Time Series Monitoring of Bush Encroachment by Euclea divinorum in Ol Pejeta Conservancy Laikipia, Kenya

摘要

Bush encroachment refers to the invasion of woody species in Savannah ecosystems driven by either anthropogenic and/or natural factors. This study sought to examine land cover changes and topographic features attributable to patterns of encroachment in Ol Pejeta Conservancy (OPC) where, Euclea divinorum unpalatable woody species has colonised former grasslands and other habitats which provide grazing grounds for herbivore wildlife species. Here, we monitored vegetation cover trends in the period 1987-2016 using five vegetation classes on Landsat images acquired during the dry season. Additionally, slope based NDVI maps and digital elevation models were used to identify topographic influences on vegetation change. Results revealed that E. divinorum increased significantly between 1987 and 2016 (Mann Kendall test for trend analysis tau 1, n=6, p< 0.01). On the other hand, Acacia drepanolobium and Acacia xanthophloea decreased from 49.72% and 5.31% in 1987 to 17.00% and 0.29% in 2016 respectively. Further, areas in low elevation were more colonised by E. divinorum. The colonising E. divinorum is unpalatable hence lacks natural predators whilst, A. drepanolobium and A. Xanthophloea which are alternative herbivory species decreased. Understanding dynamics of woody vegetation in savannah is crucial for management of healthy and sustainable ecosystems.