Where have all the mistletoes gone?

Forest and Bird, Volume 16, Issue 3, 1 August 1985, Page 10

Where have all the mistletoes gone?

by

and

Colin Ogle1

Peter Wilson2

N« Zealand’s mistletoes, some of which are notable for their brightly-coloured flowers, are becoming scarce in many regions and extinct in some. This article links mistletoe decline to the spread of possums. A European mistletoe, Viscum album, is traditionally remembered for its supposed mystical powers (as will be familiar to readers of Asterix books), and for its romantic connotations at Christmas. However, the term mistletoe can be used more widely for any plant in the family Loranthaceae which is a_ perching parasite of other trees and shrubs. Processing green leaves for stems, mistletoes presumably synthesise their own carbohydrates but must obtain water and minerals from their host plants.

(1) Wildlife Service, Department of Internal Affairs, Wellington (2) Ecology Division, DSIR, Nelson

New Zealand’s mistletoes Six of the nine endemic species are much-branched shrubs with somewhat leathery leaves in opposite pairs. Some have conspicuous, red or yellow flowers (Peraxilla, Alepis, Trilepidea) while in others (Jleostylus, Tupera) they are small and greenish-yellow. The three remaining native mistletoes are members of the genus Korthalsella. All are small herbs with semi-succulent, green, flattened or rounded stems, and only vestigial leaves. Their flowers are minute.

All our mistletoes have fleshy fruits with a single seed embedded in a sticky matrix, making them well-suited to bird dispersal. The decline of mistletoes Formerly, the species with brightly coloured flowers were widespread and abundant in New Zealand, at least in beech forests, as noted by Potts (1882), Field (1885), and Laing and Blackwell (1906). More recently, the red-flowered

Peraxilla (Elytranthe) tetrapetala was stll common in the Tararua Range. Ian Powell reports that during Christmas in 1924 he saw a lot of mistletoe on beech along the track from Totara Flat to Dalefield, and a year later mistletoe flowers littered the ground from Tauherenikau Hut to about half way up to Bull Mound. Over the past 10 years or more, Tony Druce has found very few mistletoe plants in the Ruahine and Kaimanawa Ranges, and none in the Tararua Range, although Les Pracy recently saw flowering P.(E.) colensoi in the Tararuas. Druce considers that mistletoes, once a major component of our forests, are now extremely rare wherever possums have been present for any substantial time. A 1980-84 inventory of threatened plants of the Wellington region made by members of the Wellington Botanical Society contains only one recent record of a leafy mistletoe, being a solitary plant of Ileostylus (Loranthus) on an isolated shrub of rohutu at Heretaunga. In the early 1960s, Peraxilla tetrapetala and Alepis (Elytranthe) flavida were known to one of us (C C O) and Dr Ian Atkinson on mountain beech close to the Chateau in Tongariro National Park. Dr Atkinson reports that these mistletoes were rare elsewhere in the Park at that time, but land around the Chateau was regularly trapped for possums. Both species have now disappeared from this area (Wilson 1984). Dr Ruth Mason recalls that mistletoe was very abundant in the Ureweras in the late 1920s, while Michael Greenwood and Tony Druce observed P. tetrapetala in abundance in the Ruahines

during the 1940s, where it is now rare (Wilson 1984). Trilepidea (Elytranthe) adamsii, the sole member of an endemic genus, was last seen alive in 1954, and is thought to be extinct (Given 1981). In much of the North Island and large forested tracts of North-West Nelson it is now remarkable to find a plant of Peraxilla or Tupeia, while Tleostylus, at home on a wide range of hosts, is mostly uncommon. There are some local exceptions. In a berry-fruit orchard beside the main highway at Otaki, Jleostylus occurs on eight plants of Coprosma crassifolia among a few totara trees, and occurs on totara itself where there are isolated trees in pasture of the Takaka Valley in Northwest Nelson. Jleostylus is on salt-march ribbonwood and hawthorn in Golden Bay, and on a wide range of host trees, including kowhai, silver birch and willow, in farmland of the Nelson district, where Peraxilla colensoi occurs on isolated beech trees also. Most of these sites appear to be ‘‘special cases’’, where possums are unlikely to occur, would be heavily trapped, or where there is a wide range of alternative possum foods, including pasture plants. Conditions which previously favoured mistletoe survival can change rapidly. Mark Bellingham reports that in an area of swamp of the upper Hokianga Harbour, Tupeia antarctica was common on shrubs of Coprosma propinqua up to 1982, but when water levels were lowered by draining, presumably giving access to possums, the mistletoes disappeared within a year, although they still remain in unmodified swamp nearby. Certain parts of New Zealand have never had possums, and there, not surprisingly, one or more leafy mistletoe species are common. D’Urville Island has an abundance of Tupeia on fivefinger (Ogle 1983), Jleostylus is common there, and although Alepis flavida has not been recorded there before, on 8 April 1985, PRW found large clumps of it on several large hard beech trees 50-150 m a.s.]. up a dry spur on the western side of Mill Arm, Greville Harbour.

