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Accueil du site → Master → Pays Bas → 2012 → Effects of flow alteration on Faidherbia albida stands of Middle Zambezi floodplains, Zimbabwe

UNESCO-IHE Institute for Water Education, Delft (2012)

Effects of flow alteration on Faidherbia albida stands of Middle Zambezi floodplains, Zimbabwe

Gope, E.T.

Titre : Effects of flow alteration on Faidherbia albida stands of Middle Zambezi floodplains, Zimbabwe

Auteur : Gope, E.T.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2012

Floodplain ecosystems support animal and plant species. The lifecycles of the floodplain species has evolved with natural flows of the river and is accustomed to the dynamics of the river flow. However, to support human needs such as energy, food and water, dams are being constructed worldwide modifying natural flows of rivers. The impoundment of Kariba dam upstream of Middle Zambezi floodplains and islands stabilized the river discharge to an almost constant flow irrespective of the season. As a result, the floodplain’s ecological integrity is decreasing as evidenced by the failure of regeneration of Faidherbia albida. This study aims to show the effect of the altered flow regime on the establishment and growth of F. albida on the Middle Zambezi floodplain and islands and to test the potential of dendrochronology in detecting the age and long-term growth dynamics of F. albida in response to flow regime. Field measurements of tree diameter, height and density were carried out to ascertain the current F. albida stand structure and to discuss its relationship with flow regime. In the laboratory, dendrochronology procedures were applied for the detection of age and long-term growth dynamics of F. albida trees. Results indicate a developing uneven-aged young F.albida stand on relatively ’new islands’, with more young trees and few old trees while the floodplain and the ’old island’ exhibits an even-aged stand with a lack of regeneration, coupled with a dying-off of older trees. The lack of establishment on the floodplain was argued to be related to the mis-matched and decreased flooding events associated with decrease in soil moisture and groundwater recharge as well as a decrease in alluvial deposits, which deprives F.albida of a competition-free seedbed for establishment. Browsers were argued to have influenced regeneration of the tree through their feeding habits and by being the main seed dispersers. Alternatively, the lack of regeneration on the floodplain was also argued to be a result of succession. Tree-ring analyses revealed that F. albida forms distinct annual growth rings. The species is characterized by fast growth during juvenile stage and a strong decrease in radial growth after 20 years. Floodplain trees with a diameter of 50 cm were found to be about 100 years old while an island tree of about 30cm was found to be 16 years old. Island trees are growing faster than floodplain trees did about 100 years ago. Lack of competition and possible favourable moisture conditions might be providing optimal conditions for establishment and growth on the islands. Kariba dam has negatively affected F.albida habitat on the floodplain but has had a positive effect on the new islands. Although the dam has altered the establishment of trees on the floodplain, no abrupt decline in growth rate of older trees was observed. However, with a reliable chronology from the area it is likely to deduce the dominant environmental factor(s) affecting the tree’s growth.

Sujets  : floodplains ecosystems flow regimes Zimbabwe


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