Accommodation boom – in our River Red Gums

The number of nights spent by human visitors to Alice Springs has slumped to well below 2001 levels, but local student of nature ALEX NELSON, over decades, has tracked a multitude of creatures who enjoy The Centre, living and breeding in our majestic River Red Gums.

Photo above: Even with termite soil in tree hollows they can still be used by birds, such as by this Budgerigar (left) and a Red-tailed Black cockatoo.

Extensive dieback of River Red Gums in the Todd River became evident from the early 1970s and continued well into the 1990s. Why?

Generally, it was attributed to a higher water table in the Todd aquifer due to the run of wet years in the 1970s and increased irrigation of parks and gardens for a rapidly expanding town. This also led to increased salinity of groundwater.

However, botanist Bruce Thompson observed to me in the mid 80s that River Red Gums exhibited dieback in all the major river and creek systems across Central Australia.

These photos, taken between 2009 and 2013, show roots growing in termite soil in both standing and fallen trees.

Time passed, the gum trees gradually recovered, and concern diminished over their impending demise from dieback.

River Red Gums are widespread, common and largely taken for granted. Paradoxically, understanding their long-term ecology is somewhat overlooked.

The beginning of my deeper appreciation of these trees reflects that murky background. Casual exposure over many years led to an accumulation of numerous observations but, like a jumble of jigsaw pieces, they didn't fit any coherent pattern, and at the time I wasn't looking for one.

Serendipity intervened, especially after I commenced volunteering from mid 2000 to clear Buffel Grass from the west boundary of Olive Pink Botanic Garden that is adjacent to the Todd River.

Due to frequent close proximity to the row of River Red Gums inside the fenceline, I took closer note of what I saw – termite galleries on the trunks, varying levels of dieback in each tree, and occasional old, woody sporocarps (fungal fruiting bodies) on a few branches.

Still no pattern came to mind.

In June 2004 my attention was caught by the sudden defoliation of one of the multi-stemmed trunks of a River Red Gum at the rear of the OPBG visitor centre. Examination revealed two spectacular fungal sporocarps burst through the affected trunk about 2.5 metres above ground. They were the apparent cause of the entire stem of that tree going into shock.

However, the other multi-stemmed trunks of that tree were not affected.

A year later, the affected tree was recovering with a new canopy, and the sporocarps remained as dead, woody protuberances on the side of the trunk.

In 2009, I found that termites had emerged from within the tree to consume the old sporocarps.

This confirmed a pattern I had begun to discern from other observations of River Red Gums in the Todd River. For example, in 2007 a large fallen limb beside the South Terrace bicycle path had two protruding branches trimmed back for public safety.

ABOVE: Termite activity forming a tree hollow. Idracowra Station Homestead on the Finke River, July 2018. Four years later in July 2022, the termite soil has spilled to reveal a large tree hollow.

The lower branch exhibited termite occupation, but the upper branch retained its heartwood but obviously softened from dry rot.

What appeared to be happening was that every River Red Gum hosted dry rot fungi within the dead heartwood of the tree trunks and branches. The softened heartwood in turn attracted termites that consumed the dead timber and filled the cavities with soil and frass (insect excreta and refuse) – in effect, each tree becomes a living termite mound.

But the magic doesn't stop there!

As the saying goes, it's an ill wind that blows no good; and on the spring equinox of September 2008 a tremendous windstorm (dubbed "Hurricane Alice") struck the town, causing considerable damage to thousands of trees including River Red Gums in the Todd River.

Some large, snapped limbs neatly revealed the internal pattern of live timber encapsulating dry-rotted heartwood, in turn hosting termites in the middle.

Other branches revealed inner cores completely filled with termite soil with tree roots growing through it. Some broken branches were festooned with internal roots dangling in the air several metres above ground.

I had observed this just once before, at Laura Creek 30km west of town in 1998, but "Hurricane Alice" revealed this is a common occurrence that demonstrated River Red Gums readily grow their roots upwards, internally through their trunks and branches, capitalising on the nutrient-enriched soil and frass deposited by termites.

Some old trees exhibit internal roots of considerable girth and size, usually revealed after a tree collapses from its own weight or is damaged by fire.

Mature River Red Gums have long been noted as vital habitats for a wide range of animal species, providing food, shelter and nesting spaces.

A classic poster by Peter King in 1988, for example, depicted a River Red Gum as "Nature's Boarding House", a theme later adopted in public signage as "Hotel Trees".

AccoHowever, the soil accumulated by termites within the River Red Gums enable other plant species to exploit these trees too, in effect becoming epiphytes.

These photos show roots growing in termite soil in both standing and fallen trees.

A variety of plant species have been noted, including Wild Passionfruit, Spiny Saltbush and even a Bean Tree! Most common, however, are Ruby Saltbushes. These species grew from seeds deposited by birds, with other plant species that rely on wind or insects to spread their seeds much less frequently observed.

Eventually the termite-accumulated soil collapses, giving way to hollows that provide essential habitat for many animal species, not least nesting hollows for parrots and cockatoos.

The rapidity at which tree hollows appear after termite nests emerge on trunks or branches is surprisingly quick. For example, in July 2018 a tall River Red Gum near the Idracowra Station homestead sported a large plug of termite mound several metres above ground.

Exactly four years later, this soil had given way to a large hollow, probably ideal as a new nest site for large cockatoos. Roughly the same time span has been observed with a tree in the Todd River.

However, taking into account the length of time it takes for River Red Gums to grow and mature, the total length of time for hollows to appear would be roughly in the vicinity of a minimum of 50 years and generally longer.

Typically, bracket fungi on the trunks or large branches cause limited necrosis where they emerge but little ill effect to the host trees; however, the sporocarps on upper branches usually lead to abortion of the affected stems, causing dieback.

It should be noted that dieback of tree branches is widespread, and most don't exhibit visible sporocarps – it appears the hyphae of dry rot fungi has the most impact on tree branches.

Sporocarps have been observed on River Red Gums across a wide area of Central Australia so it is clearly a common feature of these trees.

There appears to be a fine balance between the dry rot fungi and its host River Red Gum, the pathology see-sawing between contagion and strong vigour of the trees, depending much on prevailing climatic conditions.

The wet years of the 1970s, for example, appeared to favour stimulation of the fungi at the expense of its host trees, generating the extensive dieback of River Red Gums that caused so much alarm.

However, the tables eventually turned, and most trees recovered with vigorous new growth emerging from previously dormant epicormic buds on the trunk or limbs to become new branches.

The focus of this article has been on River Red Gums in the local Central Australian area, principally Alice Springs, but it's likely to be relevant over a much wider area and greater range of tree species (such as Coolibahs) that also form hollows in trunks and branches.

There is considerable scope for further exploration of this subject that is of fundamental importance to the natural environment.