Why are the cabbage trees dying?
For centuries, ti rakau (cabbage trees) have been renowned for their remarkable ability to defy age and decay, and to virtually “live forever.” Although the old trunks might perish, there were always new sprouts to carry on the life of the tree; But now the cabbage trees are dying. Particularly in the north of the country, these curious trees – among New Zealand’s most distinctive native plants – are giving up the ghost. While Maori and pakeha have grieved their loss, scientists have struggled to discover the cause of the sudden decline. Then, late last year, a breakthrough…
It was spring, 1987, when officials from Rodney County Council turned up at the door of DSIR Plant Protection’s laboratories in Mt Albert, Auckland, with a dead cabbage tree trunk—and a problem.
Why, they asked, had this tree, and an increasing number of other seemingly healthy members of the species, suddenly dropped their leaves and died.
Five years later, “sudden decline,” as the syndrome has become known, is continuing to kill off thousands of cabbage trees across the top two-thirds of the North Island. Some scientists predict that at the present rate of loss, few cabbage trees will be left in Northland by the year 2000.
The original query from Rodney County, a mixed farming/lifestyle block district just north of urban Auckland, had been sparked off by growing complaints from ratepayers that the spraying of herbicides along road edges by the council had been killing their cabbage trees.
“The council didn’t believe it was the spray, and wanted to know what was killing the trees,” recalls scientist Dr Ross Beever.
This was the first formal notification of the problem, though subsequent detective work takes the outbreak back to at least the early 1980s.
The first task for the DSIR plant disease scientists was to find material to work with.
“They’d only brought us the trunk, and in cases like this, where the leaves suddenly turn yellow, wilt and fall off, it is often a root disease that is involved,” said Dr Beaver. “The trunk wasn’t going to help our diagnosis.”
He’d noticed, however, that just outside his office window something identical seemed to be going on. For some weeks he’d been “sort of vaguely watching” the collapse of a cabbage tree in a suburban garden, on the slopes of nearby Mt Albert.
Across the road trooped the scientists, spades at the ready, and soon they were back with the first exhibit—roots and all—in what has become a long search for the cabbage tree killer.
For most New Zealanders, the cabbage tree is as much a national symbol as the kiwi, and, like the flightless bird, this tree is a bit of an oddball. Although it is obviously a tree, it has more in common, botanically, with grasses and lilies than it does with typical trees. Its common name is misleading—a 19th century steal from the nikau palm, to which the description “cabbage tree” first belonged. (See box, page 52.)
What’s more, unlike other trees, it is anchored to the ground not just by roots, but by a giant underground stem or rhizome. It is this sub-soil “branch” which gives the cabbage tree its legendary ability to defy death and live on—virtually for ever. It also provides the cabbage tree’s generic name: Cordyline is derived from the Greek kordyle, or club—a reference to the shape of the rhizome.
Just as, above ground, the cabbage tree is capable of producing a leafy shoot at the site of each leaf axil (and will do so in response to physical damage as slight as a cat’s scratching), so underground on the rhizome there are thousands of buds, each with the potential to sprout forth as a new trunk.
Thus, by the time an old cabbage tree reaches 100 or so years, and the rot that lurks in all of them finally topples it, the plant doesn’t disappear. Instead, the underground rhizome will have produced several new trunks, creating the tell-tale circle of trees seen wherever the cabbage tree is part of the landscape.
Dr Philip Simpson, who is heading the Department of Conservation’s cabbage tree rescue project, says that at least some of the trees stricken by sudden decline appear to have retained this regenerative ability.
Ethnologist Elsdon Best collected several old Maori sayings which are based on the cabbage tree’s indestructibility. “He uru a ki, he wana te ti, ka rito te ti” was one that warned of the pitfalls of passing on gossip. “The spoken word reappears, just as ti sprouts again.”
Another is a lament for a loved one. “Waiho kia tangi ahau ki taku tupapaku, kapa he uru ti e pihi ake” was translated by Best to mean “Let me mourn for my dead one; it is not as if he would spring up again as the ti does.”
Even felled cabbage trees can defy all odds and sprout again. Perhaps the most remarkable case on record is that of a Northland gumdigger who made a chimney out of hollow cabbage tree trunks positioned side by side and nailed together. “For some months a fire was kept alight continuously,” write Laing and Blackwell in Plants of New Zealand, “until the stems were burned through, and only parts of the outside bark left. The man then left the place, and within a short time, that which had been a blackened chimney, became a mass of living green.”
