Melaleuca quinquenervia: paperbark forests on the Sunshine Coast, Australia
(writing in progress)
Anyone familiar with the street trees of Perth will know the paperbark Melaleuca quinquervia, which is not native to Western Australia but instead grows naturally on the eastern coast of mainland Australia and in New Guinea and New Caledonia. Although there are several native spp. of paperbark in the Perth area, it is this broad-leafed species from the eastern states which has been widely plant as a street tree in Perth.
It is only after visiting the Sunshine Coast, just north of Brisbane, that I have come fully to appreciate this species, and I’ll mention a few of the most interesting aspects here.
Melaleuca quinquenervia is remarkably common on the Sunshine Coast, to a degree not matched by any ‘paperbark’ in Western Australia.
Melaleuca quinquenervia dominates the vegetation in freshwater swamps on the Sunshine Coast, and forms dense forests with a remarable number of boles per hectare (see the photos below). In this way M. quinquenervia is different from the paperbark trees native to the Perth area, which certain do form small patches of forest but do not give the same impression of a ‘dense plantation’. What appears to be a ‘dense plantation’ of paperbark trees, about 15 m high, is actually a natural swamp forest, seasonally waterlogged, the natural habitat of M. quinquenervia although the same species is capable of growing on well-drained ground (as it does in the uplands of New Caledonia).
At the same time, M. quinquenervia extends on to some ‘heathland’ areas where it grows as a ‘mallee’, i.e. with several stems emerging at ground level and a total height of only about 4 m.
Adjacent to the swamp-forests of M. quinquenervia, as one approaches the tidal flats where the water is brackish, we find semi-saline swamp forests of Casuarina glauca. Again, although there are plenty of trees of casuarina in Western Australia, including salinity-tolerant species, the sheer density and extensiveness of these forests north of Brisbane surprised me. A woodland of casuarina is one thing, a forest with casuarina among its various trees is another thing – but a forest thoroughly dominated by casuarina over an extensive area is a sight to behold, and consistent with the far greater extent of mangrove vegetation in eastern than in Western Australia.
So, I find that both Melaleuca and Casuarina (although seldom together) both form extensive dense stands of close-spaced trees on poorly-drained flats on the Sunshine Coast. On their seaward side the forest of C. glauca abuts mangroves. On their landward side the forest of M. quinquenervia in places abuts ‘wallum heathland) with low Banksia and sedges. In the latter case, the transition from 13 m high forest to <1m high ‘kwongan’ can be extremely abrupt despite the flatness of the topography. This is one of the starkest patterns I’ve seen w.r.t. vegetation height. One can stand in a patch of low ‘fynbos’, the habitat of the ground parrot, and look at the surrounding ‘wall’ of paperbark forest, 15 m high, a transition which has little to do with rainfall and much to do with soil nutritional regimes along subtle lines.
We happen to know a fair bit about the ecology of M. quinquenervia, partly because this species has become so invasive in Florida that it has been particularly well-studied as a problem-plant there.
Here are some aspects of the biology of Melaleuca quinquenervia, which I’ll henceforth in this email call M q.
M q is ecologically related to eucalypts despite its papery bark.
M q is remarkably able to regenerate vegetatively, by epicormic shooting. In this way it differs from ‘mallees’, which typically die above ground in fire and regenerate from the lignotuber at the base.
The epicormic regeneration of M q is partly explained by the protection afforded to the bole by the papery bark. This papery bark protects the epicormic buds.
At the same time, there is an apparent paradox, in that the papery bark also aids fire to climb from the understorey to the crown, leading to combustion of the foliage which is then rapidly followed by epicormic shooting of foliage with rapid flowering thereafter. M q is a species ‘encouraging’ crown fires owing to the oil content of its foliage.
Unlike Eucalyptus erythrocorys, which you and I examined on our trip to Dongara, the ‘mallee’ form of M q does not need to ‘bow down’ to heath-level to get fire to climb into its crown. It uses papery bark instead.
In tropical Australia, where eucalypts do not generally produce crown fires, it is possible that swamp forests of Melaleucas are the only vegetation sustaining crown fires. Even at the Sunshine Coast, which is subtropical, it seems that forest of M q carries crown fires farther north than they would occur in eucalypt forests, which tend to have understoreys of rainforest shrubs in this area.
So, M q ‘specialises’ in epicormic shooting after fire, i.e. vegetative regeneration from the bole. This fact might not have emerged clearly were it not for the scrutiny to which the species has been put in Florida, where it is a serious invader.
Despite relying on vegetative regeneration after fire, M q is as serotinous as most eucalypts, and in some sense one of the most serotinous plants on Earth. The seed-capsules, despite being relatively small, remain closed indefinitely to store the seed in the canopy of the forest rather than in the soil. The capsules so remain closed, as long as they are attached to a living stem, that it is more likely that they will open by virtue of the death and shedding of the stem to which they are attached, than by virtue of their own ‘decision’ to open.
What this means, in part, is that the capsules of M q, like those of many other spp. of Myrtaceae, remain ‘alive’ for years. Contrast this with the typical pattern in which fruit-capsules dry out and dehisce, shedding the seeds. I.e. serotiny is possible partly because serotinous ‘fruits’ (capsules) remain ‘green’ and greatly delay (to the point of the indefinite) ‘ripening’ as it were.
