Feature

Cantabrian Rata

Why are these massive trees missing?

Hugh Wilson

The Main Divide of the Southern Alps forms the provincial boundary between Westland and Canterbury. Provincial boundaries are a human conceit, of course; they seldom bear much relation to natural realities. In this case, however, the provincial boundary is also a natural boundary of extraordinary significance. There are profound contrasts that exist between the landscapes, geology, climate, soils, vegetation and fauna on either side of this elevated mountain spine.

A dramatic way to experience this is to cross the Divide on foot. On Goat Pass, one leaves the edge of the mountain beech forest near the headwaters of the Mingh, crosses the low, easy saddle and soon descends into the Deception River and into a totally different vegetation of Westland scrub and forest- rata, kamahi, neinei and cedar, not a mountain beech to be seen. (True, the same contrast can be viewed by car through Arthur's Pass, but this lacks the finesse and glow of working muscles.)

A much higher transalpine crossing is the Copland Pass (we used to refer to it affectionately as the "grope and gasp"). The Hooker Valley is left behind, with its isolated patches of silver beech and mountain totara forest dwarfed by huge open spaces and breathtakingly high mountains. The next day's tramp brings such startling contrasts that it's hard to conceive you are walking the same track as you pass beneath the massive limbs of immense southern rata (Metrosideros umbellata).

The natural ranges of many plant species in the Canterbury/Westland region are entirely restricted to either side of the Main Divide. Species at home only in the west but more or less widespread there include silver pine, yellow silver pine, some Coprosma and some tree daisies. A larger number are found only east of the Divide, even though some extend right up to it. Some examples include matagouri, many small-leafed shrubs (e.g. Coprosma and Olearia), some native brooms and many Hebes and bidibids (Acaena).

At first glance, Southern rata appears to be one of those species restricted wholly to the Westland side of the Divide. (Ask trampers if they've ever seen rata in Canterbury). Nowhere in Canterbury, so far as I know, can you walk under huge old gnarled southern rata such as I described in the Copland Valley. In fact the species is quite widespread east of the Divide. However it is seldom a tree of any stature, but is common as a shrub clinging to steep or even vertical rock faces.

One of the more unusual localities is on Mount Ararat in the Omihi State Forest, north of Christchurch, where rata occur as shrubs on a surprisingly dry limestone outcrop. I also found references to the species in the Mt Hutt Ecological District where the shrubs delineated rocky watercourses through beech forests.

The word rata crops up in at least two Canterbury place names: the Awaawa Rata Reserve is on the slopes of Mount Hutt and Hororata appears to refer to a rocky gut with rata growing in it.

Canterbury rata is probably a shrub rather than a tree due to environmental influences, such as competition between species. So far as I know, no-one has shown it to be a genetically fixed variant. Certainly it differs somewhat in size according to site conditions. A compact bushy shrub of southern rata in full flower is a visual treat of considerable horticultural merit. I grew one in a Christchurch garden for many years. Although it began as a seedling of Westland origin, it remained a compact shrub, and alas it never flowered. Conversely, there is at least one southern rata in the Christchurch Botanic Gardens considerably larger than a shrub.

Canterbury also has some fine planted specimens of northern rata (M. robusta). There are some big trees on Banks Peninsula and there is a good-sized specimen on one of the islands in Christchurch's Avon River.

Hugh Wilson is the reserve manager of the Maurice White Native Forest Trust, a private charitable trust restoring 1000 hectares of native bush on the south-eastern corner of Banks Peninsula.