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Weinmannia racemosa

Weinmannia racemosa
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Oxalidales
Family:	Cunoniaceae
Genus:	Weinmannia
Species:	W. racemosa
Binomial name
Weinmannia racemosa Linn. F

Foliage of a young Weinmannia racemosa

Weinmannia racemosa, commonly known as kāmahi, tawhero and towai belongs to Cunoniaceae, and is one of the most abundant evergreen canopy trees found in New Zealand.

Description

Kāmahi is an evergreen tree, that is abundant in lowland and montane forests throughout New Zealand.

The North Island trees occur from sea level to 1000 m, up to 900 m in the South Island, and 650 m on Stewart Island.

Kāmahi grows up to 25 m and tall. They grow up to 10 m in lowland flats and alluvial terraces. The trunk's diameter is recorded up to 12 dm. Leaves of adult kāmahi are simple and oppositely arranged of 2-10 x 1-4 cm, while the petioles are up to 2 cm in length. The sepals are of 1.5 mm length, the petals are 2-3 mm long, and the capsules are 4-5 mm long.

Kāmahi is often confused with the other New Zealand species, W. silvicola, due to similar morphology, but can be differentiated by their twig colour, and great variation in their leaf characterisation. The twig colour of W. racemosa tends to be light grey, while the petioles are glabrous, and the length of the lamina is usually 2.5 - 9 cm. The juvenile leaves are simple and trifoliolate, and flowers and racemes in W. racemosa tend to be stouter, and the adult trees are also larger, compared to W. silvicola. Whereas, the twig colour of W. silvicola are dark grey to nearly black, and the petioles are nearly glabrous, while the length of lamina in simple or compound leaves of terminal leaflets is between 2.5 - 4.5 cm. In addition, the juvenile flowers in W. silvicola are usually with 5 - 7 leaflets, but also up to 9.

W. racemosa plant specimen collected in Pahiatua, 2010

Distribution

Kāmahi is endemic to New Zealand and is geographically widespread throughout the country, and mostly abundant in Westland and Stewart Island, where annual rainfall is greater than 1000 mm. Great proportions of these dominant, woody tree species are established in highland conifer-broadleaved and hardwood forests in central Westland, New Zealand.

Native canopy forests in the North Island including Pirongia, Whareorina, Oriuwaka, Urewera and Wanganui, are selected few habitats to kāmahi, and is extended to Mt Te Aroha. Southern parts of South Island to Catlins district, and northern end of the island to Marlborough Sounds also have occurrence of kāmahi. Westland and Steward Island and surrounding islets show positive records of the species' abundance, due to suitable soils. The species population is less abundant on poorly drained soils and occurs stunted on infertile soils.

Kāmahi is grown on lowland flats and alluvial terraces. It can tolerate wet environment, prefers moist, and can sometimes tolerate dry, sunshine and shady environments. However, the seedlings are not frost tolerant.

While kāmahi has been a susceptible tree during diebacks, and were badly damaged by Cyclone Bernie in 1982, and trees of low vigour growing on stressed sites were less capable of recovering from the damage. These factors influence the population and geographical distribution of kāmahi.

Life Cycle

The active vegetative growth of kāmahi begins in early August, determined by the production of small, pale leaves. Flowering of the plant occurs during early summer, November and December. Flowers have also been recorded in May. Stages from flower buds to ripe capsules are present in August.

Propagation occurs by seed, and most of the ripe capsules are dispersed during March, leaving little to no seeds by the end of April. Seeds can also be sown directly on subsoils at planting sites with careful assessment of sowing trials.

While there is not enough information to explain the lifespan of kāmahi, the establishment and seedling development show that the tree is easily established on mineral and humus surfaces, epiphytic seedlings on logs, and tree trunks of tree ferns . It is also noted that tui, insects and honey bees visit the flowers for nectar, and develop a mutualistic relationship over the flowering period of kāmahi.

Interactions

The palatable, abundant kāmahi in native forests are a target of predation for introduced brush-tailed possum, which is an invasive pest mammal species introduced to New Zealand. The invasion of kāmahi trees and poor health of native forests have gained attention for forest surveys, and several case studies have taken place around the interaction and relationship between the possums and kāmahi.

Despite the major decline in kāmahi population due to diebacks of indigenous forests during the 1950's in the Kokatahi and Fox catchments in Westland, regeneration studies have showed that the species were successful in recovering and becoming structurally dominant species in the Westland area. Fungal attacks caused by Sprothrix due to the presence of Platypus may have also triggered diebacks. Intensive browsing by the introduced brush-tailed possum, resulted in kāmahi forests' widespread conspicuous canopy defoliation and depletion. However, forest surveys post-control of the brushtail possums have shown that palatable trees including kāmahi, cedar, southern rata and Hall's totara have increased in population over time .

The larvae of prey moths including Tatosoma tipulata (Geomitridae) and Hepialus virescens (Hepialidae) feed on the upper surface of kāmahi leaves and tunnels are made in the wood of living stems, respectively. Other ungulates such as deer, goats and cattle happen to remove bark, browse seedlings, and were also determined as a cause for rapid decline in kāmahi.

Harmless biotic relationships included the presence of fungi, saprophytes and mycorrhizae, which were recorded on living tree bark, dead stems and rootlets, respectively.

Further Information

The Westland District is a unique landscape comprising of lowland ecosystems, rimu forests and upland forests including mountain and silver beech, southern rata and kāmahi. Rare flora and fauna is also found in these forests. Data deficient species such as the South Island Kokako have also been recorded in the subalpine scrubs and kāmahi dominated forests. These features contribute to the growing tourism industry in Westland, and also hold cultural values.

Kāmahi is recognised for its economic importance. Its timber is durable, however, its weak chemical composition of tannins and catechin causes the wood to split and twist easily. Some of the uses of kāmahi timber are in logging tramways, fence droppers and sheep grating. The juvenile flowers of this tree are used for commercial purposes. In addition, some beekeepers have reported kāmahi as a honey producer.

The traditional uses of kāmahi by Maori were for medicinal purposes and dyeing. The inner bark infused in boiling water was consumed as a tonic to cure thoracic and abdominal pains. The popular historical use of kāmahi is tanning. Reddish, permanent dye is produced by boiling and soaking the bark of kāmahi, which were used to stain mats and fishing lines.

An interesting fact about kāmahi is that young leaves of this plant are less sensitive than mature leaves in response to high temperatures (from 25 to 35 °C). These heat stress tolerance results may be helpful in understanding how immature kāmahi leaves function under extreme conditions.

As a dominant tree species in major forest canopies in New Zealand, kāmahi is a palatable tree that holds great economic value and is a strong survivor through diebacks in the past. Further research around W. racemosa will support in protecting the species, as well better understanding the lifecycles and distribution.