Grass species richness decreases along a woody plant encroachment gradient in a semi-arid savanna grassland, South Afric
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RESEARCH ARTICLE
Grass species richness decreases along a woody plant encroachment gradient in a semi-arid savanna grassland, South Africa Regina Mogashoa . Phesheya Dlamini
. Masibonge Gxasheka
Received: 11 November 2019 / Accepted: 5 November 2020 Ó Springer Nature B.V. 2020
Abstract Context Savanna grasslands are undergoing rapid land cover transformation as a consequence of woody plant encroachment. It still remains unclear how increasing woody plant density and cover in savannas influences tree-grass-soil interactions. Objectives This study determined the effect of increasing tree density on grass species richness, evenness, diversity and assessed how nutrients change along an encroachment gradient of a shallow plinthic savanna soil. Methods Vegetation was quantified and soil samples analysed from eighteen randomly distributed 10 m 9 10 m plots along a tree density gradient spanning from open, transitioning into intermediate and intensively encroached savanna grassland in northern South Africa. Results We found that an increase in tree density along the encroachment gradient decreased grass Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10980-020-01150-1) contains supplementary material, which is available to authorized users. R. Mogashoa P. Dlamini (&) M. Gxasheka School of Agricultural & Environmental Sciences, Department of Plant Production, Soil Science & Agricultural Engineering, University of Limpopo, Private Bag X1106, Sovenga 0727, Limpopo, South Africa e-mail: [email protected]; [email protected]
richness (GR) by 53% in the intensively woodyencroached grassland. Higher tree density and canopy cover led to a 90% dominance of Panicum maximum grass species. Conversely, higher tree density increased concentrations of carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil layer. Conclusions This study highlights that woody plant encroachment decreases grass species richness and composition, but increases nutrients in the uppermost layer of a plinthic savanna soil. Such information is critical to improve understanding of the mechanisms underlying tree-grass interactions in savannas. Keywords Tree-grass system Grass species composition Richness Diversity Soil nutrients Savanna Vachellia species Dichrostachys cinerea
Introduction Savannas occur in the tropics and subtropics and cover about 20% of the earth’s land surface (Sankaran et al. 2005; Bond 2008; Lehmann et al. 2011) and 50% of the African continent (Osborne et al. 2018). Savannas are characterized by the co-dominance of two contrasting plant life forms, namely trees and grasses. The representation of these life forms vary considerably across savanna types (Frost et al. 1986). For instance,
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Landscape Ecol
tropical savannas are distinguished from tropical grasslands by the presence of trees (Belsky, 1994). It has been reported that C4 grasses dominate tropical and subtropical savan
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