The proposed colonisation sequence of woody species in the Sourish Mixed Bushveld of the Limpopo province, South Africa

Jordaan, J.J.

doi:10.2989/10220119.2010.502650

Cited by 5 publications

(4 citation statements)

References 4 publications

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“…There are many reports of woody plant increase in grasslands and savannas worldwide over the past century. The increases may cause biome shifts from grassland to scrub forest (Briggs et al ., ; Brook & Bowman, ; Bowman et al ., ; Wigley et al ., ), or may be gradual and reversible increases of trees within savannas (Hoffman & O'Connor, ; O'Connor & Crow, ; Archer et al ., ; Roques et al ., ; Goslee et al ., ; Asner et al ., ; Walker & Meyers, ; Van Auken, ; Jordaan, ). Large increases in tree density in grass dominated or mixed tree‐grass systems will significantly alter their functioning and biodiversity (Blaum et al ., ; Sirami et al ., ), and will reduce the yield and economic profit from rangelands (Scholes, ).…”

Section: Introductionmentioning

confidence: 99%

Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver

Buitenwerf

Bond

Stevens

et al. 2011

Global Change Biology

320

248

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For the past century, woody plants have increased in grasslands and savannas worldwide. Woody encroachment may significantly alter ecosystem functioning including fire regimes, herbivore carrying capacity, biodiversity and carbon storage capacity. Traditionally, increases in woody cover and density have been ascribed to changes in the disturbance regime (fire and herbivores) or rainfall. Increased atmospheric CO 2 concentrations may also contribute, by increasing growth rates of trees relative to grasses. This hypothesis is still heavily debated because usually potential CO 2 effects are confounded by changes in land use (disturbance regime). Here we analyse changes in woody density in fire experiments at three sites in South African savannas where the disturbance regime (fire and herbivores) was kept constant for 30 and 50 years. If global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime. Woody density remained constant in a semiarid savanna but tripled in a mesic savanna between the 1970s and 1990s. At the third site, a semiarid savanna near the southern limits of the biome, tree density doubled from the mid 1990s to 2010. Interpretation of the causes is confounded by population recovery after clearing, but aerial photograph analysis on adjacent non-cleared areas showed an accompanying 48% increase in woody cover. Increased CO 2 concentrations are consistent with increased woody density while other global drivers (rainfall) remained constant over the duration of the experiments. The absence of a response in one semiarid savanna could be explained by a smaller carbon sink capacity of the dominant species, which would therefore benefit less from increased CO 2 . Understanding how savannas and grasslands respond to increased CO 2 and identifying the causes of woody encroachment are essential for the successful management of these systems.

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Section: Introductionmentioning

confidence: 99%

Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver

Buitenwerf

Bond

Stevens

et al. 2011

Global Change Biology

320

248

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“…Over the last several decades, the remaining unploughed rangelands in the study area have been degraded by widespread encroachment of indigenous woody plants in the absence of any erosion [ 41 ]. Given this regional increase in the abundance of trees, a unique opportunity was provided by the fact that the degree of woody encroachment in the experimental site has come to show considerable variation according to the former levels of fertilization among the 60 plots ( Fig 2 ).…”

Section: Methodsmentioning

confidence: 99%

Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna

Mills

Milewski

Snyman³

et al. 2017

PLoS ONE

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The causes of the worldwide problem of encroachment of woody plants into grassy vegetation are elusive. The effects of soil nutrients on competition between herbaceous and woody plants in various landscapes are particularly poorly understood. A long-term experiment of 60 plots in a South African savanna, comprising annual applications of ammonium sulphate (146–1166 kg ha-1 yr-1) and superphosphate (233–466 kg ha-1 yr-1) over three decades, and subsequent passive protection over another three decades, during which indigenous trees encroached on different plots to extremely variable degrees, provided an opportunity to investigate relationships between soil properties and woody encroachment. All topsoils were analysed for pH, acidity, EC, water-dispersible clay, Na, Mg, K, Ca, P, S, C, N, NH4, NO3, B, Mn, Cu and Zn. Applications of ammonium sulphate (AS), but not superphosphate (SP), greatly constrained tree abundance relative to control plots. Differences between control plots and plots that had received maximal AS application were particularly marked (16.3 ± 5.7 versus 1.2 ± 0.8 trees per plot). Soil properties most affected by AS applications included pH (H2O) (control to maximal AS application: 6.4 ± 0.1 to 5.1 ± 0.2), pH (KCl) (5.5 ± 0.2 to 4.0 ± 0.1), acidity (0.7 ± 0.1 to 2.6 ± 0.3 cmol kg-1), acid saturation (8 ± 2 to 40 ± 5%), Mg (386 ± 25 to 143 ± 15 mg kg-1), Ca (1022 ± 180 to 322 ± 14 mg kg-1), Mn (314 ± 11 to 118 ± 9 mg kg-1), Cu (3.6 ± 0.3 to 2.3 ± 0.2 mg kg-1) and Zn (6.6 ± 0.4 to 3.7 ± 0.4 mg kg-1). Magnesium, B, Mn and Cu were identified using principal component analysis, boundary line analysis and Kruskal-Wallis rank sum tests as the nutrients most likely to be affecting tree abundance. The ratio Mn/Cu was most related to tree abundance across the experiment, supporting the hypothesis that competition between herbaceous and woody plants depends on the availability of anabolic relative to catabolic nutrients. These findings, based on more than six decades of experimentation, may have global significance for the theoretical understanding of changes in vegetation structure and thus the practical control of invasive woody plants.

