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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
environmental science
Increased tree densities in South African savannas: >50 years of data suggests CO
2
as a driver
Global Change Biology, Volume 18, No. 2, Year 2012
Notification
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Description
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. © 2011 Blackwell Publishing Ltd.
Authors & Co-Authors
Buitenwerf, Robert
South Africa, Cape Town
University of Cape Town
Bond, William J.
South Africa, Cape Town
University of Cape Town
Stevens, Nicola
South Africa, Johannesburg
University of the Witwatersrand
South Africa, Pretoria
The Council for Scientific and Industrial Research
Trollope, Winston S.W.
South Africa, Nelspruit
Research and Development
Statistics
Citations: 335
Authors: 4
Affiliations: 4
Identifiers
Doi:
10.1111/j.1365-2486.2011.02561.x
ISSN:
13541013
e-ISSN:
13652486
Research Areas
Environmental
Study Design
Cross Sectional Study