and trees. The seedlings of both species also at least survive under the canopy of Hoheria forest. Olearia lacunosa seedlings may be a little more shade-tolerant. The seedlings of Olearia colensoi, senecio elaegnifolius and Nothopanax colensoi become established freely in all but the deepest shade, but apparently they reach maturity only in gaps. Dacrydium biforme seedlings grow steadily under a fairly dense shrub canopy; in their light requirements they seem similar to most other podocarp seedlings. Dracophyllum traversii seedlings are rarely seen, but they too appear to be shade-tolerant.

2. The Transition from Subalpine Scrub to Forest

The subalpine scrub replaces forest in severe habitats where the canopies can attain heights of only 2–30ft. Taller scrub passes into low forest, where many of the stunted trees have massive gnarled trunks up to 20 inches diameter. The transition from forest to scrub taken as the upper limit of Libocedrus bidwillii occurs at 2,600–3,200ft.

Libocedrus bidwillii ascends highest on northerly aspects. In the top Toaroha basin, Libocedrus trees project through understories corresponding to Dacrydium biforme scrub and to Olearia forest. The altitudinal limit is 3,000ft in both cases, and would thus appear to be controlled by temperature; but whereas the trees grow to 30ft tall when associated with species of the Olearia forest, they are usually stunted to 10ft or 15ft when associated with Dacrydium biforme. The fact that Hoheria glabrata forest ascends as high as continuous scrub also shows that favourable soil conditions can outweigh climatic disadvantages.

3. The Transition from Subalpine Scrub to Grassland

The altitude of the transition from scrub to mountain grassland varies widely, largely according to the slope of the ground. Continuous scrub attains its highest altitudes (about 4,000ft) on steep slopes with northerly aspect, and patches of Dracophyllum uniflorum occur at 4,500 feet on rocky spurs. In the Hokitika catchment, the upper limit of the scrub frequently corresponds to the sharp change of slope between steep, lower slopes leading down to the entrenched valleys and moderate upper mountain slopes. Tussock grassland communities are favoured by moderate slopes and hollows; on valley flats they can descend to 1,200ft (at Price's Flat) and abut on lowland forest (Pl. 3, Figs. 1 and 2).

On gentle slopes, especially below 3,000ft, a remarkable savannah-like vegetation is frequent, with islands of scrub scattered through tussock grassland. Some 10 acres of such “savannah” occupy the highest, flattest part of the morainic area in the top Toaroha basin (Pl. 3, Fig. 3). There, hollows support bog communities, and in some there is accumulation of peat. Steeper ground, with slopes for example of 20°, have patches of dense scrub, predominantly comprised of Olearia colensoi and Dracophyllum longifolium, but also including Dacrydium biforeme. The remainder of the surface is occupied by a mixture of shrubs and such species of boggy grassland as the narrow-leaved form of Danthonia flavescens, Celmisia armstrongii, Schoenus pauciflorus, Carpha alpina and Oreobolus pectinatus. The shrubs, which range in age from seedlings to rotting stumps, include Dracophyllum longifolium, Archeria traversii, Dacrydium biforme, Pittosporum divaricatum, Podocarpus nivalis and Coprosma pseudocuneata, but not Olearia colensoi. The soils in this morainic area are old, and their drainage is impeded. But in the same basin, there is an extensive alluvial fan, sloping gently at about 5°. The soils here are freely-drained, coarse gravels which show little development of profile. The vegetation is again a mosaic, but one component is related to the Hoheria glabrata forest, and the other to a seral type of grassland. The main species are Hoheria glabrata, Olearia ilicifolia, Coprosma ciliata, C. rugosa, Aristotelia fruticosa, Polystichum vestitum, the form of Danthonia flavescens with conspicuous light-coloured midribs, Poa cockayniana, Phormium

colensoi, Uncinia sp., Cotula perpusilla, Helichrysum bellidioides, Muehlenbeckia axillaris and Hypolepis millefolium. Grassland gives every appearance of being the climax vegetation on this fan; the shrub component is maintained by the frequent changes of stream course. Thus, we have grassy vegetation conspicuous on two neighbouring areas within the scrub zone, which resemble each other in their gentle slopes, and contrast strongly in soil characters.

Topography, therefore, has a marked influence on the distribution of scrub and grassland, but it is uncertain how this influence is exerted. The two contrasting examples of mixed scrub and grassland indicate that soil factors are not necessarily responsible. The explanation may lie in microclimate factors, such as ponding of cold air and depth of snow.

The low timber line and the wide scrub belt contrast sharply with the conditions on the mountains in the adjacent part of Canterbury, where the timber line of Nothofagus cliffortioides reaches 4,600ft in places and subalpine scrub is poorly represented. The difference must be explained either by the absence of Nothofagus in central Westland, or by the marked differences in climate. The summer climates especially differ; in the Westland mountains the summer is cool, sunless, and the average annual rainfall probably exceeds 300 inches, whereas in the Canterbury mountains, the summer is sunny, and although the annual rainfall exceeds 50 inches, it is largely offset by the dry, warm, föhn winds.

4. Growth Rates, Habit and Regeneration

The growth rates of the dominant species, though falling over a wide range, are markedly less than the growth rates of the dominants of the forest below. In this respect, as in their low stature, they show adaptation to the rigorous climate. Hoheria glabrata and the composites comprise the faster growing species and Dacrydium biforme and the epacrids comprise the slower growing species. The former group all regenerate freely, at least where light conditions are suitable, and in the absence of browsing animals. Seedlings of the slow-growing species on the other hand are only occasionally encountered (with the exception of D. longifolium, whose seedlings may be abundant in well-lighted places). But longevity and, in Dracophyllum uniflorum, D. longifolium and Dacrydium biforme vegetative reproduction through downhill layering, appear to compensate for scarcity of seedlings.

This downhill layering is only the extreme development of a feature found in all the scrub species—i.e., the tendency for the lower part of the trunk to lie prostrate or inclined in a downhill direction. The very limited annual increment of wood in the stems may explain this tendency; and young stems growing in the shade of taller shrubs are weaker than those growing in the open. On steep slopes, the direct action of gravity in producing the prostrate habit may be reinforced by movement in the upper 12in of the soil mantle, and by weight of snow. The form of the shrubs is well adapted to the environment. The violent winds rarely cause breakage, and heavy snow can press shrubs at least 2ft tall to the ground without damaging them.

A large proportion of Libocedrus bidwillii trees near the upper forest limits are dead or dying, and saplings and young trees were not seen. But seedlings up to 3ft tall seem to be quite plentiful. A similar discontinuity within populations of Dacrydium biforme in the lower part of the scrub zone has already been described (p. 55). A. careful census of populations of these long-lived species would be worth while, for they may reveal a counterpart in our mountains of the “Little Ice Age” in Europe. In the European Alps the glaciers advanced from the end of the sixteenth century until the middle of the nineteenth century, and since then retreat has been rapid. The behaviour of the glaciers is related to climatic variations, which have also influenced the behaviour of the vegetation. For instance, markedly increased