The western Kaimai and Mamaku ranges are almost entirely volcanic. They are older, lower, and more weathered than the eastern ranges.
To the east lie the Raukūmara, Huiarau and other ranges in Te Urewera. They are a continuation of the chain of greywacke mountains that runs from Cook Strait to East Cape. They have several distinctive features:
- They are younger, higher and more rugged than the western ranges.
- They consist of old sedimentary rocks that have been uplifted. These rocks are mainly greywacke and argillite (hardened grey sandstone and mudstone).
- The climate is harsher than on the coast, with higher rainfall and much colder winters.
The rivers run swift. They are aligned north–south in the eastern ranges, flowing along major fault lines. The Matahina dam on the Rangitāiki River was built across a fault.
Apart from the Rangitāiki, major rivers are the Whirinaki (a tributary of the Rangitāiki), the Whakatāne and its tributary the Waimana–Tauranga. The Waiotahe, Waioeka, Ōtara, Mōtū and Raukōkore rivers all rise in the dividing range. They flow through deeply dissected hill country before reaching either the low country around Whakatāne and Ōpōtiki, or the sea itself.
The course of the Mōtū River is the most dramatic and was protected from development in 1984. The watersheds of the western ranges are closer to the coast than those of the eastern ranges. None of the rivers in the western Bay of Plenty are as substantial as those in the east.
In 1840 most of the ranges were covered in mixed podocarp–broadleaf forest, which is still found over much of the Huiarau and Raukūmara ranges. Stands of silver beech are still common above 1,000 metres. Kauri trees grew as far south as Katikati, with some further scattered stands for another 40 kilometres to the south-east.
Māori had cleared some substantial areas – for instance at Maungapōhatu in the Urewera region and in parts of the Tauranga back country. Bracken and mānuka grew on this cleared land. In places, shifting cultivation was practised, with crops being grown for a time, after which the soil was left to recover.
Europeans began milling indigenous forest in the Kaimai Range, particularly kauri, in the 19th century. Milling began in the more remote Whirinaki Forest in the 1930s.
Protecting the forests
The prospect of the indigenous forests disappearing altogether prompted the establishment of the New Zealand Forest Service in 1921. Its mandate was to gain only a sustainable yield from indigenous forests, and to plant exotic trees to meet future timber demand.
Subsequently, a new generation of conservationists opposed milling native forests. In particular Minginui, in Whirinaki, was the site of bitter disagreement in the 1970s and 1980s. On one side were the New Zealand Forest Service and locals who worked in the sawmills. On the other were the conservation activists.
A more modest but similarly acrimonious campaign to protect a small area of Kaimai native forest from being replaced by exotic trees was launched by conservationists in the late 1970s. This was prompted when a Māori incorporation leased land to New Zealand Forest Products for pine planting. The development went ahead, but some areas of bush were preserved by the Māori owners.
Birds and other animals
The eastern ranges, in particular the Urewera region, sustain populations of species that are becoming increasingly rare. One of the last reliable sightings of a significant bird, the huia (now extinct), was made there in 1907, and the area holds the largest remaining population of its relative, the kōkako. It also supports kiwi, kākā, yellow-crowned parakeets, New Zealand falcons and blue ducks, as well as more common native forest species. Other notable native animals include two species of bat, Hochstetter's frog, and giant Powelliphanta snails.
Most of these species have been under threat ever since pigs, deer, possums and other animals were released or spread into the forests. The survival of the kōkako, kiwi and other native animals has been assisted by a Department of Conservation recovery programme.
Deer cullers, possum trappers and deer and pig hunters, both Pākehā and Māori, have operated in the ranges for decades.