Positive associations involving the tussock grass Panicum turgidum Forssk. in the Air-Tenere Reserve, Niger

In the Saharan mountain range of the Aft, Niger, the tussock-grass Panicum turgidum Forssk. was hypothesized to act as a nurse plant for a number of species. Multiple spatial positive associations involving P. turgidum were detected along an aridity gradient with an index comparing species richness inside vs. outside tussocks. Associated plants were annuals and young and/or small perennials. Comparative abiotic measurements inside tussocks and in openings showed that P. turgidum diminished seriously the stress induced by vapor pressure deficit (VPD) during the harshest periods of the day (p < 0.001, randomization test). This trait identified it as a potential nurse plant. The preference for hosted plants to develop inside dead tussocks rather than in living tussocks at the driest site may indicate that the nurse plant potential of P. turgidum tussocks declined in the presence of extreme water stress. Further experimental work, especially removal experiments, should improve the understanding of the underlying mechanisms of facilitation processes in this particularly harsh environment. (c) 2006 Elsevier Ltd. All rights reserved.