Why Eucalyptus erythrocorys tends to self-amputate in cultivation

Eucalyptus erythrocorys (https://apps.lucidcentral.org/euclid/text/entities/eucalyptus_erythrocorys.htm and https://twitter.com/eucalyptaus/status/1168440927628795904 and https://www.ecovoice.com.au/the-illyarrie-wins-eucalypt-of-the-year-2020/ and http://anpsa.org.au/e-ery.html and https://alchetron.com/Eucalyptus-erythrocorys) has large blooms: bright yellow and with exotic-looking red opercula (https://www.inaturalist.org/observations/70992084).

This makes for a cheerful - even spectacular - appearance as summer becomes autumn in the mediterranean-type climate.

So it is unsurprising that this species has become a horticultural favourite as a large shrub or small tree (https://www.bgpa.wa.gov.au/about-us/information/our-plants/plants-in-focus/eucalyptus-erythrocorys). It is planted in gardens and along streets in Australia (particularly in the west) and in the similar climate of southern California.

However, cultivating the species in this way (https://cals.arizona.edu/yuma/plant_index/eucalyptus_erythrocorys.htm and https://www.eranurseries.com.au/eucalyptus-erythrocorys) brings the practical disadvantage that the several boles of each individual plant tend to grow at angles, not upright.

As a result of this leaning tendency, top-heaviness often results in partial collapse. One of the boles suddenly breaks, felling that part of the crown and its growing foliage in what looks like spontaneous auto-amputation.

Now, everyone knows that various species of eucalypts, which grow naturally as large trees, have a disconcerting habit of suddenly dropping large branches (https://treesafe.com.au/blog/tree-removal/eucalyptus-trees-dangers/ and https://sydneytreeremovals.com.au/tree-facts/widow-maker-gum-trees-clear-deadwood/). It is understandable that tall trees would benefit from getting rid of redundant lower branches and dead sticks, but what is noteworthy about eucalypts is that the branches are usually shed with the foliage still growing.

The fact that various species of eucalypts spontaneously - and dangerously - jettison branches in a state of apparent vitality has led to the term 'self-pruning' (https://en.wikipedia.org/wiki/Cladoptosis).

Seen in this context, the proneness of cultivated E. erythrocorys to partial collapse seems incongruous. This is because, on this relatively small plant, such a large proportion of the stem system is 'shed' that the action resembles not self-pruning as much as an unsuccessful attempt at suicide.

Curious about this apparently maladaptive behaviour, I visited the plant in its natural habitat near the coast south of Dongara in southwestern Western Australia (https://apps.lucidcentral.org/euclid/text/entities/eucalyptus_erythrocorys.htm and https://en.wikipedia.org/wiki/Dongara,_Western_Australia and https://twitter.com/RichardMcLellan/status/1168669607059570689/photo/1). Here E. erythrocorys is locally dominant on ridges of coastal limestone, with a heathy understorey (see photo no. 12 in http://ianfrasertalkingnaturally.blogspot.com/2014/01/i-love-sunburnt-country.html).

What I noticed immediately is that, in its natural state, E. erythrocorys is more like a mallee (https://en.wikipedia.org/wiki/Mallee_(habit)) than it appears to be in the suburbs or in photos singling out the more statuesque specimens in the wild (e.g. https://www.inaturalist.org/observations/42711339). In nature, each individual usually has more than six stems emerging at ground-level even if the lignotuber, so typical of mallees, is not particularly well-developed.

This brought me to the realization that the growth-form encouraged in cultivation is different from the wild, multi-stemmed one. Gardeners, ignorant of the natural shape of the plant, understandably tend to cull the stems to a few which resemble boles of a (small) tree.

Closer examination distinguished E. erythrocorys from all the other species of mallees with which I am familiar in the wild. The stems, although initially growing upwards as expected, sprawl down to the become prostrate in their distal sections. The result is a growth-form similar to mallee but with much of the outer foliage trailing along the ground.

Why does E. erythrocorys differ from other multi-stemmed, wildfire-tolerant eucalypts in having this oddly recurved growth-form?

One possible reason is its tenuous relationship with fire in a type of vegetation which is not easily classified as either typical mallee (https://www.anbg.gov.au/photo/vegetation/mallee-woodlands-shrublands.html) or typical kwongan (https://en.wikipedia.org/wiki/Kwongan).

Most species of mallee are not only tolerant of wildfire, but intimately adapted to a regime of periodic combustion in which all their stems die by scorching, and growth resumes from ground level. Usually the vegetation is dense enough that the flames engulf all including the crowns of the mallees. However, eucalypts tend to exclude flammable shrubs from the patch directly under the crown, which means that, if the stand is sparse, there is a chance that the fire will fail to ignite the foliage of the upper storey, simply burning through the heathy understorey and leaving the eucalypts partly unburnt.

In the case of E. erythrocorys, the vegetation is indeed relatively sparse, partly because the surface is so stony. The heathy understorey looks barely dense enough to carry the flames across the empty patches under the mallees.

What E. erythrocorys seems to arrange, by having its outermost foliage trailing back down to the ground, is a ladder whereby the flames can climb up into the crown. This would ensure the desired combustion of the foliage, which as in other mallees rejuvenates the plant and self-fertilises it with its own ash.

The problem in cultivation is that no gardener or arborist, private or municipal, wants an untidy sprawl-mallee. So the natural shape of the plant is modified in the sapling. However, this cannot correct the leaning which is 'hardwired' in the remaining few boles. This inclination of the foliage towards the ground, now made dysfunctional, results sooner or later in partial collapse.

In summary, my finding is that the auto-amputation of the stem-system of E. erythrocorys is not a case of the sort of self-pruning so well-known in tree eucalypts. Instead it is a result of artificial distortion of the natural shape of the plant.

Horticulturally desirable though this species is, its natural adaptations are such that growing it as a tree is unlikely to be achieved without selective breeding.

Posted on October 14, 2021 10:25 PM by milewski milewski


Very interesting! That makes sense.

Posted by jon_sullivan over 2 years ago

Thank you @jon_sullivan

Posted by milewski over 2 years ago

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