In many serotinous plants, this ‘canopy-storage’ of seed is combined with a tendency for the plants to die after fire (including crown-fire), the seed being released on to ash beds to result in germinative regeneration. In the case of M q, there does not seem to be a reliance on germinative regeneration because the adaptations for vegetative regeneration (epicormic buds plus paper bark) are so extreme. I suspect that in M q a main reason for the extreme serotiny is episodic germinative regeneration after unusual flooding or drying of the substrate.
Whatever the reason, what emerges is that M q is unusual globally among plants in combining extreme vegetative regeneration from boles with extreme serotiny. What needs to be further thought-through is how the extreme bark (papery) relates to this overall strategy in Melaleucas (about half a dozen spp. of forest-forming paperbark trees in various parts of Australia, New Guinea, and New Caledonia, interestingly absent from New Zealand).
As an aside, there is an odd fact about paperbarks on the Sunshine Coast, as follows. Despite the extreme abundance of M q as a native plant in this area, and the desirability of paperbark as street trees, it is not, in fact, M q which is the paperbark commonly planted as a street tree on the Sunshine Coast. There are plenty of paperbark trees planted as street trees on the Sunshine Coast, whole avenues of them in some cases, but M q is not the species used! Instead the favoured species is a tropical species not indigenous to the Sunshine Coast, namely Melaleuca leucadendra from northern Australia! This species is as tall as M q but differs in three main ways. Firstly, it tends to have several main branches from about human-height upwards, rather than a single tall bole. Secondly, it has pendulous foliage, resembling that of a ‘weeping’ eucalypt – and even the leaves have some of the asymmetry associated with eucalypt leaves. And thirdly, M. leucadendra on the Sunshine Coast during my visit is generally not in flower, this being different from M q, which is indeed generally in flower now.
What this means, in part, is that just as M q has proven remarkably adaptable as a street tree, thriving in Perth in a mediterranean-type climate foreign to it, so M. leucadendra has proven adaptable to climates cooler than those to which it is native. Perth has recruited M q from the subtropics, to its temperate climate. And the Sunshine Coast has recruited M. leucadendra from the tropics, to its subtropical climate. M q differs from its congeners, native to the Perth area, in having broad leaves instead of the ‘ericoid’ leaves of the Western Australian paperbarks. And, in turn, M. leucadendra has leaves which are not only broad but also ‘eucalypt-like’, instead of the simply broad leaves of M q.
And, by the way, the papery bark of all these spp. of Melaleucas seems similar throughout. The foliage-form of all these spp. of Melaleucas, namely the ‘paperbark trees’, differs greatly from ericoid through broad-leafed to eucalypt-like, but the bark seems to stay the same.
Coming back to M q:
Another noteworthy aspect of M q is that it is extremely ‘floriferous’. Its blooms are produced remarkably frequently and provide nectar for not only various relatively large birds but also fruit-bats. The forests of M q on the Sunshine Coast are all, at the moment, in what seems like full flower, and I get the impression that this is more the rule than the exception. And even in the ‘mallee’ form (stunted) of M q as seen by me in Lake Currimundi Conservation Park, the plants are currently in full flower.
Question:
Is it true even in the case of the street trees in Perth that M q tends to be in flower for prolonged periods?
The wood of M q has density of 0.7-0.75, which is unremarkable for an Australian species of Myrtaceae.
So, to summarise so far, M q is extreme in at least the following ways, in no particular order.
It is extremely floriferous in the sense of frequency of flowering and sizes of pollinators/nectar-robbers. M q is a major supplier of food for nectar-eating birds, as well as large bats.
It has extreme bark by any standards, but similar bark is shared with various congeners, all of which presumably benefit from occasional crown fires.
It is extremely serotinous (although not extremely bradysporous), to the extent that the only mechanism whereby the capsules open is via the death of the branch (twig) to which they are attached. I.e. they are ‘incapable of ripening’ in the sense that ‘normal’ dry fruits (dehiscent capsules of many taxa of plants) ripen and dry out with apoptosis of the tissues of the capsule wall.
It is extremely flammable, partly owing to oil content in the foliage, partly owing to the papery texture of the bark, and partly owing to the lignin content of the leaves.
Task:
I have yet to crush the leaves and smell them, to assess how ‘oily’ these leaves are compared with eucalypts.
M q has extreme adaptations for vegetative regeneration after fire, treating fire as the main consumer of its foliage (as opposed to consumption by animals, despite the facts that the koala does sometimes eat the shoots of M q and the sap is sucked by psyllid hemipterans).
An idea of the extreme nature of M q can be gained by a general comparison with North American trees. Serotiny is present in North American trees (conifers), but a) it is associated with large conifer-cones, and b) it is seldom combined with vegetative regeneration because North American crown fires usually kill serotinous conifers, leading to germinative regeneration. The conifer typical of swamps at a latitude similar to the Sunshine Coast, i.e. Taxodium distichum, is far from being flammable, being instead non-sclerophyllous and indeed winter-deciduous, and restricted to areas effectively lacking fires; and T. distichum has no flowers and is non-serotinous, its cones naturally ‘dehiscing’ promptly on maturity.
The following show forest of Melaleuca quinquenervia on the Sunshine Coast:
(writing in progress)
תגובות
הוספת תגובה