Dendrobium cuthbertsonii F. von Mueller
Wolfgang H. Bandisch
There are so many wonderful orchid species in Papua New Guinea that it is almost impossible to say which one is the most exciting. To me, one of the most remarkable experiences of observing an orchid species in its natural habitat was to see Dendrobium cuthbertsonii at the Tari Gap, approximately 2,300-m above sea level, in the Southern Highlands Province.
Dendrobium cuthbertsonii is widely distributed on the island of New Guinea from the northwestern part of Irian Jaya along the central dividing mountain range down to the southeast of Papua New Guinea, on the mountains of the Milne Bay Province. This species has also been sighted on Goodenough Island and the southeastern mountains of New Ireland.
The species appears to prefer an environment that encounters frequent rain, although the rain does not need to be daily of heavy. On the mountain slopes leading to the Tari Gap temperatures during the day may rise as high as 25 deg Celsius. At night the temperature may go down to about 5 deg C. There is constant air movement along the mountain ridges, and at higher altitudes strong winds prevail throughout most of the day. Humidity is quite low.
Apart from the outstanding flower colours the most intriguing characteristic of the species are the 'warty' leaves and ovaries. The purpose of these 'warts' is unclear but it is assumed that they serve as water traps. There is no other orchid known anywhere with a similar leaf epidermis. The flowers have an unusual feature: because of the extreme laxness of the pedicel, the weight of the flower causes it to hand or lie down on the leaves. The flowers appear 'upside down' with the labellum being the uppermost part of the flower.
Along the road from the Ambua Lodge to the Tari Gap one can clearly observe the terrestrial habit of the species. It grows abundantly in full sun on road cuttings nearly devoid of any other plants with its roots buried up to 8 cm in the clay soil. The sight is spectacular as the bright 'electric' red colour starkly contrasts with the drab surroundings. Venturing off the road on to the embankments one can find Den. cuthbertsonii growing in the bare soil along the forest edge, often as the only vegetation on the ground. In some places it is nearly impossible not to step on them because of the sheer numbers present. The embankments on the side of the road are often so well-drained that even mosses find it hard to establish themselves in that environment.
The most common colour one can find is bright red but flowers with any combination of red and yellow in the sepals and petals or labellum are frequent. Other relatively common colours are pink and combinations of pink and white in the sepals and petals. One specimen seen was pink with a pale yellow-orange labellum.
A few hundred meters up the road dramatically different vegetation is encountered with the ground covered in a thick layer of moss, patches of pitpit (a native tall grass species), shrubs and bushes. Here one can also find an abundance of Den. cuthbertsonii growing together with other plants - other species of orchids as well as dwarf species of Rhododendron. Here the roots of Den. cuthbertsonii may extend through the moss layer and find their way into the soil. Plants can be found growing in full sun or in the semi-shade of the nearby grasses and shrubs. A large variety of flower colours can be observed.
The species appears to be widespread and common in the area although out of some 400 plants observed only two had a seed capsule. Quite frequently a plant of Den. cuthbertsonii is the solitary plant on a road cutting and the next population may be hundred meters away. This raised the question of how the flowers are pollinated and how the seed is distributed. Tom Reeve, a botanist who started the Liagam Orchid Collection in the early 1970s, reported observing honey suckers of the genus Melidectes searching the flowers for nectar. Since the plants are growing on the ground, about 5-8 cm high, chances of the seed being distributed by wind appear remote, but still one can find this species in the most impossible locations.
Travelling further up the steep road towards the Tari Gap the vegetation changes from dense forest to high altitude marshlands and moss forests. The summits of the surrounding mountains are as high as 3,700 m and are almost continuously covered in clouds. The high altitude marshlands provide an eerie sight, stiff grasses, grayish-green, about 30-70 cm tall cover the plateau at the Gap.
Small plants with tiny white and others with blue flowers grow in between the grass together with small stand of Spathoglottis parviflora. A white flowered dwarf Rhododendron stands out.
The tallest plants of the marshy grasslands are cycads, some of them hosts to thick patches of Dendrobium sulphureum and the occasional Den. cuthbertsonii. The ground is soaking wet and it is most advisable to wear rubber boots as one gets frequently bogged walking through the grass.
The surrounding mountains are covered with moss forests. The marshlands are dotted with patches of these moss forests. When one enters these patches of moss forest the environment changes dramatically. Outside on the marsh the climate feels inhospitable where only the toughest of plants can survive. Inside the moss forest the lush growth is rather fascinating. Southern beech trees (Nothofagus), trunk and branches covered in thick moss, ginger plants, huge fern trees and great pandanus stands give the impression of having entered some primeval land.
The forest floor is covered with fallen trees and branches, thick layers of moss, vines and shrubs make up an almost impenetrable jungle. Everything is dripping wet; the thick water-clogged tangles of moss often get so heavy that they cause the thinner branches to break. On moss covered branches, broken off, but still dangling in mid air, held in position by the long strands of moss and starkly contrasting to the grey-brown-green colours of the vegetation, the striking fire-red flowers of Den. cuthbertsonii appear almost unnatural in such dreary surrounding. They are like glittering gems, eye catching as they grow anywhere from the forest floor to high up in the tree canopy.
They are everywhere, a flash of colour here and there. Mostly small patches with up to 20 flowers can be observed. Here the plants are not attached to the trees but have their roots entangled in the outer layers of the moss covering the trunks and branches. Plants are more abundant along the inside edge of the forest where the sun occasionally reaches through the thick layer of leaves. More then 30 m inside from the forest edge sightings are less frequent.
An intriguing observation made was that in three different patches of moss forest explored the variety of colour form appears to differ quite substantially. In one patch mostly red and red-yellow colour forms were found with only very few of the pink variety. In another patch both pink and red as well as red-yellow forms appeared to be almost evenly distributed, while yet in the third patch unusual colours like white, yellow, light pink, in plain and bi-coloured, e.g. yellow with a deep maroon outer margin of the labellum were found.
Flower size in the populations observed may vary greatly. The same variations apply to the leaf size. There does not appear to be an explanation for this as different sized plants grow in close vicinity to each other.
Cultivation of Den. cuthbertsonii in climatic zones other than those similar to its natural habitat appears very difficult. Both mounting on slabs of tree fern and pot culture have been successful, although many plants eventually die for no apparent reason. It has been suggested that the plants perhaps flower themselves to death. This dos not appear to be a supportable explanation as plants in the wild flower profusely all year round. Plants in cultivation are reported to have had individual flowers last for up to eight months.
Plants do best if their roots are constantly wet, while their foliage and flowers are kept dry. This is evident in their habitats, both terrestrial and epiphytic.