Screes or shingle slips might be regarded as a very special type of fellfield. They are widespread on the drier eastern mountains of Canterbury and Marlborough and some mountain peaks may have their mid and lower slopes completely buried in deep aprons of angular stones through several thousand metres of altitude (Fig. 111). These vast accumulations result from the already mentioned rapid disintegration of greywacke under high alpine conditions, and the relatively slow downward movement of the rock fragments in the absence of heavy rainfall.
The screes are poised at the angle of rest and are very difficult to traverse as the weight of a person will tip the balance and set the stones in motion. Altogether the screes seem an improbable habitat for plants. page 189
Apart from their instability, the stones appear soil-less and arid and when the sun shines they can become very hot, raising the temperature of the air directly above very considerably. In fact when one manages to get out onto a scree it seems at first to be entirely plantless, then one small plant is spotted, then another and another. The plants are often grey like the stones, fleshy, and tend to be scattered fairly evenly across the scree several metres apart. At first they all appear the same, but closer inspection reveals several different and mostly quite unrelated species. About a dozen of these are found only on screes and there only away from the margins, in the deeper and most mobile parts. It is clear then that conditions are not as impossible as they seem. Beneath the surface the stones diminish in size quite rapidly to a sandy soil at a depth of about 40-50 cm. The extensive root systems of the plants, arising directly from the bases of leaf rosettes or from spreading rhizomes, branch widely through this soil, which is constantly wet from water seeping downslope. Thus scree plants do not in fact have a water problem, even when high temperatures at the scree surface lead to excessive water loss from the leaves. So far, so good, but what about
Figure 111 A steep scree on the Craigieburn Range, Canterbury. The forest is of mountain beech (Nothofagus solandri var. cliffortioides).
Photo: J. W. Dawson.
the instability problem? If a plant is lucky the scree will not move in its vicinity during the growing season, but if it does the scree plants are adapted to survive the event. If the layer of stones above the soil moves substantially, although the leaves and flowers or flower heads of a scree plant will be either buried or sheared off, its root system and rootstock or rhizome will survive intact and will be able to replace the lost parts. In some scree species the leaf petioles and flower stalks taper downwards so that their connections with the rootstock are almost threadlike and can break fairly readily. If these connections were thick and tough then the whole plant might be uprooted and die. The notable example of this type is Lignocarpa carnosula,
a grey-green, fleshy plant with much divided leaves, which belongs to the carrot family (Umbelliferae). Its dry seed heads break off at the weak connection and blow around on the scree surface like a tumbleweed, scattering seeds as they go.
Figure 112 The penwiper plant, Notothlaspi rosulatum. Porter's Pass, Canterbury.
Photo: J. W. Dawson.
The best known scree species is the penwiper plant (Notothlaspi rosulatum) (Fig. 112), which belongs to the cabbage family (Cruciferae). Its rosettes of grey, fleshy leaves closely overlap in a dome-like arrangement, which reminded early settlers of the similarly arranged and sewn together diamonds of felt on which they wiped their quill pens. The quite large flowers in massed heads are ivory in colour and have a very strong perfume reminiscent of stock in the same family.
Other notable plants restricted to mobile screes are Ranunculus haastii with large yellow flowers, Wahlenbergia cartilaginea, and Lobelia roughii, which has distinctive elk's-horn-like leaves with red teeth. Stellaria roughii in the chickweed genus is also common, as are two species of Leptinella of which L. atrata, with its almost black flowers, is the most remarkable.
The scree endemics die down in winter and most are perennials. The sole exception is the penwiper, which flowers in its second year then dies.
A number of species common on the more stable parts of screes particularly near the margins are also to be found in ordinary fellfield. Among these are Epilobium rubromarginatum, Poa buchananii (a grass) and patches of Acaena glabra. Acaena glabra belongs to the bidi-bid genus, but its seeds do not have the tiresome habit of attaching themselves to clothing or the wool of sheep, as do those of its more common lowland relatives. The seeds of Acaena glabra have spines but these lack hook-like recurved hairs at their tips. Perhaps the most unusual species in this group is Craspedia incana, a ghost-like plant with all its parts densely clothed in long, white, woolly hairs.