Kanuka and Manuka

Forest and Bird, Issue 295, 1 February 2000, Page 24

Kanuka and Manuka

CHRIS WARD examines the many values of these common plants.

anuka and manuka are the stuff of controversy. Are they farmland weeds, or valuable resources? Is that hillside covered with scrub, or forest? Is it a precious example of native biodiversity and natural character, or simply a wasteland? For that matter, some layfolk will argue there’s no such thing as kanuka, only manuka. Though frequently look-alikes, and often confused with each other, there are major differences between kanuka and manuka. (Details in box). Both trees can adapt their forms according to the growing conditions. Kanuka and manuka may look much the same, but often the differences are obvious, even from a distance. Until about 20 years ago, kanuka and manuka were both identified as closely related species in the genus Leptospermum. Then, fundamental differences, especially in the flowers and seed capsules, led Australian botanists to reclassify them in different genera. Kunzea ericoides is the new scientific name for kanuka while Leptospermum sco-

parium remains the name for manuka. We might feel aggrieved at the Aussies fiddling with the names of New Zealand species in this way, but in fact kanuka and manuka are also native to Australia. Could this be the origin of the widespread misunderstanding that kanuka and manuka aren’t New Zealand natives? How often have I heard something like ‘Oh, we’re only cutting scrub, we wouldn't touch the native!’ — implying the ‘scrub’ (kanuka-manuka) isn’t native. Far from it; both were present well back in New Zealand’s geological past. That word ‘scrub’ — it can be used with the emphasis of a four-letter word to suggest the ‘weed’ status of a vegetation, its illegitimacy and lack of value, indeed its negative value. For that reason, many conserva-tion-focused people avoid using the word. But ‘scrub’ is also a straightforward technical term for closed-canopy woody vegetation dominated by stems less than 10 centimetres diameter, a purely descriptive term without any judgmental implications.

‘Shrubland@’ is an alternative word sometimes used instead of ‘scrub’ by people concerned with its negative connotations. But technically, shrubland is another form of vegetation — essentially scrub with an open canopy, where shrub cover is less than 80 percent. On this basis, there is plenty of both scrub and shrubland dominated by kanuka and manuka in many parts of New Zealand. Much of the vegetation that is commonly referred to as scrub is, however, technically forest. Scrub becomes forest when the dominant stems forming the canopy are more than 10 centimetres diameter at breast height. For kanuka this normally occurs when the stand is about 30 years old, at which time it is typically 8-12 metres tall. These are small trees, agreed, but kanuka will keep on growing to a large size if not felled or burnt. Many of us will know a corner with some large ‘old man’ kanuka. My favourite is on the high marine terrace surface of Whetumatarau, a dramatic plateau imme-

diately behind Te Araroa on the East Cape. Large kanuka trees are a component of a mixed forest there. I measured a single stemmed specimen to be 94 centimetres diameter, and estimated its height at 25 metres — a great forest tree laden with perching lilies, orchids and other epiphytes. By comparison with nearby kanuka, known to post-date 1857, I estimate these larger kanuka trees to be 300-400 years old. So while some people see all kanuka vegetation as scrub, I cannot accept this. Kanuka and manuka are ‘seral’ or successional species, which dominate key parts in the series or succession of vegetation types which follow the colonizing of a new site. Here size really does matter. Kanuka grows en masse to form dense scrub; then, as the dominant stems grow and the others are suppressed and die, it matures to form a kanuka forest. This will generally diversify to a mixed forest and ultimately be replaced in a natural succession — if we wait long enough. ill country farmers are very familiar with the most basic ecological feature of kanuka and manuka — their ability to colonize the smallest of bare patches in sparse pasture. Those tiny airborne seeds get around, and the essentially unpalatable seedlings do well in full-light conditions. They may also colonize extensive bare sites after fires or on slips. Kanuka grows well on soils of middling-to-good natural fertility and drainage. Manuka by contrast favours wetter soils and low-fertility leached soils. It is not so much an active preference for poor conditions;

rather, that with competition between the two, kanuka fails in such circumstances. In contrast, manuka often establishes with kanuka on the average or better sites but is suppressed by the faster-growing kanuka and dies out within 10-20 years, after being overtopped.

‘We wouldn't touch the native. It’s only scrub we’re cutting!’

