Metalliferous plants – the highly adapted “metalheads”  

Metalliferous plants have a unique ability to establish and grow on soils rich in certain metals like nickel, copper, cobalt and chromium, which are not essential for plant growth at high concentrations. Since plants do not have the ability to remove their roots from a toxic environment and establish themselves in more suitable soils, some plants have displayed an amazing ability to adapt, something that has been puzzling geobotanists since the turn of the 20^th century.

Over the years the curtains were lifted. Through long-term research, light has been shed on the mystical plants with the ability to grow on unusual soil types like serpentine, gypsum and limestone. These plants have developed certain physiological and morphological adaptations over time to tolerate “heavy metals” and so evolved into the “metalheads” of their unique habitats.

It is now known that metalliferous plants can either accumulate, tolerate or exclude the metals that would have been toxic to plants under normal conditions. Furthermore, many of these highly adapted plants have become so advanced and specialised that they are endemic to the unusual soil and will not be able to flourish outside of their preferred habitat. Hence, these plants are referred to as endemic edaphic (soil) specialist plant species.

A geobotanical mystery on unusual soils  

The Griqualand West Centre of plant endemism is home to 24 endemic plant species which can be considered edaphic specialists. Yet, it remains a mystery how these plants can persist on unusual soil types derived from the underlying parent geology.

The mountains of Griqualand West consist of three dominant geological types. In the east is the Ghaap Plateau, which is primarily dominated by dolomitic limestone. Towards the west of the plateau, a range of rolling hills of banded ironstone can be found. These are known as the Kuruman Hills (between the towns of Kuruman and Daniëlskuil). The Asbestos Hills lie south of Daniëlskuil towards the area of Niekerkshoop and Westerberg.

When standing on the ironstone hills, looking west or passing Postmasburg to the west, the quartzitic Langberg is a distinctive and majestic landscape feature. The Langberg extends into the Koranna Mountains in the north, passing the town of Olifantshoek, all the way down in the south towards Boegoeberg. For the planned pilot study, the targeted geologies will be the dolomitic limestone and banded ironstone.

Plants that establish and grow on dolomitic limestone must be able to tolerate soils with high calcium (> 11 000 mg/kg) and magnesium (> 5 000 mg/kg) content, contributing to high calcium: magnesium ratios (> 2) and high soil alkalinity (pH > 7). Initially, it was thought that soils associated with ironstone would be similar to serpentine soils. In general, serpentine soils have a higher magnesium than calcium content, resulting in extremely low calcium-to-magnesium ratios (< 1), with high soil pH. Additionally, plants that grow on these serpentine soils are threatened by high metal toxicity, e.g. nickel and chromium, as well as low nutrient availability. In contrast, soil analysis revealed that ironstone soils of Griqualand West have a higher calcium-to-magnesium ratio (> 1), with the soil being slightly acidic (pH ranging between 5 and 6). Yet, similar to serpentine soil, certain metals were detected at higher levels than nickel and chromium. This included iron (> 50 000 mg/kg), aluminium (27 000 – 37 000 mg/kg) and manganese (900 – 1 200 mg/kg). Under acidic pH conditions, these micronutrients may become available for plant uptake, resulting in metal toxicity.

It is highly likely that plant species occurring on these two contrasting geological types will be metal excluders (similar to previous studies conducted on unusual soil types in centres of plant endemism in South Africa). To test her ideas, Nanette will scan leaves of the plant species in the field with a portable handheld X-Ray Fluorescence (XRF) analyser. If found that certain species show high metal content in the leaves (which then suggests potential metal accumulators), samples will be collected, dried and preserved for Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for further investigation.

To confirm whether there are extractable metal concentrations from the soil, a composite soil sample will be collected close to the specific individual that tested positively for metal leaf content and analysed in a laboratory.

Vegetation surveys are currently planned, and Nanette hopes to present the preliminary findings at the 11^th International Conference on Serpentine Ecology in Kyoto, Japan, in June this year.