Alpine rock often looks barren from a distance, yet it supports a rich array of plant life, including many flowering herbaceous and sub-shrubby species, grasses, mosses and lichens.
The remarkable cushions of vegetable sheep (Raoulia and Haastia species) have adapted to avoid drying out on rock at high altitude. These plants are in effect highly compressed shrubs, their multiple branches ending in tightly packed rosettes of tiny, woolly leaves. This forms a hard, continuous surface that minimises water evaporation. Rain penetrates quite easily, saturating the peaty core, with its numerous fine roots. These roots take up water to supplement that taken up by the main roots, which anchor the plant to the rock.
New Zealand edelweiss
New Zealand edelweiss is probably the most characteristic, abundant and widespread of all the alpine rock dwellers. North Island edelweiss (Leucogenes leontopodium) occurs from Mt Hikurangi down to the northern South Island. South Island edelweiss (Leucogenes grandiceps) is found throughout the South Island and on Stewart Island. Their strong resemblance to the famous Swiss edelweiss (Leontopodium alpinum) is more than skin deep – despite growing on opposite sides of the earth, all these edelweiss species are botanically related.
Areas of fragmented rock such as fellfield, scree, moraine, bouldery torrent beds and shingle river beds are major parts of New Zealand’s mountain landscapes. Most look bare from all but the closest vantage points. But everywhere plants are colonising the debris. Mosses and lichens are often abundant. Among flowering plants, the genera Epilobium, Raoulia, Haastia, Ranunculus and Poa are well represented. They constantly risk being destroyed by moving rock.
Fellfield is the most stable of the debris habitats. A characteristic plant form on these alpine rock deserts is the cushion. Many unrelated plants (Hectorella, Raoulia, Haastia, Chionohebe, Kelleria, Phyllachne, Colobanthus and Luzula) have evolved the cushion form to make the best use of the microclimate close to the ground, and to resist damage by wind or abrasion.
Least stable is scree. About a dozen specialised scree dwellers are virtually confined to huge shingle slides on South Island ranges east of the main divide. Despite their botanical diversity, they share many features. They are small, have fleshy, waxy, blue-grey leaves, and have well-developed underground parts that lie within the firm, fine-grained layer immediately beneath the mobile surface stones.
The specialised scree plants belong to several unrelated plant families, such as the willow herbs (Epilobium, family Onagraceae), buttercups (Ranunculus, Ranunculaceae), Lignocarpa (Apiaceae), Stellaria (Caryophyllaceae), penwipers (Notothlaspi, Brassicaceae), harebells (Wahlenbergia, Campanulaceae), Lobelia (Lobeliaceae), and button daisies (Leptinella, Asteraceae).
It remains uncertain how these plants survive in the precarious shingle. One hypothesis is that they photosynthesise rapidly and send food down quickly into their underground parts. Water is never lacking on the scree sub-surface, and because the leaves of scree plants are mostly water, they are expendable – it is not a great loss if the fleshy shoots are torn off by moving stones.
This hypothesis may explain why nearly all scree plants fail to extend west into mountains with high levels of rainfall. The lower temperatures and reduced light intensities there would result in growth rates too low to compensate for losses due to scree movement.
The blue-grey colouring and waxy coating of leaves protect against high light intensities and temperature extremes (although not against drought, and not as camouflage against the stones).