Effects of invasion of fire-free arid shrublands by a fire-promoting invasive alien grass (Pennisetum setaceum) in South Africa

Authors

  • S. J. RAHLAO,

    1. Centre for Invasion Biology and Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland 7602, Stellenbosch, South Africa (Email: srahlao@sun.ac.za),
    2. Climate Change and Bioadaptation Division, Kirstenbosch Research Centre, South African National Biodiversity Institute, Claremont,
    Search for more papers by this author
  • S. J. MILTON,

    1. Centre for Invasion Biology and Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland 7602, Stellenbosch, South Africa (Email: srahlao@sun.ac.za),
    2. DST-NRF Centre of Excellence, Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch, South Africa
    Search for more papers by this author
  • K. J. ESLER,

    1. Centre for Invasion Biology and Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland 7602, Stellenbosch, South Africa (Email: srahlao@sun.ac.za),
    Search for more papers by this author
  • B. W. VAN WILGEN,

    1. Centre for Invasion Biology, CSIR Natural Resources and Environment, Stellenbosch,
    Search for more papers by this author
  • P. BARNARD

    1. Climate Change and Bioadaptation Division, Kirstenbosch Research Centre, South African National Biodiversity Institute, Claremont,
    2. DST-NRF Centre of Excellence, Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch, South Africa
    Search for more papers by this author

Abstract

Arid shrublands in the Karoo (South Africa) seldom accumulate sufficient combustible fuel to support fire. However, as a result of invasion by an alien perennial grass (Pennisetum setaceum), they could become flammable. This paper reports on an experiment to assess the effects of fire following invasion by P. setaceum. We established 10 plots (5 × 10 m) separated by 2.5 m, and added grass fuel to five plots (5 and 10 tons ha−1 to alternate halves of the plot) leaving the remaining five plots as interspersed controls. Plots with fuel added were burnt, and fire behaviour was measured during the burns. Rates of fire spread were generally low (0.01–0.07 m s−1) and did not differ significantly between burn treatments. Mean fireline intensities were higher in the high compared with the low fuel treatments (894 and 427 kW m−1, respectively). We recorded plant species and their cover before and after burning on each of the plots. After 15 months of follow-up monitoring in the burn plots, only two species, the dwarf shrub (Tripteris sinuata) and the perennial herb (Gazania krebsiana) resprouted. Most individuals of other species were killed and did not reseed during the 15-month study. The mass of added fuel load (high or low) did not influence vegetation recovery rates after fire. Should future invasions by P. setaceum lead to similar fuel loads in these shrublands, inevitable fires could change the vegetation and may favour spread of the flammable grass. Our results have important implications for predicting the effects of invasive alien plants (especially grasses) on fire-free ecosystems elsewhere. The predicted impacts of fire may alter species composition, ultimately affecting core natural resources that support the Karoo economy.

Ancillary