forming processes and the global C budget.

Volcán Barva, Costa Rica, has on its northern slope an unbroken sequence of rain forest on volcanic parent materials from near sea level at La Selva Field Station up to its summit at 2906 m. It provides a good area to study forest changes with altitude and their causes. In the present paper we describe the forests as a background for soil and litterfall studies from 1 ha plots at each of the following altitudes: 100 m, 500 m, 1000 m, 1500 m, 2000 m and 2600 m. The canopy heights (with height of the highest emergent in parentheses) ranged from 35–40 m (45 m) at 100 m to 20–23 m (32 m) at 2600 m; basal area was least (22.7 m2) at 100 m and highest (51.2 m2) at 2600 m; the tree (≥10 cm dbh) density ranged from 391 ha–1at 500 m to 617 ha–1 at 2600 m. Most trees were identified and on samples of them we recorded presence of buttresses, lianes, skiophytic climbers, vascular epiphytes and bryophytes; and drew profile diagrams. In the classification of Whitmore (1984) the two lower plots are evergreen lowland rain forests; the other four are lower montane rain forest. Species richness was highest in the plot at 500 m, with at least 135 species of tree, and least at 2600 m, with at least 35 species. The Volcán Barva forest altitudinal sequence is briefly compared with those elsewhere.

We evaluated occurrence and abundance of lianas and woody hemiepiphytes on canopy and emergent tree species in primary tropical wet forest at the La Selva Biological Station, Costa Rica. Two pioneers, Cecropia obtusifolia and C. insignis, lacked both lianas and hemiepiphytes. The seven non-pioneer species differed significantly in their loads of lianas and hemiepiphytes. For all non-pioneer species, two measures of liana and hemiepiphyte loads (the percentage of the crown occupied and the combined basal area of descending hemiepiphyte and liana roots and stems) increased significantly with tree diameter. In all non-pioneer species, most trees ∵ 70 cm diameter (50–97%) were colonized. Lianas occupied more trees and had a smaller mean host diameter than did hemiepiphytes; however, basal area of descending roots or stems was equivalent for the two life forms in occupied trees. We used partial correlations controlling for tree diameter to evaluate the relationship between annual tree diameter growth and loads of hemiepiphytes and lianas for six non-pioneer species. Five of the six species showed a significant negative correlation between loads and diameter growth. Existing published data show that the high incidence of lianas and hemiepiphytes at La Selva is paralleled in most other Neotropical wet forests.

Various aspects of the reproductive biology of a montane population of Colostethus trinitatus were quantified during the transition from a prolonged dry season to a wet season with abnormally low rainfall. One-third to three-quarters of the females collected in the dry period (March to June) had mature ovarian follicles. The proportion of ovigerous females did not increase in July or August when the number of rainy days was greater. However, breed-ing and egg laying were only observed in the latter season. The principal environmental change that stimulated oviposition was frequent daily rain which increased humidity in the terrestrial nest sites.

Pair formation, nests, and males carrying larvae are reliable criteria that indicate the breeding period of Colostethus trinitatus. The average number of mature ovules (>2 mm) was greater in the wet season (=13.1) than during the dry months (= 11.3). Female standard length and the number of yolked follicles was positively correlated. Five nests contained an average of 13.6 eggs, and males carried only slightly fewer larvae (= 11.5). One female and one individual of unknown sex were found with eggs at two different nests. Five nests were unattended.

We studied the breeding success and distribution of three species of eagles, four species of vultures and one species of eagle owl on two areas of differing primary productivity from 1973 to 1984. Densities of all species were lower in areas with lower productivity. Breed-ing success on both biomes was similar, the lower densities of birds on the least productive biome apparently compensating for the lower productivity. Eagles produced more one-egg clutches, laid later and reared fewer young in the period of poor rainfall, probably as a result of reduced productivity. Vulture data were difficult to interpret because abundant food at breed-ing time was created by ungulate population management throughout the entire study period. Concentrations of all species on the more productive biome, and solitary breeding on the least productive biome by a normally colonially breeding species, suggests that primary production also influences vulture densities and perhaps breeding success.

Field observations and laboratory experiments on unweathered and weathered dune sand beneath a tropical deciduous forest in Veracruz, Mexico were used to test the hypothesis that temporary nutrient adsorption by more weathered soil facilitates more effective nutrient recycling through the stimulation of a superficial fine root system. An analysis of soil solution composition in the field indicated that K, P and were being more effectively retained in weathered sand topsoil than in recent sand but, in a leaching experiment, only was more effectively adsorbed by this soil. Fine roots in weathered sand were more concentrated superficially than in recent sand, and a bioassay with seedlings of Cedrela odorata showed more fine root proliferation in this medium than in recent sand. In combination, these data suggest that weathering of sand in this area promotes an increased nitrate adsorption capacity which may stimulate greater root proliferation in topsoils and lead to the entrainment of other nutrients in a more concentrated topsoil-vegetation cycle. If these results can be applied to the larger domain of tropical soils generally, they suggest that nitrate adsorption may play an important role in stimulating effective nutrient recycling by forests occupying weathered soils.