The eucalypts are a strange group of species when compared with other trees around the world. This can be partially attributed to one feature of their leaf biology. You have probably noticed it before, especially if you grew up in Australia. You don’t have to go far to find it even if you’re in the concrete jungle. So I took a walk around Melbourne’s Royal Botanic Gardens to find an example (a twenty-minute walk from the Melbourne CBD).
The leaves in the images above are both from the same tree, a type of silver stringybark native to south-eastern Australia. Fully-grown trees of this species produce the long, hanging leaves that we would typically imagine when thinking of eucalypts. But when new trees grow, or when new shoots sprout from the base of an old tree, they grow as round leaves. This results in a species that grows different types of leaves depending on whether those leaves are on young or old shoots.
The shoot in the image above is from a specimen I found growing in the university car park. The tree was growing new shoots from the trunk base (possibly triggered by some damage at the top of the tree). When this shoot first began to grow, it produced small, round leaves, which you can see at the start of the shoot. Then, when it grew to a certain length, like a teenager experiencing a bunch of morphological changes, the shoot began to transition to the long leaves until any new leaves were completely adult in form. The round leaves at the base eventually die and fall off the shoot like all leaves do, which is why you’ll not see them on many fully-grown eucalypt trees.
Four species around the world that exhibit heteroblasty. Each herbarium sheet shows the juvenile (left) and adult (right) foliage of a species. (Clockwise from top left: Eucalyptus globulus (Australia); Guzmania lingulate (the Americas); Pseudopanax crassifolius (New Zealand); Tarenna borbonica (Mascarene Islands).) Images: Muséum national d’histoire naturelle
The phenomenon that involves this transition from one leaf type to another is called heteroblasty. Only a handful of other plants across the world also undergo this leaf change as they grow. They do it in different ways and for different reasons. Despite the rarity of this phenomenon, it occurs in hundreds of eucalypt species. This is another characteristic that makes our eucalypts so unique. But why is it so common in eucalypts compared with the rest of the plant kingdom? This is a question I’m trying to answer in my research.
But it can get even more complicated than that. Different eucalypt species also do it differently. Usually round leaves transition to long leaves, but often there is also a combination of other changes, such as changes in leaf thickness, leaf waxiness, leaf vein patterns, and arrangement of leaves around the stem. In some species, trees never produce round leaves at all. For example, the Manna Gum (Eucalyptus viminalis) from south-eastern Australia always produces long leaves, even as a seedling.
In other species, trees never produce long leaves, even as fully-grown trees. They always produce the small round leaves, making them neotenous, like the axolotls of the eucalypt group, because they never transition beyond the juvenile form.
To add further complication, in some species young trees produce thin, linear leaves, and older trees produce leaves that are slightly wider (as seen in Eucalyptus rosacea of Western Australia).
These different ways of producing different leaves at different times are likely to reflect different adaptations by different eucalypt species to their respective environments. Why does a tree of one eucalypt species produce long leaves its entire life, while that of another species undergoes changes from round leaves to long leaves?
This is another question for which we don’t have an answer. So the next time you’re having a wander in the bush and spot a eucalypt, try to find some differences between the young and old growth. What kind of environment is the tree growing in? Why does it need different leaves? Or, perhaps, why does it not?
Banner image courtesy of Carolyn Vlasveld
Eucalypt leaf specimens are from the online herbarium from the Museum D’histoire Naturelle