Direct evidence of possums One of us (Wilson 1984) has shown possums were a major cause of the decline of mistletoes, during a five-year study in Mt Misery, Nelson Lakes National Park. Possums were first recorded in this part of the Park around 1965 (Rowley Taylor). The study area contained three leafy mistletoe species, Peraxilla colensoi, P. tetrapetala and Alepis. Marked mistletoe bushes were examined three-monthly for signs of possum browse and flowers or fruits. The amount of browsing damage was scored on ascale of 0-4, and each November the increase or decrease in the size of each mistletoe was assessed on the same scale.

Both Peraxilla species averaged almost one browsing attack per year and in over half these attacks more than 50% of the plant was defoliated. On average, P. colensoi plants decreased slightly in size but P. tetrapetala exhibited a dramatic decrease. By comparison, six unbrowsed plants of P. colensoi and seven of P. tetrapetala on the same transect averaged 35% and 20% increases in size respectively. It was also found that flowering, and hence the availability of seed for dispersal, is markedly reduced by the small size of plants which had been browsed persistently. Even with fewer than two possums per hectare, which is four or five times lower than in some more diverse forests of New Zealand (Clout and Gaze 1984), the Mt Misery area is undergoing a marked decline in mistletoes. Waitutu forest During a survey of wildlife in Waitutu State Forest in January 1984, members of the Wildlife Service party were impressed by the variety and abundance of large mistletoe plants throughout western parts of the forest (Elliott and Ogle 1985). However, only one of the four survey teams working east of the Wairaurahiri River saw mistletoes, and it seems that in this part of Waitutu mistletoes have, at the best, a patchy distribution, and many have been reduced in numbers and range. Possums are common to the east of Wairaurahiri River, but are almost unknown to the west. New bridges build across the Wairaurahiri and other rivers is probably helping the spread of possums westward. In Waitutu, Peraxilla (Elytranthe) colensot was the most showy species. Its presence on tall silver beech (apparently its only host tree there) was often indicated by fallen scarlet flowers carpeting the ground. The flowers contain nectar, and bellbirds, silvereyes, and, unexpectedly, since it is regarded as an insectivore, a yellowhead were seen feeding in them (Kath Walker pers. comm.). It is possible, of course, that the yellowhead was taking insects from the mistletoe flowers. Less abundant, but. still widespread, was the smaller scarlet mistletoe, P. tetrapetala. It was more common on higher altitude mountain beech and silver beech, P. colensoi being absent above 600 m. Because it grows on low spreading branches of mountain beech along lake edges and forest clearings, the plants of Alepis flavida with their yellow-range flowers were the most readily seen in situ at Waitutu. On the eastern shore of Lake Poteriteri, between the two unnamed rivers on either side of Poteriteri hut, a count of mountain beeches showed that of 212 beech trees, 55 bore one or more plants of this mistletoe. By January the fourth mistletoe seen at Waitutu, Jleostylus muicranthus had finished flowering but small, green fruits were seen. While its flowers are insignificant when compared with Peraxilla and

