In search of the wood rose

Forest and Bird, Issue 267, 1 February 1993, Page 24

In search of the wood rose

Chris Ecroyd

New Zealand’s most unusual native plant, Dactylanthus, lives largely underground wrapped around the roots of trees. CHRIS ECROYD unravels the mystery of the pollinators of this rare and interesting member of our flora and explains why it is under serious threat, not only from collectors of the transformed host roots, but also from possums and rats.

OOD ROSE, flower of Hades, pua o te reinga. Whatever name you use, Dactylanthus taylorii is New Zealand’s only completely parasitic flowering plant. Strangely devoid of roots and obvious leaves, Dactylanthus consists mainly of a swollen underground stem up to 50 cm wide and looking like a large ugly warty potato. The tiny flowers are usually in dull purplish to grey-brown clusters, and although they give the plant little claim to beauty, the fluted "wooden roses" it moulds out of the roots of its hosts are a different story. These beautiful objects have long been prized by collectors, and this has added to the pressure on a rare and interesting organism. Dactylanthus does not obtain its food through photosynthesis like most plants, but lives on nutrients from the roots of about 30 species of native hardwood trees and shrubs. Mahoe, lancewood, kohuhu, wineberry, broadleaf, karamu and putaputaweta are common hosts, though it is often difficult to be sure of the host as the tree trunk may be some distance from the infected root and may not necessarily be the closest tree. Unless there are a lot

of them, Dactylanthus plants are unlikely to affect the health of the host. Growing close together and completely underground, Dactylanthus plants are extremely difficult to identify and count without digging them up. This makes it impossible to estimate accurately the number of plants in any one area, but we know that their distribution has shrunk this century and today there are likely to be only a few thousand in existence. Habitat loss from land clearance, human destruction of plants to obtain the wood roses, and browsing of the flowers by introduced animals have combined to place the species under serious threat. It 1s in the "vulnerable" category in current lists of threatened plants in New Zealand. In 1985 some live plants were found at the base of a kohuhu tree in native forest on the Mamaku Plateau, not far from Rotorua. Knowing how rare the plant was, and how very few photographs of it existed, I was keen to see and photograph the flowers. Returning to the site the next flowering season, I could find only broken flower stalks. The following season was the same but this time I inspected the broken flower stalks closely and found that the tips were consistently eaten out. An animal was very thoroughly

destroying all the flowers. In February 1989 I covered the plants with wire netting exclosures and two weeks later there was a flourishing patch of Dactylanthus flowers under the netting. Wild pigs were regularly feeding in this area and one plant had been moved by a pig rooting too close to it, but there was never any evidence to suggest that they had been attracted to the Dactylanthus plants or flowers. Pellets, browsed flowers and scratchings on the ground, however, suggested possums — a theory proved correct when the stomach contents of captured possums were found to include the remains of the sweet asparagus-like flowering shoots of Dactylanthus from outside the exclosures. The flowering season lasts about three months. At Mamaku it began at the end of February, peaking in late March to early April, and was finished by late May. Dactylanthus plants are unisexual, which means they carry either male or female flowers in clusters, known as inflorescences, on separate plants. A large mature plant can easily produce 40 inflorescences, each containing about 20 finger-like organs called spadices, surrounded by greatly modified "leaves". On each of these spadices there are at least 50 tiny simple flowers. Thus one mature female plant

can produce 40,000 seeds each season if all the flowers are pollinated. But with completely separate male and female plants, not necessarily growing close together, Dactylanthus pollen must somehow be carried from the male flowers to the female flowers. It was essential to work out who or what was responsible for pollination and to make sure that they were not excluded by the possum exclosures. At first insects were thought to be the most likely pollinators. Several different traps were used to find out exactly which insects were visiting the flowers. The only ones visiting regularly and in large numbers were the German and common wasps. The "host of small flies" around the flowers, referred to in earlier accounts of this species, was never seen. Another aspect of the mystery was the pool of sweet tasting nectar noticed inside each inflorescence. The amounts seemed particularly large for a plant supposedly pollinated by small insects. Each inflorescence contained up to 1.5 ml of nectar, a lot for any type of flower. Furthermore, each male inflorescence lasted for 10-14 days, over which time it produced an incredible 5 ml of nectar. Nectar was produced constantly throughout 24 hours, giving no clue as to whether the plant was likely to attract a nocturnal or day-time pollinator.

