2 results
Tree population changes in treated rain forest at Omo Forest Reserve, south-western Nigeria
- D. U. U. Okali, B. A. Ola-Adams
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- Journal:
- Journal of Tropical Ecology / Volume 3 / Issue 4 / November 1987
- Published online by Cambridge University Press:
- 10 July 2009, pp. 291-313
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A review of previous evaluations of long-term changes in treated Nigerian rain forest is presented, to show that these studies have concentrated on economic species with little consideration of other species, or of forest biomass and structure.
We examine regeneration and successional patterns in Omo Forest Reserve by comparing enumeration data taken before (1952) and after (1981) treatment of the forest by selective canopy opening and climber-cutting (Plot A), or clear-felling and burning (Walsh system) (Plot C).
Before treatment, about 50 species in 25 families were found of stems ≥10 cm dbh, in 4.05 ha of the forest, the Euphorbiaceae contributing the greatest number of species while the medium-sized trees Diospyros alboflavescens (Ebenaceae) and Strombosia pustulata (Olacaceae) contributed more than 40% of the stems.
Twenty-eight years after treatment (1981) the number of species and families remained similar to those in 1952, although smaller areas (0.75 ha) were enumerated; the treated plots were, however, dominated by early succession species such as Macaranga barteri, Musanga cecropioides, Cleistopholis patens, Funtumia elastica and Fagara macrophylla, and lacked an abundance of the ‘economic’ species that treatment had been expected to induce; the medium-sized trees that were dominant in 1952 were still abundant in Plot A but not in Plot C.
For stems ≥ 30 cm dbh tree diversity (reciprocal of Simpson's index) was highest (15.7) in a 1952 plot and least (4.8) in the clear-felled plot enumerated in 1981; diversity of the 1952 plots, however, fell markedly to values lower than those for the 1981 plots when computation was based on all stems ≥10 cm dbh, presumably because of increase in abundance of small-stemmed species like Diospyros spp., Strombosia sp. and Rinorea sp., each represented by a large number of stems.
Basal area was greatest (29.6 m2 ha−1) in the 1952 plots and least (12.7 m2 ha−1) in the clear-felled plots enumerated in 1981, but the relative distribution of basal area and number of stems in size-classes was similar in all the plots.
Mean annual increment, computed by dividing the mean diameter (7.50 cm) of the stems in the clear-felled plots by the number of years (28) over which they had grown, was 0.27 cm.
Nauclea diderrichii dominated the seedling regeneration from the first year after clear-felling and burning (1954) till the sixth year (1960), when seedlings of the Meliaceae entered the regeneration list and overall seedling density was 395 per hectare.
Among the plots assessed in 1981, the standing crop was greatest in an untreated Control plot (229.6 t ha−1) followed by Plot A (159.7 t ha−1) and Plot C (91.1 t ha7−1), but the relative allocation of biomass to stem, branch, leaf, root and fruit fractions was comparable for all plots.
The data are discussed in relation to other Nigerian forest studies and it is suggested that the main qualitative features of structural organization and the species composition of the top canopy synusia of mature secondary rain forest may be determined quite early in the development of the stand.
Soil–plant relations in a natural forest inviolate plot at Akure, Nigeria
- B. A. Ola-Adams, John B. Hall
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- Journal:
- Journal of Tropical Ecology / Volume 3 / Issue 1 / February 1987
- Published online by Cambridge University Press:
- 10 July 2009, pp. 57-74
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To up-date and extend knowledge of the Akure Strict Natural Reserve an assessment, block by block, of forest in the core of the Inviolate Plot was undertaken in 1974 and complemented with soil sampling. Principal components analysis indicated a gradient of soil reaction and available phosphorus through the core of the plot. Ordination (DECORANA) of floristic data revealed that a floristic trend paralleled the soil trend. In the vicinity of a drainage line, at the western end of the core, soils were lower in available phosphorus, more acid, sandier and texturally more uniform with depth. Typical trees of ferralsols were prominent on these soils but were replaced by species of ferric luvisols, especially members of the Sterculiaceae, on soils of superior nutrient status towards the eastern end. It is suggested that the soil gradients were not effects produced by the trees.
Comparison of a 1946 data set with the 1974 data supported the view that the forest had been structurally and floristically stable over the interval between assessments. This is considered justification of the original choice of the area as a Strict Natural Reserve representative of high forest in this part of Nigeria.