Date of Graduation

1999

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

James B. McGraw

Committee Member

William Peterjohn

Committee Member

Jeffrey Skousen

Abstract

Land-use history in Zimbabwe has caused dramatic differences in erosion severity between Commercial and Communal Lands. High population numbers have been found to be positively correlated with the extent of eroded area in Communal Lands. Dambo wetlands are particularly vulnerable to this population pressure as they are isolated wet areas supporting plant growth during the six to eight months of the dry season. In Zimbabwe, they cover 1.3 million hectares of the country. This study examines the plant composition and seed bank of these wetlands and compares vegetation in dambos with contrasting land-use histories, communal versus commercial. Communal Lands are the former reserves that were set up during the colonial era. They support high human and livestock populations. The Commercial Lands are divided into large farms where lighter livestock densities are kept. Six dambos with the two land-use systems adjacent to each other were used to reduce environmental variability. Three 10 x 10 m plots were set up on either side of the fence in April/May 1994. Ten randomly selected .5 x .5 m quadrats were sampled within the plots. Plant species were harvested, oven-dried at 65oC for 48 hours, and their dry weights used to compare biomass and species diversity on either side of the fence. Additional data was collected using the dry-weight rank method to assess the dominant species on either side of the fence. Soil samples were collected at 2 cm intervals to a depth of 10 cm in the 10 x 10 m plots for seed bank assessments. Viability of seeds was established by spreading out the soils in trays and watering. Community coefficients were used to compare the two land-use systems.

Biomass was significantly lower on the Communal side of the fence and ranged from 656 to 2,310 kg/ha while it ranged from 1,975 to 6,594 kg/ha on the Commercial side. Overgrazing on the Communal lands have caused this reduction in dambo vegetation biomass although this does not necessarily imply decreased primary productivity of the plants. No general pattern was observed in terms of species richness and species diversity between the land-use systems. Open patches in Communal Lands are colonized by immigrants and species from the seed bank. Four of the six sites showed a definite shift in dominant species contributing to the total biomass in the Communal versus the Commercial areas. The taller and more palatable grasses seem to be replaced by weeds and more grazing tolerant grasses. Grasses that do well when there is under-grazing or selective grazing dominated the Commercial Lands. Land-use history did not have an overriding effect on seed bank species richness, however it did affect viable seed densities. There were more seeds/m2 on the Commercial side. Soil profiles had variable species richness and seed density patterns. The majority of species in all the seed banks were herbs rather than the grasses that dominate the aboveground composition. Community coefficients indicated more similarity between the land-use systems at a site in the seed bank than in the aboveground plant communities.

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