Multiple nutrients control threatened grassland vegetation in eastern South Africa

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Abstract

Grassland types associated with highly weathered upland soils derived from igneous dolerite and sedimentary sandstone in eastern South Africa are rich in restricted range plant species, and have been extensively destroyed for agriculture and other land uses. Anthropogenic increases in nutrient supply, including atmospheric N deposition, and changes in herbivore abundance may alter vegetation and threaten species in remaining fragments. We investigated the influence of multiple nutrient limitation and herbivory on the productivity, diversity, and composition of these distinctive grasslands, as part of the world-wide Nutrient Network. We examined nutrient limitation using three years of factorial additions of N, P + Ca, and K + micronutrients at two sites, one dolerite-derived and one sandstone-derived. Adding N and P + Ca increased aboveground productivity at both sites by 33–55%, indicating that these nutrients co-limited productivity. Adding N reduced species richness (by 11%) and effective species richness (by 24%) at the sandstone site; all nutrient additions also tended to reduce diversity at the dolerite site. At both sites, adding N increased the abundance of grasses at the expense of (N-fixing) legumes, which declined in relative abundance by 44–60%, whereas adding P + Ca tended to increase the abundance of legumes. At the dolerite site, a factorial fencing and nutrient addition experiment showed that large herbivores did not significantly influence vegetation during three years. Widely increasing atmospheric N deposition, and perhaps warming-induced increases in N mineralization at high elevation, will likely increase productivity, reduce diversity, and reduce legume abundance in these dolerite and sandstone grasslands, posing an additional challenge to conserving them.

Original languageEnglish (US)
Pages (from-to)225-236
Number of pages12
JournalSouth African Journal of Botany
Volume112
DOIs
StatePublished - Sep 2017

Bibliographical note

Funding Information:
Ralph Katzwinkel (Summerveld) and Mondi Shanduka Newsprint (Gilboa) supported these experiments on their land. Jane and Mike Wragg, Johannes Ncgobo, Caroline Bell, Lynette Bingham, Stuart Hall, and Mark Taylor helped establish and sample the experiments. Kevin Kirkman, Nicole Hagenah, Terry Everson, Craig Morris, Martin Hampton, Alison Young, and Steven Johnson (University of KwaZulu-Natal) helped with materials, equipment, and lab facilities. Trevor Edwards, Christina Curry, David Styles, Richard Boon, Kathleen Gordon-Gray, Rob Scott-Shaw, Debbie Jewitt, Kevin Kirkman, and Craig Morris helped identify plants. David Tilman, Eric Seabloom, Sarah Hobbie, and Jeannine Cavender-Bares commented on the manuscript. Funding: PDW was supported by a Scholarship for Doctoral Study Abroad from the National Research Foundation (South Africa) and by Graduate School and Doctoral Dissertation fellowships from the University of Minnesota. This work was generated using data from the Nutrient Network (http://www.nutnet.org) experiment, funded at the site-scale by individual researchers. Coordination and data management have been supported by funding to E. Borer and E. Seabloom from the National Science Foundation Research Coordination Network (NSF-DEB-1042132) and Long Term Ecological Research (NSF-DEB-1234162 to Cedar Creek LTER) programs, and the Institute on the Environment (DG-0001-13). We also thank the Minnesota Supercomputer Institute for hosting project data and the Institute on the Environment for hosting Network meetings. Network soil analyses were coordinated by Lydia O'Halloran, Missy Rudeen, and Elizabeth DeLorenze. Eric Lind compiled and managed Network data. Elsa Cleland extracted roots from soil samples.

Keywords

  • Biodiversity
  • Drakensberg Foothill Moist Grassland
  • Eutrophication
  • Grassland
  • Herbivory
  • KwaZulu-Natal Sandstone Sourveld
  • Legumes
  • Nitrogen deposition
  • Nutrient Network (NutNet)
  • Nutrient co-limitation
  • Nutrient limitation
  • Primary productivity
  • Species richness

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