A wander through the tropical forests of North Queensland, and for that matter any tropical forest in the Southeast Asian and Pacific region, would be so much more richer an experience if one could recognize or at least tell between the multitude species of lily pillies and satinashes (Syzygium spp.) that inhabit these forests.
If eucalypts dominate the vast open woodland vegetation of Australia, Syzygium could be considered its rainforest counterpart, at least in terms of the number of species found in the tropics (100 spp.). Both eucalypts, Syzygiums and allied genera are from the large Myrtle family. So it turns out that Australia really specialises in Myrtles. As a group also, Syzygium must play an immensely importance role to the ecosystem of the rainforest by providing food. Most of the species of Syzygiums are probably edible (…most really are just edible).
The difficulty of identifying Syzygiums without fruit (or even with) is well known among botanists. Any botanist with a modicum of modesty would admit to countless nightmares of green-jobs with no other than opposite leaves. My utmost respect goes to Bernie Hyland who did the most comprehensive revision work on the Syzygiums of North Queensland.
But even without the eye of a fully fledged taxonomist we can still try to recognize at least a selection of the more common or distinctive species by leaf whispering (here taken to mean by just knowing how the leaves look like).
So here are a few features:
– The leaves usually have a thick or leathery texture
– the leaves of many species are a deep dark green
Having said this I must clarify that these feature apply for most species in the related Myrtaceae genera Backhousia, Eugenia, Decaspermum, Gossia, Psidium, Pilidiostigma, Rhodamnia, Rhodomyrtus, Uromyrtus, etc., but members of many of these genera have some subtle distinguishing features themselves which I will go into later in another post.
So here is an overview of some groupings of selected species with potentially easily recognizable leaf characteristics.
Building on the typical thick-leaf feature of Syzygium leaves, some species have a somewhat shiny underside, with the nerves not very obvious. Examples are S. papyraceum and S. johnsonii. In the case of S. kuranda, S. hemilampra and S. fibrosum, the leaves are not shiny underneath but the leaf blades are rather leathery.
Syzygium johnsonii with shiny leaf undersides. This species also has a rather strong distinctive “lolly” smell emitted when leaves are crushed
Some Syzygiums have twigs with angular cross-sections (especially near the newest leaves are borne) or twigs with flanges. Syzygium australe is a common example. Flanged twigs also occur on the often coastal growing S. angophoroides, which in addition has a habit of the leaves that turn red or orange before shedding. The character of having flanged twigs is strongly expressed in S. alatoramulum, S. apodophyllum and S. hedraiophyllum and S. trachyphloium.
Some species have small cute leaves with extended pointy tips. Examples are S. luehmanii, S. canicortex and S. dansiei. The related genus Decaspermum humile can look very similar but the leaves in this species has a raised midrib on the leaf upperside. Uromyrtus also looks similar, but often have hairs.
Many species of Syzygium can be distinguished by their venation, and how far one takes this will be a testimony to one’s “monomaniacism”. The common cultivated S. cormiflorum has a certain ladder-like vein pattern obvious on the leaf underside. This ladder-like venation feature can be useful for recognizing some Syzygiums in the field in a sterile state. Very similar to S. cormiflorum is S. bamagense, which is so similar that I had to stare for a very long time to tell the difference, even with both of them in my hands. After a while I realized that S. bamagense has longer petioles, and the veins don’t seem to form loops that are as complete.
S. sharoniae, a species more or less restricted to the Daintree region, has venation pattern also resemble S. cormiflorum, but has a somewhat slight silvery sheen, and smaller leaves. S. monospermum and S. erythrocarpum, probably best identified by their fruits borne on stems, have similar ladder-like patterns with S. cormiflorum. These two species, S. monospermum and S. erythrocarpum, often have have cordate leaf bases.
S. tierneyanum has a similar sort of vein pattern but has narrow leaves. Syzygium glenum is a very restricted species endemic to Cape Tribulation and so are probably not going to be seen by most but the species has a superficially similar venation pattern to S. tierneyanum.
On the feature of “longish” leaves, there are a number of species that look rather alike.
Some species clearly have two intramarginal veins. An example is Syzygium graveolens, which can be distinguished by the dark red-brown trunk. S. divaricatum has very similar leaves. Syzygium gustavoides has a very strong two intramarginal veins with a very distinctive pattern of venation between the inner and out intramarginal veins.
Syzygium divaricatum leaves. The photo is intentionally darkened to highlight the venation. Note the appearance of two intramarginal veins. This particular species also has rather leathery leaves and very thick leaf stalks.
Most Syzygiums have entire leaf margins (i.e. bearing no teeth), but when the “entire leaf margin” rule is broken, the species can be identified almost instantly. Syzygium sayeri is one such species that breaks the cardinal rule of the Syzygium alliance of being toothless.
Some species have wavy leaf margins, such as S. unipunctatum and S. claviflorum.
Syzygium claviflorum is a rather non-descript species, but the wavy leaf margins and rather closely spaced and almost parallel secondary venation seems to be a fairly good combination of features to help with it’s recognition.
“Leaf whispering” Syzygiums can be immensely fun (sometimes in an exasperating way), and the learning never ends. Enjoy.
Hyland BPM. 1983. A revision of Syzygium and allied genera (Myrtaceae) in Australia. Australian Journal of Botany., Supplementary series; Suppl. no. 9.