Alepis, its ripe fruits are yellow, 5-8 mm in diameter. Most remarkable at Waitutu was the abundance of this species and its ability, unlike the other species of mistletoe there, to use a wide range of host plants. The 19 different host species recorded included mountain totara, pink pine, six species of smallleaved Coprosma, horopito, tree fuchsia, southern rata and the liane, Rubus schmideliordes. South Westland and Fiordland Some mistletoe species are still abundant in parts of South Westland. Possums are known there, but according to Less Pracy there were very few before 1960, and even now they are in very sparse, scattered populations south of Paringa to Jackson Bay. He believes they were spread deliberately, including very recently into the Lake Ellery and Haast Road regions. During a search of beech forest adjacent to the road from Paringa River to Haast Pass in February 1984, PRW noted that Peraxilla colensoi was abundant, and, on average, in much larger clumps than in the Nelson Lakes study area. Peter Gaze reports that Alepis was quite widespread in forested areas he visited during 1984 between the Karangarua and Paringa Rivers. It was mostly growing on the outer branches of Coprosma propinqua bushes. Over the first week of January 1985, while touring South Westland and Fiordland, Rowley Taylor found that P. colensot was plentiful, in large clumps and in heavy flower, from Paringa to Haast; on both sides of Smoothwater River; at Cascade River on bush edges around Smiths Ponds and Colin Creek and through the forest canopy on surrounding country; from Haast to the Gates of Haast; and, with P. tetrapetala, at Haast Pass. Possum numbers in these areas still appear to be too low to have any noticeable effect on mistletoes. Taylor notes there were many fewer mistletoes in the parts of Fiordland that he visited than in South Westland. From west of The Divide to Milford, where possums were liberated before the turn of the century (Pracy 1974), Taylor found only six P. colensoz plants, all large and in flower. On 5 January, he found P. tetrapetala on silver beech in the Eglington Valley, four plants at Lake Gunn and one, past flowering, at Knobs Flat. However, at the latter site about 10 days earlier both P. tetrapetala and P. colensoi were reported by Kath Walker and Graeme Elliott to be common, but heavy rain destroyed many of the flower buds and open flowers. Possums were in the Eglington Valley by 1960 (Pracy 1980). The contrasting observations on mistletoes at Knobs Flat highlight several problems in assessing the abundance of mistletoes, particularly when an abundance rating is made from sightings of flowering plants. Different observers, varying climatic conditions, and precise time of year can all give apparent differences in mistletoe numbers at a given

site, or between sites. Furthermore, individual bushes of mistletoe which are browsed but not killed can recover if browsing pressure is reduced. Les Pracy reports that near Lake Wanaka in the Makarora Valley up to Camerons flat, mistletoes were severely defoliated by possums by 1950, but that in 1961 he observed some regrowth, and by 1978 mistletoe plants were again quite commonly seen. In other areas, too, reported gains or losses of mistletoes might sometimes be the result of changes in size and flowering state of plants rather than from the establishment of new plants or death of older ones. Korthalsella mistletoes Although possums browse leafy mistletoes, little is known about their effect on the three species of tiny, almost leafless, Korthalsella mistletoes. All occur in areas infested with possums, and the rarity of reported sightings of Korthalsella probably reflects the difficulty of seeing the plants. Nevertheless, since all are species of scrub and shrublands, Korthalsellas are likely to have suffered from extensive clearance of their habitat in the past few years. K. salicornioides occurs on the majority of older bushes of manuka and kanuka in an area on the west shore of Lake Wairarapa, but it takes careful searching to spot it. Near Taihape, the flatstemmed K. lindsayi is similarly difficult to see where it grows on Melicope simplex. There are only two records of the third Korthalsella, K. clavata, in the North Island, so that it may have always been a rare plant there, but it occurs locally in parts of the South Island. Hugh Wilson (1978) reports it as being fairly common on Coprosma, mountain wineberry, matagouri, and kohuhu in Mount Cook National Park. Mistletoe conservation There remain many unanswered questions about New Zealand’s mistletoes. For example, how important were they to indigenous wildlife for nectar and fruit, and, conversely, how important were birds to mistletoes for pollination and seed dispersal? Until recently there were few answers, but in 1985 in South Westland, Colin O’Donnell’s study revealed tuis and kakas taking nectar of both red mistletoes, and _ silvereyes, bellbirds and blackbirds feeding on fruits of Peraxilla colensot. PRW found bellbirds eating Alepis fruits on D’Urville Island. Many beech forests are poor sources of nectar and fleshy fruits, the more so since deer, possums and other browsers have depleted plants such as_ fuchsia, broadleaf, Pseudopanax species, and large-leaved Coprosma species. One can still study bird/mistletoe relationships in South Westland and in Waitutu forest but this may change rapidly as possums spread. Why are mistletoes not more common on certain islands with suitable host plants but without possums? If possums

