Doctoral Dissertations

Date of Award

3-1984

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

H. R. DeSelm

Committee Members

Murray Evans, Edward R. Buckner, Frank Woods

Abstract

The tropical rainforest ecosystems of the world are currently being altered (or destroyed) at a rate never before recorded for any forest ecosystem. Recent estimates put the destruction rate at 2% per year, which is equivalent to 11 million hectares. The primary reason for the destruction include rapid population increases, economic development, and agriculture-related activities. Unfortunately most policy planners in the countries in which these endangered ecosystems are located are presently too concerned with short-term economic interests to worry about long-term ecological consequences such as loss of soil fertility and genetic impoverishment.

Compared to other forest ecosystems, very little is known about the rainforest; for example, of an estimated three million species of plants and animals in the tropics, only 500,000 have been described. There is, therefore, an urgent need for studies which will elucidate the complex interactions which occur in disturbed and undisturbed rainforest ecosystems.

The specific objectives of this study were: (1) to examine the effects of partial and complete forest disturbance on litter and understory biomass production, (2) to evaluate differences in species diversity between moist evergreen and semideciduous rainforests, (3) to evaluate the relationship between rainforest species diversity and precipitation, (4) to determine the effects of slash-burning on rainforest soil fertility, pH, cation exchange capacity, movement of macroelements such as carbon, nitrogen, phosphorus, and potassium, and the cumulative impact of the changes on forest regeneration and succession, and (5) to compare the results obtained to similar results obtained elsewhere in Asia and South America.

To accomplish these objectives, forest stands at different successional stages were sampled, and their floristic richness, species diversity, importance values, understory biomass and litter accumulation rates were compared. In the fire study, results from early (December) and late (March) burned plots were compared with those from an unburned control.

Data from the above evaluations were obtained by random sampling. In the species diversity study six 25 x 25 m plots were used to sample all trees with gbh 15 cm or more. In the litter and biomass studies, five 1 meter square plots were utilized. The plots were located in an (1) undisturbed "virgin" forest, the so-called Strict Nature Reserve or SNR, a (2) 30-year old secondary forest, and a (3) 30-year old partially cut forest, the so-called tropical shelterwood system, or TSS. Understory biomass and species' presence data from early and late burned and control plots, and understory biomass and litter data from a Strict Nature Reserve, a Tropical Shelterwood System and a 30-year-old secondary forest, were collected monthly and compared. Variations in soil element concentrations caused by fire were laboratory analyzed and compared. Representative plant specimens collected during the species diversity study were mounted and preserved for herbarium use. Climatic data relevant to the study area were obtained from various Nigerian government agencies and published sources.

A number of observations were made: (1) Most Nigerian rainforest species are represented by only a few individuals per hectare. (2) Single-species dominance as noted in temperate deciduous forests are rare, while multiple-species dominance with combinations of Diopyrus, Cola, Celtis, Drypetes and Sterculia were more common. (3) Extremes in moisture conditions reduce plant species diversity components and intermediate moisture conditions give rise to maximum species diversity. (4) High species diversity is a function of high species eveness or equitability. (5) Moisture availability is the major factor that controls litter and biomass production patterns. (6) Moisture (in the form of rainfall) exerts a positive influence on biomass production and a negative influence on litter accumulation. (7) Fire in general altered and reduced plant species distribution by direct destruction of seeds, roots and other plant organs, but early burning in December improved soil fertility by increasing the concentrations of nitrogen, phosphorus and exchangeable magnesium, and (8) early burning and complete protection from fire encouraged plant species growth and regeneration, while late burning encouraged forest reversion to grassland.

The major conclusion drawn from the above results is that, in order to maintain continued high species diversity and prevent the extinction of important plant species in the Nigerian rainforest, changes must be made in present forestry and cultural practices. The present practice of leaving disturbed forest to naturally regenerate itself is inadequate and needs to be supplemented by artificial regeneration practices, especially agroforestry. Forest clearing for agricultural, industrial and urban development should be confined to secondary forests, while primary forests are set aside as parks and reserves to serve as outdoor scientific laboratories and to protect representative species from total extinction. Forest reserves should be established mainly in the high species diversity areas where moist and dry rainforests interface.

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