In another instance felled cabbage trees lay on a beach for eight months “rolling up and down in the salt tide, or baking high and dry in the sun” before being returned to a paddock. There they promptly sprouted.
The genus Cordyline includes about 15 species, with representatives in India, South-east Asia, eastern Australia and across the Pacific to South America. There are five New Zealand species, referred to by Maori collectively as ti rakau or simply ti:
- Cordyline australis, the common cabbage tree (ti kouka or ti whanake), is found throughout the three main islands in swamps, clearings, forest margins and suburban gardens, from sea level to around 600m. Growing to a height of 12-20m, older trees can reach a trunk diameter of 1.5m or more. The leaves are long and narrow (30-100cm by 3-6cm), and in spring the tree bears panicles of strongly scented white flowers.
- Cordyline indivisa, the broad-leafed or mountain cabbage tree (ti kapu or toii), is found from South Auckland to Fiordland, in wet areas from 450m to 1350m above sea level, usually in well lit forest clearings. It grows to about 8m in height, usually with a single trunk. The leaves, at 12m by 10-15cm, are longer, broader and thicker than those of australis. Dense flower panicles hang down beneath the tuft of sword-like leaves.
- Cordyline pumilio, the pygmy or dwarf cabbage tree (ti rauriki or ti koraha), is found in scrubland from the Waikato and Bay of Plenty northward. With little or no trunk, it is often mistaken for a broad-leafed (30- 100cm by 1-2cm) grass.
- Cordyline kaspar, found naturally only on the Three Kings and Poor Knights Islands, grows to about 10m and has a multi-branched trunk. It is similar in appearance to australis, but the leaves are broader (5-7cm) and less fibrous. The flowers are highly scented.
- Cordyline banksii, the forest cabbage tree (ti ngahere or ti parae), is found from sea level to 1000m in wet forest margins, from Westland north. It is multi-trunked and grows to about 4m. Leaves are about 1-2m by 4-8cm, typically broader in the middle. Distinctive flower panicles grow up to 2m long.
Of the two huge groups of flowering seed plants, the dicotyledons (which include most of the world’s woody shrubs and hardwood trees) and the monocotyledons (which are predominantly non-woody, and thus stemless, like grasses, orchids, rushes and sedges), the cabbage tree, along with the palms, belongs to the latter.
Traditionally, cordylines have been considered part of the lily family, and C. australis was regarded as the largest lily in the world.
However, a review of the whole monocotyledon group in the 1950s resulted in the cabbage tree and related genera—those capable of forming trees, with stems mechanically able to support aerial branches—being separated from the true lilies into the family Agavaceae.
They then found themselves in the company of 20 other genera totalling some 700 rhizomatous, woody plants found mainly in the drier, warmer parts of the globe. Family members included the New Zealand flax, the century plant (source of the Mexican drink mescal), the Joshua tree (a species of Yucca), Dracaena (from which comes a red resin called “dragon’s blood”) and sisal hemp, a source of cordage.
Over the past decade there have been further attempts to refine “lily” classification, and cabbage trees have been placed in the family Asphodelaceae (which includes the asphodels of Europe). Some taxonomists believe even further splitting is warranted—with cabbage trees going into the Asteliaceae, a family with a southern hemisphere geographic focus.
New zealand’s original Polynesian settlers must have found the widespread distribution of cabbage tre
es a welcome sight. Throughout the Pacific, one species of ti (Cordyline fruticosa) had been cropped and cultivated for the strength of its leaves as cordage, for its medicinal properties and, especially, for food.
Recent research has proved scientifically what the ancient Polynesians well knew: the young stem and the rhizome contain carbohydrates which exceed cane sugar in sweetness.
C.fruticosa (formerly called C.terminalis) is indigenous to Southeast Asia and India, and is found throughout the Pacific Islands. Whether C. fruticosa is native to these islands or was introduced is a matter of some debate, but there is little argument that the early Polynesian voyagers to New Zealand brought this important crop with them, along with kumara, taro, hue (the gourd) and aute (paper mulberry).
Because of its tropical origins, ti pore (as C. fruticosa became known) proved hard to grow in New Zealand’s harsher climate. Indeed, it seems only to have thrived in the Far North, and then only under cultivated conditions.
