124 ; The Plant Formations. 
The forests fall naturally into the two classes of hygrophytic rain-forest 
and mesophytic southern-beech (Notkofagus) forest, but between the two there 
are transitions. Generally speaking, the lowland tree-associations are rain-forest, 
the species of Nothofagus, being wanting in many localities, or, if present, not 
forming pure associations. 
The principal ecological conditions which govern New Zeajand lowland- 
forest are: — ı. A rainfall exceeding 75 cm distributed more or less equally 
throughout the year. 2. A sufficient supply of available water at all seasons 
in the surface-soil. 3. A winter temperature not falling below — 10°C. 4. The 
special conditions .brought about by the plants themselves, which define, in 
large measure, the wind, light, heat, air-moisture, soil-moisture and humus 
factors. These last Mielkaned conditions come gradually during the evolution 
of forest and render possible the various phases of succession. Usually,. the 
rainfall is far greater than as given above, but with its increase neither the 
ecological nor the floristic composition of a forest is greatly affected. The 
even distribution of rainy days throughout the year is a matter of prime im- 
portance, for a drought of several months duration will actually cause the death 
of several endemic species of trees. As for the heat-factor, each species has 
its special requirements and limitations, and the degree of cold, cited above, 
is far greater than many plants can tolerate. = 
The ecological conditions of any forest-area are far from uniform. Much 
depends upon the topography, — slopes, ridges, flats and gullies, each possess 
their special plant-coverings in harmony with the differences in ground-water, 
depth of humus, air-moisture, light-intensity and shelter from wind. An in- 
teresting case is presented by the forest of the Wanganui coastal plain, where, 
on the slopes, there is hygrophytic tawa (Beilschmiedia tawa) forest and on 
the ridges a pure semi-xerophytic ässociation of Nothofagus Solanderi, even 
if these trees be almost in single file. Nor isthe lowland forest, climax-form- 
ation though it be, in any great state of stability. As the dominant tall trees 
