&lt;i&gt;Virgilia divaricata&lt;/i&gt; may facilitate forest expansion in the afrotemperate forests of the southern Cape, South Africa

Corli Coetsee,Benjamin J Wigley

doi:10.4102/koedoe.v55i1.1128

Abstract

Virgilia divaricata is a fast-growing nitrogen-fixing tree species often found on the margins of forest in the southern Cape of South Africa and is particularly abundant after fire. However, V. divaricatamay invade fynbos even in the absence of fire and it has been described as a forest precursor. We investigated whether V. divaricata enriches soil fertility after its invasion into fynbos areas adjacent to forests. We measured soil organic carbon and soil nutrients at four sites. At each site, three vegetation types (forest, V. divaricata and fynbos) were examined on the same soil type and at the same elevation. Our results showed that, on average, soils taken from V. divaricata stands had higher nitrogen and phosphorus values than the adjacent fynbos soils, with either lower or similar values to the adjacent forest soils. Higher soil fertility under V. divaricata, together with their shading effect, may create conditions favourable for shade-loving forest species dependent on an efficient nutrient cycle in the topsoil layers, and less favourable for shade-hating fynbos species, which are generally adapted to low soil fertility. We suggest that the restoration of the nutrient cycle found in association with forest may be accelerated under V. divaricata compared with other forest precursor species, which has important consequences for the use of V. divaricata in ecosystem restoration.Conservation implications: Alien plantations in the Outeniqua Mountains are being phased out and the areas are being incorporated into the Garden Route National Park. Fynbos areas are increasingly being invaded by forest and thicket species owing to fire suppression in lower-lying areas. An improved understanding of the fynbos–forest boundary dynamics will aid in efficient management and restoration of these ecosystems.

Highlights

In South Africa, natural forest covers about half a million hectares, which equates to only 0.5% of the land surface
We suggest that an alternative mechanism for facilitation of forest sapling growth may be that V. divaricata increases soil fertility, as both Virgilia spp. are members of Fabaceae, which have root nodules containing nitrogen-fixing bacteria (Palgrave 1981)
The three vegetation types were arranged along a gradient of soil fertility, according to which forests were found on the most fertile soils, V. divaricata was associated with intermediate soil nutrients, and fynbos was found on low-nutrient soils

Summary

Introduction

In South Africa, natural forest covers about half a million hectares, which equates to only 0.5% of the land surface. The largest forest complex is the afrotemperate Knysna forest, which covers 65 000 ha of the southern Cape. Heathlike vegetation that is adapted to frequent fire, known locally as fynbos, forms a matrix with these forests (Manders, Richardson & Masson 1992). Geldenhuys (1994) has shown that environmental factors such as rainfall and substrate determine the potential limits of forest distribution, actual forest distribution is determined by the interaction between prevailing winds during dry periods, terrain physiography and the effect of this interaction on fire pattern. Extensive plantation forestry, which has protected fynbos against fire, has led to forest expansion in the southern Cape in recent times In the prolonged absence of fire, forest species start to invade the surrounding fynbos and the dominant vegetation may switch to closed-canopy forest (Kruger 1984; Masson & Moll 1987)

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