Wonderfully rich botanically

This article is an extract from Classic British Wildlife Sites The Lizard Peninsula by Andrew Byfield in British Wildlife, 3 (2), pp. 92105 (1991).

So Why is The Lizard so wonderfully rich botanically?

As long ago as 1848 the eminent botanist the Rev C A Johns commented `to the sharp eyed botanist I firmly believe that no extent of the country of equal dimensions, except the sub-alpine regions, produces so many rare and characteristic plants as The Lizard district`. No-one has ever questioned this observation! The Peninsula has 20 `Nationally Rare Red Data Book` vascular plant species, and over 50 `Nationally Scarce` species (with national strongholds for many species) – as such it ranks alongside the Breadalbanes (Ben Lawers and adjacent mountains) as the botanically richest area in Britain.

So why is this flora so remarkably diverse and rich in rare species? A number of reasons have been given of which the most quoted is the presence of serpentine. Serpentine gives rise to soils which are characterised by a number of unusual properties in relation to plant growth. Whilst it has a very low calcium content, it is nevertheless rich in bases with high levels of magnesium. As a result, whilst true `calcicoles` – calcium demanding – species are absent, many apparently lime-loving (but in fact merely demanding high base levels) are present.

Cornish Heath (Erica vegans)

Conversely many `calcifuge` – calcium hating – species typical of `acid` conditions can thrive due to the absence of high calcium levels. Thus species typical of chalk and limestone, such as Bloody Cranesbill (Geranium sanguineum), Spotted Cats-ear (Hypochoeris maculata), and Dropwort (Filipendula vulgaris), thrive with species typical of heathlands, most notably the heathers themselves.

Serpentine-derived soils also contain particularly high levels of heavy metals such as chromium and nickel. This proves toxic to some species, whist others such as Spring Sandwort (Minuarta verna), so characteristic of the lead mine spoil heaps of Northern Britain, are locally plentiful. Finally, serpentine soils are generally low in the major plant growth nutrients and are frequently shallow and skeletal. All in all not at all fertile, a fact which has mercifully helped save their vegetation cover from the plough.

Serpentine is a very rare rock nationwide – indeed worldwide- with only tiny exposures in areas such as Anglesey, Aberdeenshire, and Shetland. The Lizard exposure, covering 64 sq km, represents by far the largest in Britain.

People are quick to explain away The Lizard’s floristic riches merely as a manifestation of the presence of serpentine. Whist this is in part true – indeed a number of vegetation types found at The Lizard on serpentine are unique – it is by no means the whole story. The diversity of geological formations is of equal importance.

The geology of The Lizard is complicated: the southern half of the peninsula is made up of a complex of igneous and metamorphic rocks such as gabbro, mica, hornblende schists and granite-gneiss as well as the serpentine itself. The northern half is remarkably different, comprising a range of sedimentary shales – rocks very typical of much of the rest of Cornwall. Contrary to popular belief, the special flora of the peninsula is by no means confined to serpentine. Of The Lizard `classics`, Cornish Heath (Erica vagans) is restricted to serpentine and gabbro; Land Quillwort (Isoetes histrix), Twin-headed (Trifolium bocconei) and Upright Clovers (T.strictum) are confined to serpentine and the schists; Pigmy Rush (Juncus pygmaeus) is restricted to Crousa gravels, gabbro and serpentine; and Long-headed Clover (T. incarnatum ssp. molinerii) avoids serpentine altogether, occurring on the schists and shales. Fringed rupturewort (Herniaria ciliolata) is abundant on serpentine and schists, whilst also occurring on gabbro, shell sand and shales.

Climate also has an equally important role to play. Owing to its extreme southerly position on mainland Britain, The Lizard has a climate characterised by a number of exceptional features. It has the highest mean temperature of any location on the mainland. Although summer temperatures are not excessively high, the winter ones are exceptionally mild, with the highest mean daily January and February temperatures of any location on the mainland. To plants this means two things: frosts are very rare, and the growing season is very long – in fact the temperature rises above the level needed for active growth (6 °C) every day of the year.

Another important factor is rainfall, which is not high (880 mm per annum). Much falls in the winter, with droughts being a frequent feature of spring and summer. These elements all contribute to what is best described as a `Mediterranean-type` climate: mild, moist winters with active plant growth, and hot dry summers when many species become dormant. Accordingly, annual `therophytes`, such as the clovers and vetches, together with bulbous species such as Autumn Squill (Scilla autumnalis), are important components of the flora.

One final climatic feature contributes to the special flora. With no land masses sheltering the Peninsula from the Atlantic Ocean, The Lizard receives the full brunt of Atlantic gales during the autumn and winter. As such it is one of the few places in Britain to experience more than 30 gale days per annum. This has had a marked effect on the flora. The physical force of the wind means that trees are rare, whist many herbaceous or sub-shrubby species show adaptation to avoid the worst excesses of the gales. Many plants have become genetically dwarfed or prostrated: eco-types of Ox-eye Daisy (Leucanthemum vulgare), Betony (Stachys officinalis), and Saw-wort (Serratula tinctoria), at times only 3 cm high, are amongst the many examples of species whose stems have simply shortened, whilst prostrate varieties or sub-species of Asparagus, (Asparagus officinalis ssp. prostratus) Broom (Cytisus scoparius) and even Black Bog-rush( Schoenus nigrans) take on the latter form. Yet others have become hairy to avoid the effects of gales – Heather (Calluna vulgaris var. hirsute) and Sorrel (Rumex acetosa var. hirtulus) spring to mind.

Associated with the occurrence of high winds are the excessive loads of salt spray that they carry , often far inland. Severe gales with salt deposition, can result in gross damage to stands of vegetation on The Lizard’s west coast. But perhaps the most telling manifestation of these gales is the existence of a suite of typically `maritime` species growing, often abundantly, in the centre of the Peninsula, over 5 km from the sea. Such plants include Spring and Autumn Squill, Sea Plantain (Plantago maritima), and Sea Campion (Silene maritima).