The arrival of European traders, with their sweets and crystallised sugar, led to a rapid abandonment by northern Maori of their ti pore plantations, with the result that the plant, unable to seed in New Zealand, died out.
Naturalist Rev. Canon Walsh recorded that in the late 1880s he had been shown examples of a, to him, hitherto unknown variety of cordyline while visiting Waimate North. They had been found “in a long-deserted native cultivation” and were “almost the last survivors of a variety that is practically extinct so far as New Zealand is concerned.”
A local settler confirmed that the plant, ti pore, had been commonplace in Maori settlements 50 years before. “He was quite familiar with the cooked article, as the Maori nurses and other retainers of the mission families used often to bring small quantities as presents to the children.”
Walsh describes it as having a “short, slender stem .. . surmounted by a handsome head of soft glossy leaves” from 45-60cm long by 8-10cm wide. In older plants the trunks forked into several heads at about one metre high.
“The root was by far the most important part of the plant from the Maori point of view. It is a mass of greenish-white pulpy fibre, of such a consistency as to be easily cut through with a sharp spade.”
By “root” he is of course referring to the rhizome which, he records, is “out of all proportion to the rest of the plant. One that I transplanted was nearly 3ft long with a principal diameter at the upper third of from 3-4 inches.”
To propagate ti pore “the usual plan was to cut off and replant the stalk with a small portion of root attached, in the same manner as is done with taro.” Advantage was also taken of “offsets which often spring up at the foot of old stocks . . .”
To prepare it for eating, the rhizome was pounded with a wooden club until the fibre was broken up, and then steamed in a hangi for 12 to 24 hours.
“The substance then presented the appearance of a glutinous mass, and the taste is described as of a sugary sweetness far beyond that of the ti rauriki [the dwarf cabbage tree] but, like that root, with a slightly bitter after flavour.”
The cooked ti pore kept well, and was handy in times of war as emergency rations within the pa and as takeaways for war parties. However “it was most generally used merely as a sweetmeat.
“In fact, the Maori say that in the old times the chewing of a piece of prepared root when one had nothing else to do, gave the same satisfaction as is now afforded by a pipe and tobacco.”
One use for ti, seemingly undiscovered by the Maori, was its use as a base for alcohol—an application which T. F. Cheeseman, curator of the Auckland Museum, noted in 1900 had not escaped the attention of the Sandwich Islanders (Hawaiians). By bruising the rhizomes and leaving them in water to ferment, “an intoxicating drink” was produced. The Samoans were equally aware of this process.
It was left to a later wave of migrants to New Zealand to put cabbage trees on tap. Reverend Richard Taylor, who arrived in the Bay of Islands in 1839, was later to note, after observing that the cooked rhizomes had a “bitter sweet taste”, that “the early missionaries brewed excellent beer from them.”
The indigenous varieties of ti were also adopted as an important food source by the Maori—especially C. australis, C. pumilio and a mysterious plant which was cultivated in Hawkes Bay, the Waikato,Taranaki and Whanganui. This latter plant,known as ti para in Hawkes Bay, ti tawhiti in Taranaki and ti kowhiti in
Whanganui, has long been considered extinct.
Writing in 1880, explorer-botanist William Colenso noted that ti para was then so rare, the only specimens he knew were in his own garden “raised from a single plant I found in an old Maori cultivation . . . in 1845.” None had ever flowered.
“Thirty years ago, whenever some of the oldest chiefs here should happen to see the plant growing in my garden, they would invariably longingly beg for its stems to cook for a meal, saying how much they liked it.”
Recent research by DSIR Botany Institute’s Peter Heenan and Warwick Harris indicates that this plant has not been lost, but has been sold for many years in the nursery trade under the name Cordyline kirkii. It is a distinctive dwarf plant that has not been known to flower, and its precise relationship to our other cabbage trees remains obscure.
The proliferation of Maori names for ti—nine for the widespread C. australis alone—reflects the status the cabbage tree had as a food source.
In pre-European times, the cultivation and preparation of the stem and rhizome for food was a major industry—one that was both time-consuming and labour-intensive. Yet within a few years of the first settlers arriving, it had been abandoned.
This was partly the result of the settler-introduced market economy, which totally altered Maori life. Trading fish, potatoes, grain and flax for bags of sugar was a much simpler way of gaining sweet carbohydrates than farming ti trees through a three-or four-year cropping cycle.
