The extent of forest before human settlement commenced is now impossible to determine, but it is thought that at about the time white settlement began, at least two-thirds of the country was forest clad. Both Maori occupation and other influences brought about constant changes in the forests. These will be described briefly. European settlement has, of course, brought about major changes, which will be described as the forest types are dealt with. Lastly, the introduction of mammals, of which the country was completely free, except for two bats, has everywhere changed the virgin condition of the forests. No forest is now free of their influence. Readily detectable effects are brought about by the ground-browsing animals, deer, goats, and escaped domestic animals, and the canopy browser, the opossum. These effects vary from complete destruction of certain types of forest by a combination of animals to slight effects on bird life and thus on the regeneration of the forest.

Before any human settlement took place, forests – as well as other vegetation – were undergoing slow but constant changes associated mainly with climatic changes. Beech forest invaded broadleaf, and vice versa. Botanists over the past decade have shown that changes of this nature now taking place indicate the development of a colder and wetter climate. These changes are occurring within the life spans of some of our major forest trees, rimu, totara, kauri, etc., which can be as much as several hundred or even a thousand or more years old.

The great central North Island volcanic eruptions of the past few millennia brought about cataclysmic changes in the vegetation throughout certain districts of the North Island. There is ample evidence, in the form of charred remains, of the widespread destruction of forest by fire and by burial under pumice. It seems that the first vegetation to cover raw pumice consisted mainly of grasses and some low pioneering shrubs. Because the region is, by and large, a forest one, forest vegetation gradually spread through the lower growing forms of vegetation from pockets of forest that escaped destruction and from the undestroyed forest around the edges of the showers. The result was a peculiar pattern of forest, related in part to nucleus forest areas and in part to the nature of the spread of the forest – the quick spread of podocarp-broadleaf forest, and the slow spread of beech.