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Relation to Slope and Soil.
The aspect of the slope in the forest, at any rate, does not appear to be of much consequence, provided climatic factors have been considered separately. A much greater effect on vegetation is brought about by the degree of slope, as this is closely associated with soil development and moisture content. As the slope becomes steep trees become unable to obtain sufficiently strong root-hold. Shrubs persist in crevices even on almost vertical slopes provided the moisture supply is adequate. The steep slopes in the subpolar belt are generally well supplied with percolating moisture. Their soil is usually thin, but densely covered by the so-called “rock garden.” The following usually abound in such places: Anisotome dissecta, Leucogenes leontopodium, Celmisia spectabilis, Ranunculus insignis. On the less inclined slopes the soil is much thicker and is covered by meadows of Danthonia antarctica, which is usually associated with Astelia Cockaynei in somewhat steeper places. In many places steep slopes run into screes which attain extensive proportions on the eastern side of Mount Matthews ridge. Plants establish themselves with difficulty in such places and often their associations do not reach the climax before the next soil movement begins. On some slopes of Mount Matthews it never seems to end, even under forest.
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On more or less flat surfaces and around springs the soil is often almost permanently wet and is hummocky and spongy. The vegetation here is usually very dense but not more than 10 cm. tall. This peculiarity is apparently due to deficiency of soil air. At any rate, the soil is quite warm, indeed, much warmer in the direct sunshine than under the adjoining tussocks, and it does not appear to be stagnant, water in the pools being as fit for drinking as in the streams issuing from them. All plants inhabiting such places send roots out more than 10 cm. down, and a large portion of the species is composed of liverworts and mosses.
There are few places which can be termed bogs. The outstanding examples of these are known near the top of Mount Omega and near the top of Mount Oriwa (900 m.), the Oriwa Lake-Hollow, about 3 km. south of Mount Waiopehu. On Mount Omega (1100 m.) this was once a Dracophyllum Urvilleanum association in a Sphagnum bog of about one hectare or so in extent. It is now very much altered by the continual trampling of deer. Oriwa Lake-Hollow is in reality but a slightly depressed elongated area of a few hectares in extent, surrounded by Nothofagus Menziesii forest. It appears to have once been a continuous Sphagnum bog with but few spermatophytes in its association. Now, however, about half of it is drained (there is an outlet) and is occupied by scanty growth of Agrostis Dyeri, Uncinia compacta, Epilobium pedunculare, Gnaphalium Mackayi and a few others together with Celmisia gracilenta. The latter species is known to occur in abundance only in this place and a somewhat similar one on Mount Kaiparoro, otherwise it is a rare species occurring along the eastern side. Of shrubby species only Pittosporum rigidum and Coprosma parviflora occur in the drained area as stunted shrubs.