Feature

Save a Rata  Kill a Possum

Australian immigrants seem to be killing off our northern rata forests.

Bruce Burns, Phil Cowan, and Phil Knightbridge

One of the most distinctive trees in the New Zealand landscape -- the scarlet-flowered northern rata (Metrosideros robusta) -- is under threat.

Northern rata, a relative of pohutukawa and other Pacific forest trees, is a characteristic tree of New Zealand lowland rainforests in the North Island and the northern half of the South Island. Commonly its massive trunks -- up to four metres in diameter and 30 metres tall -- penetrate the canopy so that its layered foliage and distinctive scarlet flowers stand emergent.

Northern rata often gets to this lofty position by establishing as an epiphyte, or perching plant, on an existing canopy tree of another species, then growing roots to the forest floor that ultimately coalesce to form a self-supporting pseudotrunk, with the rata often outliving its original host. Less dramatically, northern rata also establishes on forest floors after catastrophes like forest fires or blowdowns.

Any tree of this size, with its proportionally large contribution to ecosystem nutrient and energy flows and influence on biodiversity, must be considered a keystone forest species. The loss of such species are likely to have significant consequences for ecosystem composition and processes. The many conspicuous stagheads of dead northern rata now commonplace amongst northern forests are therefore an alarming sight to forest ecologists.

Dangerous Dieback?

Dead trees occur in all forests and may simply reflect natural and inevitable mortality within a healthy population. However, the simultaneous death of trees within a forest at an unusually high frequency is recognised as a special process -- forest dieback.

Dieback of northern rata in North Island forests has been reported since at least the 1930s and has now occurred almost throughout its range. Severe defoliation of many trees in an area is common, for example in forest on Coromandel Peninsula, but northern rata has almost been eliminated in the worst hit areas, the Tararua Range, Aorangi Range, and Mamaku Plateau.

What is the cause of this dieback? Is it affecting all of the population or only a conspicuous but susceptible part of it? Is there effective regeneration to eventually replace those that have been lost?

Dieback of Metrosideros species has been observed throughout the Pacific. In Hawaii, extensive dieback of Metrosideros forest has been attributed to a process whereby an even-aged and old cohort of trees, that originally established together on a lava flow, all become susceptible to a dieback trigger such as drought or a storm, at the same time.

Some dieback of southern rata forests in the South Island may also have a similar explanation in cases where a single age class established on an area following landslides. Similarly, synchronous northern rata dieback was reported on Mt Taranaki before possum invasion. In terms of survival strategies, such extensive dieback can be seen as advantageous to establishment of another generation of these light-demanding trees in the large opening caused by the death of the original Metrosideros canopy.

Rata Under Attack

Most of the dieback of northern rata currently observed, however, does not fit this pattern and appears directly attributable to browsing by introduced Australian brushtail possums. Wherever recent dieback of northern rata in New Zealand is examined, it occurs soon after a build-up in possum numbers and is not confined just to the largest, and therefore probably the oldest trees.

Research on northern rata dieback in Orongorongo Valley, east of Wellington, has found that northern rata is the second most preferred possum food there, making up about 30-35% of the leaf diet. Possums account for 95% of damage observed on northern rata and heavy browsing can kill a healthy tree in three years. Isolated, heavily browsed trees whose trunks were banded to exclude possums recovered within 3-5 years.

The fates of fifty large northern rata have been followed in the Orongorongo Valley since 1969. By 1974, 10 had died and several more were seriously browsed. At this rate, it was calculated that all trees would be dead by 1993. However, by 1994, only a further three had died. So why did a proportion of the rata survive? Two possibilities have been suggested.

First, the mortality recorded between 1969 and 1974 occurred during a period of several years of above-average possum densities. The additional browsing pressure may have been more than the trees could sustain. However, the rata mortality also coincided with a regional drought. Therefore northern rata may be able to survive long-term possum browse under normal environmental conditions, but has increased susceptibility to death when there are additional stresses, such as rare extreme climatic events, especially those that tend to favour a build-up in possum numbers.

Second, surviving trees may be those with larger quantities or specific types of leaf chemical constituents, such as terpenes, phenols and tannins, that make them relatively unpalatable to possums. Is there a portion of the northern rata population genetically less susceptible to possum browse? These hypotheses are the subject of current research.

As well as the death of mature trees, another concern with northern rata is the apparent absence of young trees in populations with a long history of possum presence. Northern rata populations in the Orongorongo Valley and the Waitakere Ranges are composed almost entirely of large, old trees with very few seedlings or saplings. In contrast, populations on Little Barrier Island, still without possums, contain trees of all sizes. The fear is that even if we prevent further death of mature northern rata by techniques such as banding, will we be left with an aging population and no seedling recruitment?

Fortunately, possum control operations currently widespread in New Zealand give some relief to lowland forests and the species they contain. Where possums have been substantially reduced in number or even eliminated, as on Kapiti Island in 1986, northern rata trees quickly recover their foliage and vigour. The widespread distribution, conspicuousness of foliage, and rapid response to both increasing and decreasing possum populations of northern rata have led to its identification as a potential indicator of possum damage at an ecosystem level. Research to test the usefulness of northern rata for this purpose and to identify damage thresholds -- the level to which possum populations need to be reduced to alleviate rata damage in the long-term -- are continuing.

Northern rata trees bloom from November to January with spectacular masses of scarlet nectar-rich flowers. These floral characteristics (red colour, conspicuousness, and high nectar output) are associated with principally bird-pollinated plants. New Zealand honeyeaters, such as tuis and bellbirds, are or have been the most important northern rata pollinators. The sweet nectar also attracts insects, and probably geckos and native bats, that could also pollinate some flowers.

After fruit set, tiny northern rata seeds are released from their capsules between April and June in the thousands. They weigh only 0.06 milligrams, which means that in every gram there are about 17,000 seeds. However, on average only 3% of mainland northern rata seed are viable. Although there are still plenty of viable seeds even at this level because of the huge total seed output, this low percentage may be a sad indication of the decline in honeyeating birds, and perhaps bats and geckos, that are now not available as pollinators.

The importance of northern rata to the conservation of New Zealand's forests has been recognised by its adoption, along with three other Metrosideros species, within the umbrella of Project Crimson in September this year. This organisation, supported by Carter Holt Harvey Ltd and Department of Conservation, has successfully championed community initiatives for conservation of pohutukawa on upper North Island coasts.

Now, it is planning to extend its restoration and protection activities to revitalise rata populations in lowland forest. Further understanding of the interactions between northern rata populations and possums will help this cause and also provide possible means to monitor total ecosystem health of our native forests.

Bruce Burns is a Manaaki Whenua -- Landcare Research scientist.
Phil Cowan is a Manaaki Whenua -- Landcare Research scientist.
Phil Knightbridge is a Manaaki Whenua -- Landcare Research scientist.