Hyper-emergence in eucalypts

Tropical savanna in the Northern Territory
(Photo: John Morgan)
 

Sub-tropical rainforest, Dorrigo National Park
(Photo: John Morgan)

Q: What's the most obvious difference between a rainforest you'd find in the tropics and a eucalypt forest you might find in temperate areas?

The difference between these two is so obvious, most of us don't even notice it.

And it struck me recently that the explanations for it are also poorly understood.




Mountain Ash - Eucalyptus regnans
The tallest flowering tree on the planet.
(Photo: John Morgan, Wallaby Creek, 2006)

One of the most obvious differences is that Eucalyptus (and this would include some closely related genera such as Corymbia), are characteristically emergent trees of many vegetation types in Australia. Typically, they are considerably taller than all other species in these communities, often by an order of magnitude or two. As such, the trees tower over all the other plants. Rainforests, by contrast, tend not to have such obvious tiering or separation of the canopy and the mid-under storey, with lots of different species comprising the canopy. Emergent trees, when they do occur, do not tower over the rest of the plant community. This seems a really obvious distinction, but have you ever stopped and asked "why"?

A recent review sheds some light on why eucalypts grow so tall.

Tng et al. (2012) New Phytologist 196: 1001-1014, while focusing on the reasons for the evolution and occurrence of gigantic eucalypts (i.e. those species that grow >70 m tall, a fascinating topic for another post),  highlight that 'hyper-emergence' of eucalypts is a trait that extends across climates and clades and can be found in many Australian vegetation types such as heath, mallee, dry sclerophyll, subalpine and savannah communities. In some cases, some giant eucalypt trees are >60 m taller than the underlying canopy. The nearly ubiquitous nature of hyper-emergence in eucalypts suggests that this trait is an ancestral feature of the eucalypt lineages, and if this is true, would have arisen >60 million years ago!

In some respects, growing really tall above your competitors seems like a dumb idea. There's the risk of wind damage (or worse still, lightning strike!), tall trees need to sustain their own weight once bent, there's a lot of investment into woody structures that are diverted away from reproduction, and of course there is the not insubstantial problem of getting water up to great heights. So, why would trees - and eucalypts in particular - bother growing so tall? What are the benefits of far exceeding the heights of competitors?

Alpine Ash (E. delegatensis) are hyper-emergent
trees at high altitudes in the Australian Alps
(Photo: John Morgan, near Mt Hotham)

Tng et al. suggest a couple of reasons, but clearly this is an area where more research is needed.

First, tall trees tend to have very rapid early growth (relative to lots of other woody plants in the same community), allowing them to escape a 'fire trap' such that this growth would allow saplings (or resprouts) to reach heights that allow them to avoid the effects of high intensity ground fires. Such processes could apply to eucalypts in general (e.g. such as those of savannah and forest) but may be less applicable to eucalypts where fire return intervals are very long (decades to centuries). Indeed, in Mountain Ash forest, infrequent but high intensity canopy fires are actually necessary for stand replacement.

Second, most eucalypts are shade-intolerant. Intense intra- and inter-specific competition provides a strong selection pressure for tall growth, hence allowing them to overtop their neighbours. By overtopping slower growing, shade-tolerant trees, often quickly after disturbance, early reproductive maturity can be assured.

While both explanations undoubtedly play a role, they are not particularly satisfactory answers when asking the question "why grow so much taller than all the other plants in your community"? I think this is a fascinating topic that really needs a bit more thought. So next time you're in the bush, marvel at the eucalypts that undoubtedly overtop all else and ponder why this might be so.


Further reading
Tng et al. (2012) Giant eucalypts - globally unique fire-adapted rain-forest trees? New Phytologist 196: 1001-1014.
Larjavaara (2014) The world's tallest trees grow in thermally similar climates. New Phytologist 202: 344-349.





.