Also, often the land containing the ti plantations passed out of Maori hands. One detailed account by an old South Island Maori of how they used to prepare kauru—the food from the ti—ends bluntly:
“Such was the work on the kauruproducing lands every year from very ancient times right down to three years after Mantell came, when the Maori stopped preparing the food because the Pakeha had settled here and burnt the kauru-producing plains, and they were all destroyed.”
(Walter Mantell was sent to Canterbury and Otago in 1848 as land commissioner, with the express purpose of “extinguishing native claims in the Middle Island.”)
Cultivation methods varied from the intensive farming of C. fruticosa in sheltered spots in the north to the more informal cropping of natural and planted plantations of the common C. australis.
Ownership rights to plantations, even those many kilometres from a tribe’s home base, were jealously protected—and respected. The same South Island account notes that in late September, when harvesting the ti begins, “each hapu goes straight to its section on its working territory. A hapu never works on the territory of any other hapu without permission—the Maori have the strictest customs with regard to a person or hapu trespassing.”
Both the stems of young (three- to four-year-old) trees and the rhizomes were cooked, and sometimes the young leaves, though the latter appear to be something of an acquired taste.
Explorer William Colenso, lost and “hunger-impelled,” in the Hawkes Bay bush in 1847, broke off some cabbage tree leaves “and roasted the stalks of the young leaves, which, though both tough and bitter, served to allay our pangs.” Wrong recipe, it seems.
Chief Hone Tare Tikao from Rapaki (near present-day Lyttelton) recalled to Elsdon Best that “the rito [the crown of young undeveloped leaves] was plucked off and cooked and,when saturated with the fat of eels or birds cooked with it, was good eating.
“In like manner was used the tap root of the ti. It was dug up and roasted at a fire, buried in the hot ashes until cooked.
“It was then pounded to soften it, spread out on flax mats, sprinkled with the sweet water (nectar) of flax blossoms and, when so saturated the edible matter was shaken into bowls . . . this process was adopted during the month of November.
“At other times it might be cooked in a steam oven.”
Steaming in a hangi for up to 48 hours, especially where the young stems are concerned, seems to have been the main method of cooking.
In South Canterbury and Otago, the remains of many huge cabbage tree cooking ovens, umu ti, have been discovered. Most are circular, with diameters up to 7 metres and oven stones weighing up to 16kg.
The South Island report quoted above describes a pit of two chains (40 metres) in length, and nearly two metres in depth, that took 20 men two days to dig and stack with fire wood. Much ritual attended the process.
West Coast explorer Thomas Brunner noted in January 1847 that “this morning opened our oven which smelled like a sugar-boiling establishment. Found the ti excellent, but rather too sweet for a diet; however, this and the fish make a fine meal.”
University of Otago chemist Donald Brasch and anthropologist Barry Fankhauser have pinpointed scientifically what Brunner and the ancient Polynesians discovered by experimentation. The common cabbage tree is a rich source of carbohydrate which could be used for the preparation of fructose syrup, an excellent sweetening agent now widely used in the food processing industry. Being twice as sweet as sucrose, it is popular for diet foods because fewer calories are required to achieve the same level of sweetness.
The cabbage tree “contains more usable carbohydrate on a fresh weight basis than either sugar cane or sugar beet.”
As well as its potential as a commercial sweetener, the researchers suggest that cabbage trees could be a useful feedstock for a fuel ethanol industry.
The cabbage tree’s underground sugar factory is just one of the features that fascinate Department of Conservation scientist Dr Philip Simpson about this remarkable tree.
Not only is it a rarity amongst the monocotyledons, being one of only about 100 species (out of something like 60,000) that have developed a tree form, but its vertical rhizome system is shared with only one other species, its American cousin the yucca.
Simpson explains the process by which the rhizome forms: “When the seed germinates into a tiny seedling, a bud in the axil of one of the first leaves grows out and straight down.”
What we get is not a continuation of the main stem but “a side branch which has come out of one of the earliest leaves and grown straight down into the ground.”
Underground, it adapts to its special roles: to act as the plant’s water and food storage chamber, to anchor the tree and to provide a continuous new surface for the roots that radiate out in great numbers. The roots, most plentiful near the surface, are 4-5mm thick and grow up to 10m in length. Simpson says that on a mature tree there are thousands of these strong, robust roots spreading out from the upper portion of the rhizome.
