Relations between soil nutrient availability, the occurrence of root symbioses and tree species composition in tropical Africa are discussed in the light of new evidence. Transects from humid areas at the equator towards the deserts can be summarized as follows:

High availability of nitrogen, low availability of phosphorus, low pH and aluminium toxicity probably restrict the competitiveness of nitrogen-fixing species in the forests on highly weathered and leached soils in humid west and central Africa. Ectomycorrhizal species attain dominance in species-poor forests on the soils poorest in phosphorus, while species with vesicular-arbuscular (VA) mycorrhiza form diverse forests on more fertile soils. Nitrogen-fixing species, which mostly are VA mycorrhizal, are more common in woodlands, where nitrogen is lost in grass fires, than in forests, which have a more closed nitrogen cycle. Low availability of phosphorus, however, limits the growth of nitrogen-fixing species in moist savannas. Ectomycorrhizal species are dominant, possibly because of a competitive advantage conferred by the large storage capacity of the fungal mantles under conditions of discontinuous nutrient supply. Nitrogen-fixing species become more abundant towards drier areas with an increase in soil phosphorus and a decline in soil nitrogen. These conditions culminate at the desert fringe, where nitrogen-fixing species reach their most conspicuous development.

It is concluded that soil nutrient availability and root symbioses interact and strongly affect the composition of vegetation communities.

Two stands of mature evergreen wet forest in Mauritius (Mascarene Islands) were sampled for floristic composition to assess the extent of invasion by weedy exotic phanerogams (Brise Fer, 550 m alt.; Bon Courage, 200–260 m alt.). All individuals ≥ 2.5 cm dbh were recor ded in 50×2 m transects totalling 0.1 ha at each site, and 2×2 m seedling plots totalling 40 m were also sampled at each site. Both forests showed a high degree of invasion. Although exotics constituted only 5% of woody species ≥ 2.5 cm dbh at Brise Fer and 14.5% at Bon Courage, they comprised 34.8% and 20.8% of the individuals, respectively. Seedling plots at both sites were dominated by exotics, which comprised 20.6% of the species and 97.4% of the individuals at Brise Fer, and 22.2% of the species and 73.9% of the individuals at Bon Courage. Comparisons are made with Macabé forest, sampled nearly 50 years ago. These data suggest that unless steps are taken to check the spread of exotics, floristic composition at these sites will shift towards total invasion and degradation as has occurred elsewhere on Mauritius.

The distribution of the nutrients N, P and K in soil and vegetation and their mobility through litterfall and decomposition in mature and successional stands of a subtropical dry forest were studied in Guánica, Puerto Rico. Soils of the Guánica forest have high total amounts of N (9100 kg/ha), P (1820 kg/ha), and K (7460 kg/ha). However, high extractable Ca (>4000 mg/g) and pH (> 7–8) may explain why only 1.3 and 25% of the total P and K, respectively, were extractable. Total ecosystem storage of N, P and K was 10,300, 1900 and 7700 kg/ha, respectively, of which vegetation stored only 10, 2 and 3%, respectively. Litterfall returned 26, 18 and 180% per year of the N, P and K stored in the ground litter compartment. Trees retranslocated about 30 and 65% of the N and P required to satisfy aboveground net primary production and immobilized P in dead roots. Slow leaf decomposition (7.3 yr for 95% decomposition) released K faster than mass, P as fast as mass, and ash and N slower than mass. The use efficiency of P by litterfall was high compared with other tropical forests, while that of N and K was similar to other tropical and temperate forests. Cutting and regrowth of vegetation resulted in differences in the nutrient concentration in litterfall and nutrient use efficiency of successional vegetation.

Phenological patterns of flowering and fruit-set were studied in cocoa trees (Theobroma cacao) (Sterculiaceae) at monthly intervals in two contrasting habitats in Costa Rica for a one-year period. One of these habitats, a well-maintained plantation, had irregular and broken shade cover {Erythrina trees in particular) while in the other habitat, a ‘cocoa forest’, cocoa trees were heavily shaded by Huara crepitans (Euphorbiaceae). ‘Matina’ variety cocoa trees of about the same age (50–60 years) were censused in both habitats. Cocoa-pollinating midge (Diptera: Ceratopogonidae and Cecidomyiidae) availability was estimated by sampling immature stages in experimentally-distributed ground-cover breeding substrates, comparing overall abundances and species numbers between the two habitats over three census dates (dry, mid-rainy, and late-rainy seasons), along with examination of fungal-rotted (diseased) cocoa pods both on trees and the ground.