HEN A PLANT produces large quantities of nectar it usually means it has large pollinators. But what large native animal could be attracted to such dull coloured flowers, smelling like fermented corn and found only on the ground? Birds are generally attracted by brightly coloured flowers, rather than by smell, and although bright red and yellow Dactylanthus inflorescences have been found, these are rare freaks. Large insects such as weta are not known to feed on nectar. This seemed to leave bats, lizards and tuatara — our only other large native land animals. The bat theory was strongly supported by a record of Dactylanthus pollen in guano from short-tailed bats in Northland’s Omahuta Forest. This rare bat feeds on nectar and fruit as well as insects and, having evolved in the absence of terrestrial mammalian predators, is well adapted to feeding on the ground (see Forest & Bird August 1992). Little Barrier Island was the only site where short-tailed bats and Dactylanthus were known to co-exist but Dactylanthus had only been found there twice in the last 40 years. Then in 1991 came a report that Dactylanthus had been found on Little Barrier. On a hastily organised trip I managed to find only two live plants and

numerous dead ones in the general area suggested. The dead plants were the first indication that this island was not the safe haven for Dactylanthus that it was thought to be. Another trip to Little Barrier was planned to resolve the pollination question. A special video camera was borrowed, with infra-red lighting for nocturnal monitoring and a time-lapse video recorder which could monitor a site for up to six days without human interference. The equipment was tried out for a night at Pureora where a ship-rat was filmed visiting, but not harming, female Dactylanthus flowers. With some difficulty the heavy equipment was taken to Little Barrier Island in March last year. Only eleven plants were found, and only unopened buds. After a week of monitoring, the video caught a kiore or Polynesian rat chewing and damaging the nearly open buds. All the flowers produced from Little Barrier plants that season suffered this fate and there was no sign of bats near the plants. HE BREAKTHROUGH occured when the video equipment was subsequently set up in an area of magnificent native forest at Pureora. The tape captured a shorttailed bat fluttering onto a tree trunk,

climbing down to the ground, crawling through the netting exclosure to the Dactylanthus flowers and pushing its head into the flower. Upon finishing this feast of nectar the bat flew up through the 50-mm mesh netting without hesitation. Amazingly this bat visited the same flowers 40 times that night from 8pm until nearly dawn. Over the next four weeks we made further visits to this site and set up a 35-mm camera and flashlight alongside the video camera. From a comfortable spot nearby we used the video as a closed circuit television system, and remotely triggered the camera and flash when the bat was in position for a photograph. Although frightened by the flash, the bat kept quickly returning to the site. These photographs and the video tape provided the proof that Dactylanthus is indeed a bat-pollinated species. N EXCITING RESULT of this discovery is the opportunity now to use Dactylanthus flowers, or an artificial lure imitating the nectar, to help locate short-tailed bats. The bats are completely nocturnal, extremely hard to locate and very difficult animals to study. We still do not know, for example, whether they are killed during operations to control possums using 1080 poison.

Dactylanthus is the only plant in the world producing flowers at ground level that is known to be bat-pollinated. This special relationship is not so surprising because the short-tailed bat feeds on the ground more than any other bat species and it has developed from its ancient Gondwanan ancestors alongside Dactylanthus over millions of years. The remains of extinct larger species of shorttailed bat have been found in ancient Maori ovens and these larger bats may also have been pollinators of the Dactylanthus flowers. Unfortunately, instead of bats these flowers are now attracting introduced mammals such as possums and Polynesian rats which are destroying the flowers and preventing seed production. Frequently thought to be "just a fungus" or disease, Dactylanthus is now recognised as being a very distinctive member of our native flora. Not only a rare and interesting plant in its own night, it also has great ecological significance by producing nectar which contains ingredients important for the diet of the short-tailed bat. A limited number of plants can be protected from possums by a very simple wire netting exclosure with the mesh coarse enough to allow bats through. Possum control using traps or poison is expensive and if small areas are involved,

rapid reinvasion of the site will be a problem. Eradicating kiore from Little Barrier Island would help ensure the survival of that Dactylanthus population. The evidence suggests that ship-rats, on the other hand, do not destroy the flowers and may even help in pollination — female flowers, for example, visited by these rats have set abundant seed. Therefore introducing Dactylanthus to another island free of kiore

and possums but with ship-rats present is a now a possibility due to a recent breakthrough in cultivation. Dactylanthus seed requires chemical stimulation from a host root to germinate and if the seedling is unable to attach itself after growing only a millimetre or two towards a host root, it will exhaust its food reserves and die. Now for the first time seeds have been germinated by sowing them adjacent to the roots of known

host plants and it is thus possible to transfer the Dactylanthus to possum free sites. Unfortunately, moving short-tailed bats to a new location is not currently a practical option. Dactylanthus is a most unusual plant and is worthy of extra effort to ensure its survival. New Zealand’s short-tailed bat has features which make it distinctive among the 1,000 or so bat species in the world and it is placed in a family of its own. It is one of our two surviving native land mammals and in recent years has been seen in very few locations. The survival of these two closely entwined but increasingly rare species presents a challenge to us all and will ultimately depend on maintaining suitable habitats free from the effects of introduced mammals such as possums and rats. Members of Forest and Bird can assist by reporting any sightings of the bats and of Dactylanthus to DoC and reporting anyone digging up these plants from reserves. %

is a scientist

with a special interest in rare and endangered plants and is curator of the New Zealand Forest Research Institute herbarium. He is on the Bay

of Plenty Conservation Board and a pastpresident of the Rotorua branch of Forest and Bird.