Acacia melanoxylon

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Author: Tim Tunison, Global Invasive Species Team, The Nature Conservancy

Contents


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Taxonomy
Kingdom: Plantae
Phylum: Magnoliophyta
Class: Magnoliopsida
Order: Fabales
Family: Fabaceae (Leguminosae)
Genus: Acacia
Species: A. melanoxylon
Scientific Name
Acacia melanoxylon
R. Br. ex Ait. f.
Common Names

black acacia, blackwood, Australian blackwood, blackwood, blackwood acacia

Overview

Appearance
Acacia melanoxylon is a straight trunked, medium sized tree of the legume family (Fabaceae) with a dense crown. It is a thornless, evergreen tree 26-49 ft. (8-15 m) high. It can occasionally get up to 148 ft. (45 m) tall. Native of rainforest areas in southeastern Australia, it was introduced as a forestry planting to Hawaii, New Zealand, and South Africa. It was introduced to Hawaii as a forestry planting (Nelson and Schubert 1976).
Foliage
The leaves are bipinnate on seedlings. In mature trees coppice shoots usually become phyllodes. Phyllodes on Acacia melanoxylon are about 3-4 in. (7-10 cm) long. Foliage starts out greyish turning dark dull-green as it matures. The leaves are straight to slightly curved, with 3-7 prominent longitudinal veins and fine net-veins between.
Flowers
The pale yellow to whitish colored flowers have a powder puff appearance due to the presence of numerous stamens. Each one ranges from 0.2-0.4 in. (5-10 mm) in diameter. The flower clusters are borne on peduncles from 0.2-0.6 in. (5-14 mm) long. They are alternately arranged and arise from the phyllode axil. Flowering can occur year round.
Fruit
The fruit of Acacia melanoxylon is an elongated, flattened pod approximately 1.6-6 in. (4-15 cm) long and 0.1-0.3 in. (3.5-8 mm) wide. The pods are strongly curved, twisted or coiled. These pods are mostly glabrous with only a slight constriction between each seed. The pods are green and leathery when young, maturing to brown to reddish-brown in color and become woodier. The seeds are elliptic and about 0.1-0.2 in. (3-5 mm) long and 0.07-0.1 in. (1.7-3 mm) wide. Seeds are glossy, black and are encircled by a large pink to dark red aril.
Ecological Threat
Acacia melanoxylon is native to eastern Australia. This tree grows fast and tall, up to 148 ft. (45 m) tall. It tolerates a wide range of conditions and soils but grows better in colder climates. It invades areas such as natural vegetation, commercial timber plantations and farmland.

Stewardship summary

Blackwood acacia is a potentially invasive species in Hawaii. However, its potential is limited by its apparent lack of regeneration from seed. Massive recruitment from seed following disturbance is possible. Production of viable seed and seedling recruitment should be monitored. Until then, the most logical management program is to quarantine blackwood acacia within the pine plantations while controlling root suckers that escape from the plantation

Natural history

A medium sized tree of the legume family (Fabaceae). Native of rainforest areas in southeastern Australia, it was introduced as a forestry planting to Hawaii, New Zealand, and South Africa. It was introduced to Hawaii as a forestry planting (Nelson and Schubert 1976). It has been planted sparingly in wet to mesic habitats [1] and is best adapted to cool, moist sites (Nelson and Schubert 1976). It is found in Waikamoi Preserve but not Kamakou Preserve.

The ecology of this species in Hawaii is poorly understood. There is a plethora of literature on this species from Australia, South Africa, and New Zealand dealing with its wood qualities and use in forestry.

Blackwood acacia is a moderately fast growing species in Hawaii (Nelson and Scubert 1976, Little and Skolmen 1989). Its main and possibly only means of reproduction in Hawaii may be vegetative through root sprouting. Viable seeds have been produced in Hawaii, at least from trees used as a seed source for forestry plantings (location unknown). Blackwood acacia regenerates from seed in its native range, and copious seedling recruitment is responsible for its invasiveness in South Africa.[2] Blackwood acacia is apparently fire-stimulated, with prolific regeneration from seed after fire.[3]

Smith (1985)[4] lists blackwood acacia as one of 33 alien species which should be monitored as potential threats to native ecosystems. This species is spreading vegetatively in pine plantations in Waikamoi Preserve, and has begun to escape from the plantations into adjacent drainages. The magnitude of its threat to native ecosystems is limited if its spread is exclusively vegetative. If reproduction from seed becomes common, the potential for invasiveness is considerably greater.

In Hawaii, invasive in some areas; spreading vegetatively in pine plantations in Waikamoi Preserve, and has begun to escape from the plantations into adjacent drainages.

Management/Monitoring

Control of root sprouts escaping the plantations into drainages in Waikamoi should first be controlled. Then all blackwood acacia near the edges of the plantations should be treated.

Effective control methods have not been developed yet. Some preliminary studies to identify biocontrol agents in its home range have been conducted.[5] However, biocontrol is not feasible or warranted in Hawaii.

Production of seed and recruitment from seed should be monitored.

If apparently viable seed are produced, hot water treatment should be used to test germination. This consists of placing seeds in boiling water, removing the heat source, letting seed soak overnight, and then germinating. Recruitment from seed can be evaluated by uprooting young plants to determine if they are connected to long, lateral roots.

Research

An effective cut-surface herbicide is needed. Undiluted Garlon 3A and Roundup should be tested.

If fruiting trees are located in Waikamoi, a quick examination of the soil seed bank for the presence of viable seeds may be useful to management. Blackwood acacia seeds require scarification to germinate.[6] A soil seed bank may have developed that could be stimulated by disturbance such as fire, resulting in recruitment and spread of this species.

Resources

Information sources

Bibliography

  1. Skolmen, R. 1991. Retired USDA Forest Service forester. Telephone conversation with Tim Tunison, P. O. Box 754 Volcano, Hawaii, 96785. May 31.
  2. De Zwaan, J. G. Is a seedless blackwood tree a possibility? South African Journal of Forestry 113:59-61.
  3. Hill, R. S. Rainforest fire in werstern Tasmania. Australian Journal of Botany 30:583-589.
  4. Smith, C. W. 1985. Impact of alien plants on Hawaii's native biota. pp. 180-250. in C. P. Stone and J. M. Scott (eds.). Hawaii's terrestrial ecosystems: preservation and management. Univ. Hawaii Coop. Natl. Park Resour. Studies Unit, University of Hawaii Press. 584 pp.
  5. Den Berg van, M. A. 1982. Coleoptera attacking Acacia dealbata Link., Acacia decurrens Willd., Acacia longifolia (Andr.), Willd., Acacia mearnsii De Wild., and Acacia melanoxylon R. Br. in Australia. Phytophylactica 14(2):51-55.
  6. De Zwaan, J. G. The effects of hot-water-treatment and stratification on germination of blackwood ( Acacia melanolylon) seed. South African Journal of Forestry. 105:40-42.

Original Document

Element Stewardship Abstract; Tim Tunison, 1991


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