Rainfall and Soils, Not Grazing Intensity, Determine the Composition and Productivity of Annual Plants in a Biodiverse Arid Winter Rainfall Region

Martha Konje ,Neil Griffin ,P K Muoria ,E Wabuyele ,Susanne Vetter

doi:10.7176/jnsr/12-10-04

Abstract

Concentration of grazer activity around watering points and stock posts has led to well-documented vegetation impacts in arid and semi-arid rangelands. Effect of grazing and abiotic factors on perennial plant diversity have been reported in the bio-diverse winter rainfall vegetation of the Succulent Karoo in South Africa, but the impact on annuals had not been investigated. The aim of this study was to assess the effect of rainfall, soil nutrients, land forms and grazing on forage depletion, soil chemistry and the composition, diversity, richness and biomass production of annual plants in the Richtersveld National park, which is a contractual national park used by semi-nomadic pastoralists to herd goats and sheep. A grazing gradient away from stock posts at 100m, 500m and 1000m on sandy plains and rocky foothills at five study sites with different mean annual rainfall and vegetation types were used. Distance from stock posts corresponded to a gradient of forage depletion and resulted in changes in soil chemistry with distance. Biomass production, richness and diversity of annuals were correlated with rainfall but not significantly affected by landform or distance from stock posts. Rainfall and soil variables had a greater influence on species composition than grazing in this arid ecosystem. The decrease in perennial cover and richness near the stock posts was not accompanied by increase in biomass production or richness of annuals, which is expected to have adverse effects on overall plant diversity and forage availability. Keywords: Piosphere, Pastoralists, Rangeland, Succulent Karoo DOI: 10.7176/JNSR/12-10-04 Publication date: May 31 st 2021

Highlights

Drylands support the livelihoods of millions of pastoralists
Rainfall variability which is common in Succulent Karoo affects the productivity, diversity and composition of annual plants leading to variability in forage availability and distribution in different landscapes
Since annual plants are unreliable source of forage due to their high dependence on rainfall, decrease of perennials cover and biomass production near the stock posts that is not accompanied by increase in biomass production of annual plants will have unfavorable effects on the ecosystem as well as to the herbivores especially during the dry periods

Summary

Introduction

Drylands support the livelihoods of millions of pastoralists. Pastoralist systems are adapted to tracking high degrees of spatial and temporal climatic variability, with livestock grazing being spatially and seasonally constrained by the availability of water. While traditional nomadic pastoralists had very limited year-round impact in most arid regions due to the absence of perennial water; their population growth and sedentarization around settlement and permanent watering points have resulted in unprecedented livestock densities that are able to access an increasing proportion of the world’s arid vegetation. This has resulted in rangeland degradation, loss of biodiversity and conflict with conservation in many areas. Grazing movements around stock posts and watering points causes grazing-induced gradients in soil nutrient availability due to increased deposition of dung and urine around the stock posts

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