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“…There are many examples of other, e.g. broadleaved, species becoming established through mainly bird dispersal under, but not extending much beyond, the canopy of an Acacia (Potts and Tidmarsh 1937, du Preez and Venter 1990, Smith and Goodman 1987, Skowno et al 1999, Jordaan et al 2004, Jordaan 2010 Figure 12: Box-and-whisker plot of fruit size diameter (mm) for woody species (n  41) found in bush clumps (n  40) in the Kei Road region of the Eastern Cape bush clumps form through nucleation around an Acacia (or other) initial, with the clump attaining a size well in excess of the size of the initial tree. A spatial pattern of bush clumps of this nature is apparently not commonplace in South Africa.…”

Section: Why Bush Clumps?mentioning

confidence: 98%

“…An increased number of case studies would contribute a broader foundation for improving understanding of underlying processes. In southern Africa, bush encroachment is most commonly associated with an increase of microphyllous species such as Acacia (van Vegten 1983, Friedel and Blackmore 1988, O'Connor 2001, Roques et al 2001, Wigley et al 2010, which may facilitate the later entry of other woody species, including broad-leaved, bipinnate microphyllous, and evergreen species (Potts and Tidmarsh 1937, du Preez and Venter 1990, Hoffman and O'Connor 1999, Skowno et al 1999, Smith and Goodman 1987, Jordaan 2010. Successional change may promote development of bush clumps in certain instances (Gower et al 1992).…”

mentioning

confidence: 97%

Bush clump succession in grassland in the Kei Road region of the Eastern Cape, South Africa

O’Connor

Chamane

2012

African Journal of Range & Forage Science

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Bush encroachment is a vegetation dynamic of global interest. This study describes the pattern of succession of bush clumps in the Eastern Cape, South Africa, using a space-for-time substitution. Clumps formed following establishment of mainly Acacia karroo in grassland through nucleation via animal dispersal. A total of 49 tree or shrub species were recorded in 40 clumps. With increasing clump size, there was a corresponding increase in woody basal area, species richness and diversity, but not evenness. Correspondence analysis showed that the pattern of compositional variation was closely related to clump size; differences resulting from clump establishment disappeared as clumps developed. Population size structure of 20 (of 21) species was dominated by small individuals, indicating that regeneration has increased over time. Coddia rudis was prolific on a clump periphery; Scutia myrtina and A. karroo were dominant within clumps. Seven shrub and nine tree species were encountered more frequently within a clump than on its periphery. All species other than A. karroo had fruits suited to dispersal by birds or mammals. The overall pattern of bush clump succession shows close parallels with that initiated by invasion of Prosopis glandulosa into grassland in Texas, USA.

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The proposed colonisation sequence of woody species in the Sourish Mixed Bushveld of the Limpopo province, South Africa

Cited by 5 publications

References 4 publications

Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver

Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver

Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna

Bush clump succession in grassland in the Kei Road region of the Eastern Cape, South Africa

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The proposed colonisation sequence of woody species in the Sourish Mixed Bushveld of the Limpopo province, South Africa

Cited by 5 publications

References 4 publications

Increased tree densities in South African savannas: >50 years of data suggests CO2 as a driver

Increased tree densities in South African savannas: >50 years of data suggests CO2 as a driver

Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna

Bush clump succession in grassland in the Kei Road region of the Eastern Cape, South Africa

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Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver

Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver