Alien tree invasion into a South African montane grassland ecosystem: impact of Acacia species on rangeland condition and livestock carrying capacity

Abstract

The degree to which invasive Acacia species affect South Africa's livestock production has received little attention. We investigated the ecological impacts of Acacia mearnsii invasion on forage quality and quantity and on soil resources, along A. mearnsii invasion gradients, on South African rangelands and the subsequent conditions following clearing. Grazing capacity was reduced by 72% in densely invaded sites, whereas clearing improved grazing capacity by 66% relative to densely invaded sites within 5 years. In densely invaded sites total grass species basal cover was reduced by up to 42%. As a result, A. mearnsii reduced grazing capacity, from 2 to 8 ha required to support one large stock unit (ha/LSU) in uninvaded and densely invaded sites, respectively. Soil moisture content was lower in densely invaded sites compared with lightly invaded and cleared sites. Plant litter increased from 1.3% to 4.2%, carbon content of the soil increased from 2.0% to 4.0% and nitrogen concentrations increased from 0.1% to 0.2% in response to invasion by A. mearnsii. Clearing resulted in improved grazing capacity within 5 years. These results also showed that, if left uncontrolled, wattle species can reduce livestock carrying capacity within montane grasslands in South Africa. ARTICLE HISTORY

An Evaluation of the Impacts of Alien Species on Biodiversity in South Africa Using Different Assessment Methods

“…They found significant declines in native plant species richness at the scale of the sample quadrats used in their study (4-256 m 2 ), notably when the cover of alien plants exceeded 50%. Similarly, Yapi et al (2018) recorded marked declines in the cover of native grass species with increases in the canopy cover of alien Acacia mearnsii (Black Wattle) trees in the Eastern Cape. Such declines in the abundance and richness of native plant species, and associated faunal species, are likely to take place where any alien plant species becomes dominant.…”

Diverse trends in observed pan evaporation in South Africa suggest multiple interacting drivers

“…One explanation for locally declining wind speeds is the increasing density of trees in South African landscapes. [35][36][37] Woody encroachment -the spread of invasive alien trees across biomes and planted trees -likely causes lower surface wind speeds and is very likely a cause of changes in fetch conditions, possibly also accounting for attributions of global stilling for the evaporation declines postulated by others.…”

Opportunities and challenges in using remote sensing for invasive tree species management, and in the identification of restoration sites in tropical montane grasslands

“…Invasion by exotic trees not only reduce the extent of grassland but also threaten endemic grassland species (Allan et al, 1997;Robin et al, 2014), alter hydrological regimes (Buytaert et al, 2007;Le Maitre et al, 2000;Sikka et al, 2003), reduce wildlife grazing capacity (Yapi et al, 2018) and impact livelihood practices of traditional communities (Cordero et al, 2018;Shackleton et al, 2019). Exotic tree invasions have also been reported to increase fuel loads and fire intensities (Van Wilgen and Richardson, 1985), which degrade soil quality (Lazzaro et al, 2014) and lead to soil erosion (Van der Waal et al, 2012).…”