Assessment Of Woody Plant And Grass Competitive Interactions Using Reciprocal Transplants Along A Rainfall Gradient In Namibian Savannas

Abstract

Savannas are mainly characterised by co-existence of woody and herbaceous vegetation. In Namibia, savannas occupy about 64% of the country’s land area. They are characterised by the arid to semi-arid climatic conditions and play a vital role in livestock and vegetation production. Most vegetation communities in savannas are influenced by rainfall and species interactions. Change in climatic pattern has shown that a decrease in rainfall can cause more frequent droughts and ultimately decrease the productivity of rangelands and can increase competition among the plants. Savannas present problems for management such as the increase in shrub thickening which can reduce grass production leading to a reduction in the carrying capacity of rangelands. However, there is no consensus yet on the main drivers of shrub encroachment, making it hard to set conservation goals and management recommendations in the savannas.

This research intended to assess competitive interactions between woody plants and grasses in the savanna’ along a rainfall gradient in Namibia using plant neighbour manipulations. Reciprocal transplant experiments were conducted at three “Options for sustainable geo-biosphere feedback management in savanna systems under regional and global change’’ (OPTIMASS) sites along a rainfall gradient from November 2016 to April 2017. Seeds of Senegalia mellifera (tree), Anthephora pubescens and Stipagrostis uniplumis (grasses) were collected and sown at their origin site and a neighbour drier site. Germination, survival rates and height growth were measured.

Clipping was done before seeding and within the experiment period by removing all emerging plants surrounding the respective pipes. This study revealed that clipping did not have a significant effect on the germination, survival fractions and height of all the three species. There was a higher germination fraction of S. uniplumis and S. mellifera at Tsumeb than at Okakarara. This can be attributed to the fact that high amount of rainfall was received in this area. Only S. mellifera survived in Tsumeb and significant interactions occurred in the seed origin and site with clipping. The grass plants (S. uniplumis and A. pubescens) did not survive at Tsumeb, this can be due to the clay soils found around Tsumeb and these limited the roots to penetrate deep into the soils to get water. Additionally, these species could not compete with

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other perennials found in that area. The study concluded that, clipping (neighbour manipulation) does not have an effect on germination and survival fractions neither on height growth.

Furthermore, S. mellifera grew well despite the decrease in rainfall and steeper gradients are needed to determine the effect of severe drought conditions. There is a need to put in place interventions to control S. mellifera.

Keywords: Anthephora pubescens, clipping, rainfall, Senegalia mellifera, Stipagrostis uniplumis.