Although total flower production was much greater in the plantation habitat, total production of new pods was similar between habitats. Flowering followed a cyclic temporal pattern in the forest but not in the plantation. Sudden leaf drop of forest shade trees in the dry season probably triggered a cyclic response in which flowering peaked in the first half of the rainy season.

There was an inverse relation for frequencies of mature cocoa pods killed by squirrels and pathogenic fungi (Monilia roreri and Phytophthora) between the two habitats: squirrel-killed pods were far more abundant in the plantation than in the forest, and the opposite for fungus-killed pods. Fungus-killed but otherwise intact pods, and not squirrel-killed pods rotting on trees, were a major breeding site for midges, particularly during the late rainy season. Ceratopogonidae were most abundant in the dry season and frequently encountered in cocoa pod husks and banana tree trunk sections in both habitats, and much more so in the forest habitat. The abandoned cocoa plantation (cocoa forest) supported a more diverse assemblage of pollinating midges than the plantation.

In the plantation but not in the forest, a negative correlation was discovered between distance from shade trees and the numbers of pods on trees, suggesting greater pollinating activity by midges in cocoa trees beneath shade trees than away from them. The uniform dense shade cover in the adjacent forest probably obliterated such a pattern.

‘Times Species-counts’ are a simple method of comparing the avifaunas of extensive areas by sampling representative habitats. The method has been evaluated in south central Kenya but could be applied to most terrestrial regions of tropical Africa. The essential feature is that the observers move anywhere within a sample site of about 1 km2, recording all of the species seen in a fixed time, e.g. 1 h.

Counts are made in all seasons and at various times of day. Reasonable estimates of the total number of species in an area can be derived from a series of 15 counts, but such estimates tend to be low in non-forested habitats because ‘wanderers’ (non-resident species) continue to be recorded almost indefinitely. Open sites have far fewer species than well-wooded areas. The numbers of resident species in non-forested terrestrial habitats is related both to the amount of woody vegetation and to a measure of plant growth. However, total species numbers seem to depend solely on the amount of woody vegetation. Road counts, from a vehicle, were compared to those made on foot. They are less satisfactory, especially for cryptic species.

Zygonix iris is widespread in tropical Asia, and larvae are sprawlers/clingers on rock surfaces in fast-flowing streams and rivers. In the Lam Tsuen River, Hong Kong, this species is univoltine; emergence occurs prior to the summer monsoon and larval recruitment during the wet season. Studies on larval dietary composition in four habitats indicated that Z. iris is a generalist predator, consuming epibenthic prey taxa in proportion to their abundance in the environment. Larval Chironomidae (Diptera) and Baetis (Ephemeroptera) were the commonest food items at all sites and there was little consistent evidence of preference for individual taxa. Larger Z. iris larvae tended to consume more prey taxa than did smaller larvae, and Baetis prey size was positively correlated with predator size. No size selection of chironomid larvae was apparent. Despite its unusual larval habit, Z. iris is a generalist feeder resembling lotic and lentic temperate-zone Odonata.

In January 1983, two 50 m × 50 m rain forest plots in the University of Ife campus in southwestern Nigeria were demarcated for a baseline study of species composition, litterfall and tree girth increments. By accident, a severe ground fire burnt one of the plots on 31 January, barely two weeks after litter traps had been set and species listing and first girth measurements completed.

The effect of this fire was assessed in the burnt plot after 14 months, in April 1984. The fire affected small trees especially, and there were indications that some species were more sensitive than others.

Manihot glaziovii, which was present in the plot and abundant in its vicinity before the fire showed a big increase in density after the fire because it was able to germinate in the openings created by the fire. These observations were related to succession, and it is concluded that the observation by some workers that the initial composition of a regrowth forest may be partly predicted from a knowledge of the seed bank in the soil may be applicable in the present case.

Community patterns along a successional sequence of abandoned fields within a tropical rain forest environment are described in terms of floristic composition and structure. Temporal patterns were summarized and main axes of variation extracted by multivariate methods.

External factors – length of cropping period, regrowth age, and proportion of the perimeter occupied by forest communities or remnants (here called forested perimeter) – were related to floristic and structural data on 40 sites. Multivariate direct gradient analysis showed overall floristic development to be positively related to regrowth age and forested perimeter, and negatively related to length of cropping period.

Structural development was shown by multiple regression to be positively related to regrowth age and negatively to length of cropping period; but forested perimeter had no significant relation to site structure. The type of crop grown showed no significant relation with floristic or structural development of the regrowth.

These results indicate that the conditions of the vegetation around a site have a great effect on community recovery, particularly in its species composition, whilst structure seems to be affected more directly by parameters which imply time.