Tree cover and biomass increase in a southern African savanna despite growing elephant population

Abstract

Abstract. The growing elephant populations in many parts of southern Africa raise concerns of a detrimental loss of trees, resulting in overall reduction of biodiversity and ecosystem functioning. Elephant distribution and density can be steered through artificial water points (AWPs). However, this leaves resident vegetation no relief during dry seasons. We studied how the introduction of eight AWPs in 1996 affected the spatiotemporal tree-structure dynamics in central Chobe National Park, an unfenced savanna area in northern Botswana with a dry-season elephant density of ;3.34 individuals per square kilometer. We hypothesized that the impact of these AWPs amplified over time and expanded in space, resulting in a decrease in average tree density, tree height, and canopy volume. We measured height and canopy dimensions of all woody plants around eight artificial and two seasonal water points for 172 plots in 1997, 2000, and 2008. Plots, consisting of 50 3 2 m transects for small trees (0.20-3.00 m tall) nested within 50 3 20 m transects for large trees (≥3.0 m tall), were located at 100, 500, 1000, 2000, and 5000 m distance classes. A repeated-measures mixed effect model showed that tree density, cover, and volume had increased over time throughout the area, caused by a combination of an increase of trees in lower size classes and a decrease in larger size classes. Our results indicate that the decrease of large trees can be attributed to a growing elephant population. Decrease or loss of particular tree size classes may have been caused by a loss of browser-preferred species while facilitating the competitiveness of less preferred species. In spite of 12 years of artificial water supply and an annual elephant population growth of 6%, we found no evidence that the eight AWPs had a negative effect on tree biomass or tree structure. The decreasing large-tree component could be a remainder of a depleted but currently restoring elephant population.

Some ecological side-effects of chemical and physical bush clearing in a southern African rangeland ecosystem

“…The removal of mature trees from savanna ecosystems leads to changes in intra-and inter-specific tree competition, often allowing new tree seedlings (of either the encroacher or other tree species) to establish (Smit, 2004;Teague and Smit, 1992). As a result, a transition in the dominant tree species can occur, depending on factors such as shade tolerance (Ben-Shahar, 1991) or herbivore population dynamics (Kalwij et al, 2010). Our results suggest that P. leubnitziae benefitted from the reduced competition, especially for water, following removal of A. mellifera.…”

“…The importance of combatting bush thickening is therefore apparent. However, the removal of large woody species can also lead to the rapid establishment of new woody seedlings (Kalwij et al, 2010;Smit, 2004). In this study, P.…”

Patterns of forest composition and their long term environmental drivers in the tropical dry forest transition zone of southern Africa

“…The long-term effect of fire on forest composition remains however controversial, especially because it is influenced by other variables, such as soil, browsing and rainfall (Bond et al 2003;Sankaran et al 2008). The impact of elephant populations on woody vegetation is well studied in the eastern Baikiaea woodlands (Makhabu et al 2006;Holdo 2007;Loarie et al 2009;Kalwij et al 2010;Aarrestad et al 2011), however, elephants are less common in the western parts and cattle are the most important large herbivores. This study combines data of different forest inventories collected over four decades to identify tree communities over a large area and study their long-term interactions with site variables, including disturbances.…”

Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position

“…Quadrat 1 (2 x 40 m), where only seedlings (≤ 0.5 m) were recorded, was nested within quadrat 2 (30 x 40 m), where individuals > 0.5 m were recorded, which was nested within quadrat 3 (50 x 100 m), where individuals of additional species, i.e., species not encountered in either quadrat (1) or (2) were recorded, including all height classes. This sampling design is similar to Kalwij et al [10]. In each quadrat, all woody individuals were counted, identified to species, and tree height of the individual was recorded.…”

“…In savanna ecosystems, much of the spatial heterogeneity of herbivory is understood as a function of the distance to major water sources, such as rivers and waterholes, at the landscape level [9,10]. However, ecohydrological conditions can vary at much smaller spatial scales, such as within hillslopes.…”