Encroachment dynamics of Acacia drepanolobium in Maswa game reserve

Abstract

Habitat degradation, caused in particular by woody plant encroachment, has been a common phenomenon in savanna ecosystems. With increasing woody plant cover in open savanna grassland, structural changes and their associated impact need to be assessed and quantified. We analyzed the extent of woody plant encroachment via remote sensing and assessed encroached sites and open grassland of Maswa Game Reserve on the ground to identify and examine encroaching woody plant species and their associated impacts on herbaceous plant and herbivore species. We found that woody plant cover increased by 0.5% to 2.6% per annum. Acacia drepanolobium dominated the woody encroaching species, and the number of stems in heavily encroached sites was seven times and three times higher than in open grassland and at medium encroached sites, respectively. Our results show that woody plant encroachment reduces grazers and mixed feeder species occurrence. Furthermore, the number of herbaceous species slightly increased with increase of woody plant encroachment, while herbaceous cover was negatively correlated with increase of woody plant cover. Fire significantly reduced germination success of A. drepanolobium. Intermediate simulated rainfall was more beneficial to seedling emergence success than infrequent simulated rainfall, while seedling shoots growth was significantly higher in frequent and intermediate simulated rainfall treatments than under infrequent simulated rainfall. Hence, our study suggests that the recent bush encroachment phenomena might have been stipulated by suppression of bush fires and frequent low rainfall. This understanding will contribute in management of vegetation specifically woody plants in savanna ecosystems that are important wildlife habitats.