Vegetation-environment relations along an aridity gradient on Kalahari Sands in central Namibia

Abstract

The distribution of the world’s flora and fauna is shaped by different environmental factors. Such factors operate either locally, regionally or globally to influence species distribution. Studies of vegetation-environment relationship are very crucial especially under current and future changes in climate. The Kalahari basin also known as ‘thurstland’ offers an excellent basis for these studies at a sub-continental scale due to its variability and uncertainty of rainfall, high day temperatures and a uniform soil substrate that makes the Kalahari highly susceptible to climate change. This study was aimed at determining the main environmental factors influencing the distribution of vegetation, and investigate changes in the main plant functional and structural attributes in response to the detected main environmental factors along the Kalahari aridity gradient. Five study sites were selected along the Kalahari aridity gradient in central Namibia. Ten 50 m x 20 m plots were demarcated at each site in which grasses and phanerophytes were sampled using PCQ and BECVOL sampling methods respectively. For each site, grass sward density and for phanerophytes species richness, diversity, total basal area, total canopy volume, leaf volume production at 1.5 m, 2.0 m and 5.0 m, and density were calculated. Results revealed that mean annual rainfall was the main environmental factor influencing vegetation distribution on Kalahari sands. Phanerophytes in high rainfall areas were mainly evergreen with big leaves whereas phanerophytes in low rainfall areas were mostly decidious with small leaves. Phanerophytes species richness, diversity, total basal area, total canopy volume, leaf volume production at 1.5 m, 2.0 m and 5.0 m and density increased with rainfall while grass sward density decreased with increasing rainfall mainly due to competition from increasing phanerophyte density. It was concluded that overall phanerophyte productivity declines with decreasing rainfall and vice versa, and is anticipated to decrease further under projected change in climate.