When sheoaks take over

I recently had an opportunity to go for a few hours’ wander on the Queen’s Domain in Hobart, a huge park covering a hill near the city, which still contains some great native vegetation. Having often walked and played there as a kid in the 1980s I have a little historical perspective on the place, and the recent domination of former grassy woodlands by thick stands of drooping sheoak (Allocasuarina verticillata, previously known as Casuarina stricta) saplings really stood out.

I recall a family outing on the Queens Birthday firecracker night, for which we walked up onto the western side (above the Brooker Highway) and let off our fireworks while enjoying the view across the suburbs where everyone else was doing likewise. We spent the evening sitting in the grass in an open woodland environment with some trees and shrubs around. It was easy to walk through. Some areas on the north side still look a bit like the west side did in the mid 1980s.

2016-07-20 11.21.03
Looking west across the north slope of the Queens Domain, Kunanyi/Mt Wellington in the distance. This is how I remember a lot of the hillside being in the 1980s.

The area I remember sitting in on the west is now covered with young sheoak regrowth, “so thick a dog couldn’t bark in it” as they say. Underneath, there are a few Lomandra (aka Mat-rush or irongrass) tussocks, occasional cranberry heath (Astroloma humifusum) clumps and a few herbs, but mostly just the discarded “needles” of the sheoaks. Their thick canopy doesn’t let enough light through for much else to  grow.

2016-07-20 11.08.26
Dense regrowth of drooping sheoak on Queens Domain western face

There were always sheoaks on the domain. They are a beautiful tree, and the old specimens especially so. Their needles sigh peacefully as the winds pass through them. The relatively open areas on the north face of the Domain have much less of the thick regrowth, and retain good patches of the threatened Kangaroo Grass (Themeda triandra/previously T. australis) grassland/grassy woodland ecological community (with veteran sheoaks scattered throughout). These areas must have been burned often enough to keep the understorey open for the grasses. I found one patch of decimated sheoak saplings that had been burned recently with grass recolonising the ground beneath. Little winter-growing herbs that pop up between grass tussocks were beginning to show. Sheep-burr (Acaena echinata) was particularly prominent.

Sheoaks, which come in many species, were probably more widespread across southeastern Australia before the European invasion. Their great utility for firewood and timber did not work in their favour: useful trees do not last long! Birds like the black cockatoos that feed on their seed have not done well either, and the various sheoak species are uncommon in many areas where one suspects they were once widespread.

Given a chance, though, the thick growth of sheoak saplings, as seen on the Hobart Domain and many other woodlands, can crowd out other plants. Eucalypts are taller, but if their seed can’t germinate and grow, eventually Eucalypt woodland may turn into sheoak scrub. Maybe a lack of native herbivores and digging mammals also favours the thick sheoak regrowth.

Jamie Kirkpatrick published a paper in 1986 detailing a ten year ecological study of the Domain finding that the fires in the later part of his study seemed to have promoted greater diversity of species in the grassy areas and kept the woodlands open by suppressing seedlings of Eucalypts and sheoaks developing into saplings. He also noted that mature sheoaks

“are not readily damaged by fire. Their photosynthetic organs have the least ability to propagate fire of any of the dry forest species tested by Dickinson and Kirkpatrick (1985). Eucalypts mixed in with dense thickets of C. stricta experience high mortality rates during droughts (Kirkpatrick and Marks 1985)…

“The dominance of C. stricta can be the result of at least two alternative fire histories. Where there is long-term lack of fire C. stricta may gain dominance (Withers 1979). C. stricta is also favoured by low-intensity fires. Such fires usually set small eucalypt saplings back to ground level, because of the high flammability of their tissues (Dickinson and Kirkpatrick 1985), whereas the growing tips of the less flammable C. stricta saplings often survive. C. stricta seedlings also establish more easily than eucalypt seedlings in competition with perennial grasses.”

Ben Zeeman, Ian Lunt and John Morgan (2014) discussed a long term study of a coastal woodland undergoing encroachment by sheoaks at Ocean Grove in Victoria, where fire had been excluded for a long time. They reported in their paper that the highly drought-resistant sheoaks continued to encroach (at the expense of the Eucalypts) even in the long millenium drought (1997-2009).

The Mouheneenner Indigenous people who lived around the area of the Queens Domain probably used fire in some manner to keep the woodlands open enough to gather plants for food and to hunt. Native animals (especially wallaby, bettong, bandicoot) are now absent* from the domain (I didn’t even see a lot of evidence of rabbits) and they also would have reduced the growth of young saplings as well as other shrubby plants by eating or uprooting them. Their presence was probably another crucial, but now missing, element of keeping woodlands open (sheoak seedlings are a favourite for little nibblers). Modern land managers only have easy access to fire as a tool in most of the country (like here in Victoria) where bettongs and bandicoots are locally extinct, and wallabies becoming more scarce.

But what a tree! Sheoaks are drought-hardy yet fire-resistant, attractive to look at and to listen to. They provide good shade, but don’t grow too high (A. verticillata typically in the 5-10m range). Like  wattles, they host a symbiotic bacteria on their roots (Frankia sp.) that fixes nitrogen from the air, enabling them to grow in poor sandy soil. Mysteries of ecological or vegetational balance aside, sheoaks should be more widely planted!

* Edit (9/9/2016): I’ve been told by a Bushcare coordinator with Hobart City Council that I am incorrect: “Wildlife monitoring over the last few years, including at the BioBlitz last weekend, shows that native animals are still prevalent at the domain including eastern barred bandicoots, little forest bats, southern brown bandicoots, potoroos, ringtail possums and pademelons.
A full list of the species discovered at the bioblitz is available here.  Which is a welcome correction to my vague and unreferenced “native animals are now absent”. It appears there is some quite special native wildlife present, including a couple of sightings of painted button-quail.

References

Kirkpatrick, J. (1986), The Viability of Bush in Cities – Ten Years of Change in an Urban Grassy Woodland. Aust. J. Bot., 34, 691-708

Zeeman, B. J., Lunt, I. D., Morgan, J. W. (2014), Can severe drought reverse woody plant encroachment in a temperate Australian woodland?. Journal of Vegetation Science, 25, 928–936. doi: 10.1111/jvs.12153

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One thought on “When sheoaks take over

  1. Great to see a post about sheoak grassy woodlands! Thanks Ben. They once covered vast areas of South Australia too, but are all but gone now, thanks to the effects of clearing, sheep, roos and rabbits (and the extermination of the dingo from the agricultural zone). But they would have been so important to the native biota of this area. E.g. It’s no coincidence that glossy black cockies are all but extinct over most of SA. I touched on some of the efforts to restore them to the landscape in this story about the bush stone curlew http://www.paperbarkwriter.com/rewilding-weeloo-the-enigmatic-bush-stone-curlew/
    Cheers, Paula

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