N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species
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N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species Michael D. Cramer & William J. Bond
Received: 26 February 2012 / Accepted: 6 September 2012 / Published online: 16 September 2012 # Springer Science+Business Media B.V. 2012
Abstract Background and aims Below-ground grass competition limits woody establishment in savannas. N2-fixing legumes may, however, have a nutritional advantage over broad-leaved species. We hypothesised that broad-leaved non-legume savanna thicket species would be more severely constrained by grass competition for N and consequently respond more to Nfertilization than the legume, Acacia karroo. Methods A. karroo and five non-legume thicket species (Maytenus senegalensis, M. heterophylla, Euclea divinorum, Ziziphus mucronata, Schotia brachypetala) were grown together in an irrigated competition experiment with clipped-, unclipped-grass and without grass with/without N-fertilizer. The biomass, foliar
Responsible Editor: Hans Lambers. Electronic supplementary material The online version of this article (doi:10.1007/s11104-012-1456-4) contains supplementary material, which is available to authorized users. M. D. Cramer (*) : W. J. Bond Department of Botany, University of Cape Town, Private Bag X1, Rondebosch 7701, South Africa e-mail: [email protected] M. D. Cramer School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
nutrient, δ13C and δ15N of grasses and woody species were determined. Results Growth of both A. karroo and the non-legume species was equally sensitive (c. 90 % reduction) to both clipped- and unclipped-grass competition, regardless of N-fertilization. With grass competition, however, foliar [N] increased and δ15N decreased in response to N-fertilization. Grass biomass accumulation was also unchanged by fertilisation, despite increases in foliar [N] and decreases in δ15N. Conclusions The N2-fixation capacity of A. karroo provided no growth advantage over non-legumes. The lack of responsiveness of biomass accumulation by both the woody species and the grasses to Nfertilization, despite evidence that plants accessed the N-fertilizer, indicates limitation by other nutrients. Keywords Africa . Bush encroachment . Competition . Grass . Legume . N2 fixation
Introduction Savannas represent a delicate balance between woody plants and grasses (Frost et al. 1986). Although grasses form a near continuous cover, woody plants vary in density so that savanna structure varies along a continuum from treeless grasslands to woodlands. The balance between savanna tree and grass life-forms has also changed over recent decades with increasing densities of woody species in many savannas (O’Connor and
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Crow 2000; Hoffmann and Ashwell 2001) and encroachment of savannas by scrub forests (‘thickets’) (Wigley et al. 2010). The woody species composition of savannas is thought to be dictated by the ability of the seedlings
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