Elephant‐mediated compositional changes in riparian canopy trees over more than two decades in northern Botswana

Abstract

Questions: How has the composition and diversity of canopy tree species in a riparian woodland changed over time? How are the compositional changes related to impact of elephants? Does the composition of juvenile plants indicate that the woodland retains the potential to recover its former composition? Location: Northern Botswana adjoining the Linyanti River. Methods: We assessed the species composition of the riparian woodland in 2007/2008 along belt transects, recording living and dead individuals in different size classes plus signs of elephant damage and the presence of juvenile plants. We related this current composition to the composition recorded in a previous survey in 1991/1992 and reconstructed the earlier composition by combining living and dead trees recorded in 1991/1992. We established the association between mortality and impact agent, severity, year and size class using model selection statistics. Changes in species diversity were assessed using the Shannon diversity index. Results:The composition of canopy trees changed from the initial dominance of two Acacia spp. towards the current situation with these two species forming <5% of the woodland canopy. Dead trees were strongly associated with severe damage inflicted by elephants, including bark stripping and felling. As the acacia trees declined, elephant impacts shifted onto other canopy tree species. The woodland canopy became progressively more open because recruitment from juvenile and sapling stages to replace trees that had died was also suppressed. Nevertheless, the tree species that had decreased in abundance in the canopy remained abundant as juvenile plants. Conclusion: Substantial changes in woodland composition can occur in the presence of high elephant concentrations because of the selective damage that elephants impose on particular tree species and size classes. The loss of functionally important species may not be reflected by changes in compositional diversity measures. K E Y W O R D S Acacia, Chobe National Park, Colophospermum mopane, debarking, demographic bottleneck, disturbance, diversity, elephant impact, non-equilibrium, regeneration, wind throw 586 | Journal of Vegetation Science TEREN ET al.

Long‐term high densities of African elephants clear the understorey and promote a new stable savanna woodland community

“…The vegetation changes observed were modest despite the long‐term exposure to high elephant browsing pressure. The relatively modest vegetation changes revealed here are highly different from previous studies done in the Chobe National Park riverfront, Botswana, where local elephant densities are extreme during the dry season because this is the only source of water in the region during the dry season (Rutina & Moe, 2014; Teren et al, 2018), but are in accordance with another study carried out further into the Chobe National Park (>50 km from the river) where there are small effects of elephants on the woodlands (Kalwij et al, 2010). Taken together, these and our results suggest that elephant might have a drastic impact in large ecosystems that have reached an equilibrium, but only close to water sources.…”

Conservation with elevated elephant densities sequesters carbon in soils despite losses of woody biomass

“…Our results of 33% losses are in line with these studies, and support our first hypothesis that higher elephant densities lead to substantial loss of woody biomass and related C stocks. Apart from the direct impacts of elephants also other drivers such as fire, a selective browsing of nutrient‐rich trees (Teren et al, 2018) and the potential interactions of these drivers with elephants may have contributed to the losses of aboveground woody biomass (Pellegrini et al, 2017). In this study, we cannot decipher the contribution of each individual factor, but following Davies and Asner (2019) we consider the impact of elephants to be the dominant one.…”

“…Increasing numbers of megaherbivores impact ecosystem functioning at different spatial and temporal scales (Dirzo et al, 2014; Fullman & Bunting, 2014; Hempson et al, 2017). Previous studies have often outlined effects on vegetation characteristics such as on aboveground biomass (Guldemond et al, 2017) including vegetation structure and composition (Cromsigt et al, 2018; O’Connor & Page, 2013; Teren et al, 2018). Those studies found that browsing megaherbivores typically exerts strong negative effects on woody aboveground biomass, for example, by increasing the mortality rates within tree and shrub populations, thus also reducing the amount of carbon stored in aboveground biomass (Smit & Putman, 2011).…”

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Woody vegetation damage by the African elephant during severe drought at Pongola Game Reserve, South Africa

“…debarking) and mortality to this species from elephant browsing are common and have been recorded at various locations (e.g. Northern Botswana (12% and 13%, Teren, Owen‐Smith, & Erasmus, 2018), Mkhuze Game Reserve [5.3%, 3.2%, White & Goodman, 2010]). However, unlike other species (see below), no studies have reported complete extirpation, and significant regeneration from elephant damage has been recorded (e.g.…”