The effect of altitude on arid inselbergs along a bioclimatic gradient

Abstract

Twenty-four inselbergs in four distinct landscapes along a biogeographic gradient from mesic winter rainfall in the southern Namib to arid summer rainfall in the northern Namib were analysed. Declining similarity of vegetation between matrix (surrounding plains) and inselbergs with altitude were exhibited in most inselbergs, irrespective of biogeographic region, rock types and height above surroundings. Some inselbergs showed the least congruence with matrix vegetation on upper slopes, others with the top of inselbergs. This is probably related to the nature of the individual inselbergs, e.g. with steep cliffs in the upper reaches supporting plant species adapted to more mesic conditions. Most trends of declining similarity between matrix and inselberg habitats were maintained across the investigated bioclimatic regions. Even low inselbergs clearly supported vegetation very distinct from the surrounding area. Their contribution to species richness of a landscape is therefore unquestionable and adequate protection of such landscapes is needed where human impacts are severe.

Keywords:

• altitude gradient
• matrix
• mountain habitat
• species pool
• species richness

Acknowledgements

The field work in 1998 and 1999 was supported with funds from the EU's INCODEV programme. Namibia's Ministry of Environment and Tourism granted permission for this research, and the National Botanical Research Institute in Windhoek assisted with plant identification. The fieldwork would not have been possible without the unwavering support of my late husband, John. Two anonymous reviewers provided very useful comments which improved the manuscript.