Effect of CLimatic Variation and Agricutltural Landuse on Woody Plant Diversity and Growth of Riparian Forests in the Volta Basin of Ghana

Abstract

Conservation measures such as creation of buffer zones are used to protect riparian forests (RF) under the Freshwater Buffer Zone Policy of Ghana. Nevertheless, scientific information on the impact of climate change and agricultural landuse management on the buffer zone is scarce. This study used Afram (HS, Humid savanna) and Tankwidi (DS, Dry savanna) riparian catchments within the Volta basin of Ghana as research sites to assess the effect of climatic variation and agricultural landuse on woody plant diversity and growth of RF. The research integrated field inventorying, remote sensing, stable carbon isotope and dendrochronological techniques for the RF assessments. Analysis of time series Landsat images showed that forests area in the Afram and Tankwidi riparian catchments have been lost at a deforestation rate of 1-4% per annum from 1986 to 2014. The effect of the spatial deforestation was confirmed by significant reduction in the richness and diversity and degrading structural characteristics of woody plants in RF from protected areas to farmlands in both catchments. These degrading conditions put the farmland riparian landscape at a greater risk of harm through climatic disturbances such as fires and flooding. When sample riparian trees (Afzelia africana [evergreen] and Anogeissus leiocarpus [deciduous]) were tested, the results as it was revealed through the analysis of growth ring width and carbon-13 isotope showed that a similar climatic forcing caused by the movement of the intercontinental convergence zone is driving the growth of riparian trees in both the HS and DS. The study further showed that A. leiocarpus has higher growth rate and resilient to water stress than A. africana. A significant correlation of the trees with the equatorial Pacific Ocean Sea Surface Temperatures revealed that the riparian forests are impacted during El Niño years which are normally responsible for drought in the West African sub-region. Because of the negative effects of agricultural landuse intensification and climate change, the study recommends the enforcement of the Freshwater buffer zone policy of Ghana in order to rescue farmland riparian buffer from further degradation. Furthermore, there is the need to reforest degraded riparian areas on farmlands. Such reforestation activities should consider A. leiocarpus owing to its high fitness and growth rate in order to speed up the recovery of the riparian forest cover.