1. School for Environmental Research, Charles Darwin University, Darwin, NT 0909, Australia
2. Present address: Department of Biological Sciences, Macquarie University, NSW 2109, Australia
3. Natural Resources and the Environment, CSIR, PO Box 395, Pretoria, South Africa
4. Department of Plant Science, North Carolina State University, Raleigh, NC, 27695, USA
5. Department of Botany, University of Cape Town, Rondebosch, 7701, South Africa