The net result is that kanuka dominates in some areas, such as most of the Gisborne District. Manuka persistently dominates on wetland margins, and on some particularly hard, ‘bony’ or burnt sites. It also flourishes in areas with consistently high annual rainfall, and at higher altitudes. The different lifespans of manuka and kanuka is the basis of another important distinction between them. Manuka is comparatively short-lived — generally to about 60 years. As a stand approaches this age,

there is a progressive breakdown of the canopy as individual manuka die or fall. This allows seedlings or saplings of other species to come through. Now there is an early succession to forests dominated by broadleaved species such as rewarewa or kamahi. In some cases, where browsing by stock or wild animals is excessive, this natural succession may fail — then the manuka may be replaced by mingimingi and bracken, in patches, or a second generation of manuka establishes itself. Kanuka by contrast is long-lived. Stands dating from the abandonment of land during the economic depression of the 1930s, or before, are widespread. Whether the plants beneath them are heavily browsed by animals or not is to some extent immaterial as far as survival of kanuka forest is concerned. The kanuka will still be there at the end of another century. Removing browsing animals from the understorey would, however, allow a diverse forest to establish and eventually succeed the kanuka. anuka and kanuka have other values, too. While some iwi leaders have declared that manuka (including kanuka) have no worth, others consider that its former use for prized tools and weapons represents a cultural value of high importance. On the utilitarian front, perhaps the best known value, for kanuka especially, is as a source of firewood. Alas, in the absence of sustainable management systems, this use tends to be an opportunistic ‘mining’ of the resource. The quality of kanuka timber also suits it to machining for tool handles, with far higher value-added potential. While there have been encouraging thoughts of harvest-

ing kanuka for such purposes, using sustainable management practices on quite modest areas, no one has yet got that off the ground on a commercial basis. There has been a tendency in the past to regard kanuka and manuka as significant only as a ‘nursery crop’ allowing a ‘real’ forest to develop, but there is far more that makes kanuka-manuka vegetation valuable. High density kanuka-manuka scrub/forest is very effective at holding the land in severe rainstorms — in maintaining slope stability on the steep hillslopes so prone to soil-slips when in pasture. Landcare Research has shown that the combination of canopy interception of the rain, and the strong interlocking roots, means kanuka stands 16-20 years old or more are as effective at erosion control as close-planted pines of eight or more years old. With kanuka there is the added opportunity for that stability to be maintained for centuries. Kanuka-manuka scrub/forest won't stop an existing gully eroding out, but it will stop a gully initiating on a slope that in pasture would be vulnerable to catastrophic gullying. Vigorously growings stands, of kanuka especially, extract large quantities of carbon dioxide from the atmosphere to form woody branches, stems and roots. This is a valuable ‘carbon sink’, tending to reduce the overall effect of greenhouse gases. Could that value become tangible? If the concept of tradeable carbon ‘credits’ gets off the ground internationally, a good kanuka stand could have a very tangible value, perhaps up to $200 per hectare a year by a recent calculation. Both kanuka and manuka yield honey in

large volumes. This is generally sold as manuka honey with a significant price premium over clover honey. Spectacularly effective antibiotic activity has been tested in some manuka honey (but not from kanuka). There is great potential here for converting ‘scrub’ into dollars, with careful marketing of reliably tested honey, from undisturbed stands. The oil extracted by steam distillation from manuka leaves from the East Coast also has striking anti-bacterial and fungicidal properties. Manuka oil is being used in a variety of medicinal and cosmetic products, commercially produced and marketed from Te Araroa. This use too involves nondestructive annual harvesting of the foliage of standing manuka. (Another smaller-scale operation harvests manuka tips and distills oil at Port Fitzroy on Great Barrier Island.) A remarkable recent discovery is that manuka oil varies dramatically in chemical composition and properties from district to district. This illustrates firstly that manuka goes a long way back into the geological past and has evolved locally, and secondly how much we have still to learn about New Zealand’s plants and biodiversity generally. Kanuka and manuka-dominant vegetation also provide habitat for a remarkable variety of other plants and animals — they are major repositories of New Zealand’s indigenous biodiversity. Many native orchid species, for example, particularly favour older kanuka or manuka-dominant vegetation. Twenty-five species of orchid have been recorded in one small area of the Kakanui block near Te Araroa — so far. The more this forest is studied, the more we find. Measured by species numbers and ecological complexity the most important elements of biodiversity are the invertebrates — moths, beetles, millipedes, spiders, snails and the rest. Research work in the Gisborne

district shows that the significance and diversity of the invertebrate fauna in 60-year-old kanuka forest is as great as that in primary forest. Extensive kanuka-manuka areas support large numbers of the commoner forest birds, and less common species such as native pigeon and the insecteating whitehead/popokatea. In some places, shorter manuka supports the threatened fernbird. To some, kanuka-manuka areas are but signs of recent farming gone wrong; an essentially unnatural phenomenon of the past century. But there is a long history of forest modification, extending back to the first Maori occupation and long before, due to fires and other disturbances. So kanuka and manuka scrub forests are an important part of the natural character of most districts. The character of kanuka and manuka vegetation ranges as a continuum, from nuisance weeds invading pasture at one

end, to a treasure trove of indigenous biodiversity and natural character at the other. The challenge is deciding at what point in the continuum do the positive values become dominant. There is no simple answer, and to a great extent it must depend on the specific context of an area. Landowners will commonly argue that their own views on the subject must be paramount. I would like to think it possible to stimulate increasing respect among landowners for kanuka and for manuka. It would be nice not to hear again: ‘Oh, we wouldn't touch the native, it’s only scrub we're cutting!’

CHRIS WARD is the conservancy advisory scientist at Gisborne with the Department of Conservation.