by Piter Kehoma Boll
If you walk through eucalyptus forests in eastern Australia, you may find today’s fellow in its natural environment. Its name is Acacia pycnantha, commonly known as the golden wattle and, as obvious by its scientific name, is a species of acacia.
The golden wattle is a peculiar tree. It reaches a height of about 8 m, although most individuals grow only up to 6 m. As common among Australian species of the genus Acacia, the golden wattle does not have true leaves. Instead, it has modified leaf stems, called phyllodes, that are widened to look and function like leaves. The phyllodes have a lanceolate and falcate shape, i.e., they look like a typical leaf that is slightly curved to one side, like a sickle. The outer side of this “sickle” has an extra-floral nectary, a structure that produces nectar and attracts insects and birds that feed on it.
The plant produces flower buds all year round but only those produced between November and May will develop further and open between July and November of the next year. The flowers occur in inflorescences and have a strong yellow color and the typical fluffy aspect of acacia flowers caused by the very long stamens.
Despite the huge amount of flowers that a single tree produces, this species is self-incompatible, meaning that it cannot fertilize itself and needs its pollen to be taken to the flowers of another individual of the same species. It has been shown that birds are very important pollinators of the golden wattle and the tree uses the extra-floral nectaries to aid that. When a bird visits the tree, it feeds on the nectar from the extra-floral nectaries and, in the process, brushed against the flowers, becoming covered with pollen. When the birds visit the next tree and brush against its flowers, part of the pollen of the first plant passes to the flowers of the second one.
The bark of the golden wattle produces large quantities of tannins, more than any other Australian acacia, which led to its cultivation for this purpose. When stressed, the trunk exudes a gum (resin) that is similar to the gum arabic produced by African species of acacia.
The golden wattle has been introduced in several other countries, especially in Europe and Africa, for ornamental or economic purposes. In South Africa, its cultivation for tannin production made it spread quickly through the native ecosystems, becoming invasive. And now, as always, we have to deal with the consequences of our irrational acts and run to solve this problem.
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References:
Hoffmann JH, Impson FAC, Moran VC, Donnelly D (2002) Biological control of invasive golden wattle trees (Acacia pycnantha) by a gall wasp, Trichilogaster sp. (Hymenoptera: Pteromalidae), in South Africa. Biological Control 25(1): 64–73. 10.1016/S1049-9644(02)00039-7
Vanstone VA, Paton DC (1988) Extrafloral Nectaries and Pollination of Acacia pycnanthaBenth by Birds. Australian Journal of Botany 36(5): 519–531. doi: 10.1071/BT9880519
Wikipedia. Acacia pycnantha. Available at < https://en.wikipedia.org/wiki/Acacia_pycnantha >. Access on 9 August 2019.
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