As well as trying to unravel the cabbage tree’s morphological secrets, Wellington-based Simpson heads a task force set up to prepare a conservation plan for this, our most distinctive native tree. While the outbreak of sudden decline sparked off the project, the reality is that the cabbage tree, though still common throughout the country, has been declining in numbers since the introduction of pastoral farming last century.
Simpson says that while general observation puts the cabbage tree’s natural habitat as lowland wetlands and forest margins, human intervention by both Maori and pakeha over several centuries makes it difficult to pin down its precise distribution.
He suggests populations could have actually increased in pre-European times, with Maori developing huge plantations in South Canterbury, for instance, and in Hawkes Bay. With the arrival of pakeha, however, the plantations were abandoned, and much of the land was turned into farms.
The newly cleared land initially proved excellent for cabbage tree colonisation, but once livestock was introduced, regeneration ceased. The problem was simple: the stock loved the tender shoots of the new seedlings and the sprouts emerging underground from the rhizome.
So, over vast areas of farmland, all that are now left are the ancient trees dating from about the time the area was cleared.
The picture then, is of a plant population in some stress before the emergence of sudden decline.
Explains Simpson: “It’s a wetland plant, so that’s where it is at its best, but there are very few nice healthy wetlands left in New Zealand. Most of the cabbage trees are out in the hills on farms, where they’re geriatric, or in urban centres where they have to put up with other stresses such as soil and water problems and the urban population explosions of insect pests. Just have a look at the cabbage trees around the city—many are in terrible condition because of insect attack.”
Gordon Hosking, forest entomologist with the Forest Research Institute, Rotorua, has travelled the country investigating the distribution of the cabbage tree as part of the over-all study. He confirms Simpson’s view “that in many areas, the population dates back to the original breaking in of the land. As you go down through the Wairarapa, all the hills are dotted with widely spaced old cabbage trees. But because of the browsing animals, none are regenerating.
“So most of the young trees are along roadsides, railways or places where cattle can’t get at them, and in the forests.”
An early DSIR survey showed that sudden decline was confined to an area north of Taumarunui, and both Hosking and Simpson agree that the disease shows a distinct geographical pattern. Hosking says it’s hard to find evidence of it south of a line drawn across from Wairoa to Awakino. It also tends to be worse to the north and in the east.
Simpson takes the line further south, and remembers driving up from Wellington and reaching Norsewood on the edge of the Hawkes Bay plains, “and whammo, it suddenly becomes a major feature of the landscape—most of the cabbage trees are dead.”
To both men, the localised nature of the problem suggests that some environmental or climatic factor is playing at least a part in sudden decline.
Recent mild winters could be having an effect, suggests Simpson, by allowing disease organisms to persist in the trees year-round. Other observers have suggested that sudden decline may be connected with the apparent over-flowering of cabbage trees in recent years. Flowering is an exhausting process for the plant, which, instead of building up its reserves and strength, has to direct its energies into flowering.
Simpson rejects the theory that cabbage trees are flowering themselves to death. Rather, he suggests, it could be the natural response of a distressed tree to put all its resources into a mass flowering in a last desperate endeavour to reproduce before death.
After visiting more than 700 sites, Hosking, while concerned by evidence of sudden decline, also expresses alarm “that pastoral farming is really eliminating cabbage trees from a large part of the country. Year by year, the residual population of trees that may be 150 years old continues to disappear. And on flat land the trees are still actively cut down—if you’re making hay you don’t want a tree in the middle of your paddock.”
Because of such concerns, DOC has taken the key sites Hosking’s survey work has revealed and moved in to try and protect them. Many farmers, when made aware of the general attrition of the cabbage tree population, have agreed to help, often by fencing their best stands to allow natural regeneration. Wetland areas are increasingly being recognised as important repositories of biological diversity, and are being afforded high conservation priority.
Where areas are low in cabbage trees, DOC is encouraging the propagation and replanting of local offspring—plants which, by virtue of adaptation, should have the best chance of survival in each particular location.
For the North Island’s cabbage trees, though, survival is a matter not just of replanting, but of halting the disease that is afflicting this special New Zealand tree.
At this stage, it is premature to predict what control measures might be successful, but the DSIR has identified two broad approaches. First, to reduce the population of the insect that is spreading the disease by using biological control. Second, to identify resistant forms of cabbage trees.
Unfortunately, there are few prospects for a quick fix, and in the mean time we are going to see a lot more cabbage trees dying.