were involved in the presumed extinction of Trilepidea adamsii, then why did that mistletoe apparently disappear from Great Barrier Island, which has no possums? If Tvilepidea still occurs, then Great and Little Barrier Islands would seem likely places to search. Why do leafy mistletoes persist in some eastern and southern parts of the South Island, despite the presence of possums? Are there other animals which eat the flowers or fruit without depositing seed on new hosts or do some introduced birds such as_ chaffinches destroy mistletoe seed if they eat the fruits? Is disease a factor? How long would it take to establish the relative importance of various factors, if more than one is involved, and is there time to do this if mistletoes are already under threat? What can be done now to conserve mistletoes? Some have been successfully protected on isolated host trees by constructing possum-proof barriers around the trunk, but this protects no more than a few mistletoe plants. Establishing mistletoes on host trees in urban areas and perhaps around dwellings in forests where possum numbers are kept down could have similar local success. While it might be unusual in New Zealand to speak of ‘‘marooning’’ as a technique for saving threatened plants from extinction, the technique is now well-known for certain native animals (Williams 1977), and could be equally valid for preserving some plant species.

As with vulnerable birds, the main prospects for large populations of mistletoes might lie in establishing the species on islands which are free of the problem animals, or on islands such as Kapiti and Codfish if they can be cleared of possums. Ian Atkinson reports that heavily-browsed plants of Jleostylus on karo bushes on Kapiti Island have recently begun to produce new leafy shoots, following the massive reduction in possum numbers there through hunting in 1983-84. Whatever the answers to these questions, we believe that time is running out for at least the two Peraxilla species, Tupeia antarctica and Alepis flavida. Although not mentioned in Rare and endangered plants of New Zealand (Given 1981), Peraxilla colensoi has been added to a new working draft of Dr Given’s checklist of threatened plants, with a rating of vulnerable. We recommend that the conservation status of all mistletoe species be re-evaluated and that action be taken to preserve, and if possible enhance, their remaining populations. ACKNOWLEDGEMENTS Our thanks go to all those named in the text for their personal communications on mistletoes and/or possums, and to Dr M. Crawley for his editorial comments.

REFERENCES Allan, H. H. 1961. Flora of New Zealand Vol 1. Govt. Printer, Wellington. Clout, M. N; Gaze, P. D. 1984. Brushtail possums (Trichosurus vulpecual Kerr) in a New Zealand beech (Nothofagus) forest. New Zealand Journal of Ecology 7: 147-155 Elliott, G. P; Ogle, C. %. 1984. Wildlife and wildlife habitat values of Waitutu Forest. Fauna Survey Unit Report No. 40. NZ Wildlife Service, Wellington. Field, H. C. 1885. Notes on Loranthus fieldii, Buchanan. Trans. and Proc. NZ Inst. 17: 288-90. Given, D. R. 1981. Rare and endangered plants of New Zealand. Reed. Laing, R. M; Blackwell, E. W. 1906. In Plants of New Zealand: 138-145. Whitcombe and Tombs, Christchurch, New Zealand. Ogle, C. C. 1983. The flora of d’Urville Island and Stephens Islands. Chap. 16, pp. 306-328; in O. Baldwin: Story of New Zealand’s French Pass and d’Urville Island, Book Three. Potts, T. H. 1882. Out in the open. In Lyttleton Times: 133-140. Christchurch, New Zealand. Pracy, L. T. 1974. Introduction and liberation of the opossum into New Zealand. NZ Forest Service, Wellington, New Zealand. Pracy, L. T. 1980. Possum distribution map defined and updated by L. T. Pracy for Agricultural Pest Destruction Council, Possum Survey Report, 1980. 37 pp. Williams, G. R. 1977. Marooning — a technique for saving threatened species from extinction. Jnternational Zoo Yearbook Vol. 17: 102-106. Wilson P. R. 1984. The effects of possums on mistletoe on Mt Misery, Nelson Lakes National Park. Proc. of Section A4E, 15th Pacific Science Cong., Dunedin, Feb. 1983 (P.R. Dingwall, compiler). Dept of Lands and Survey, Wellington.