Koalas don’t only need eucalyptus feed trees, they use a broad range of trees in the Australian Bush, and one, the Cherry Ballart, is particularly important.
The Native Cherry, or Cherry Ballart Exocarpos cupressiformis is a small native Australian tree that lives throughout eastern Australia, in mostly the same areas that koalas live*. The genus has representatives throughout Southeast Asia, the Pacific Islands, New Zealand and Australia. They are part of the Sandalwood family Santalaceae, that also includes some mistletoes.
Not much scientific research has been done on this tree. But it is known to be a hemiparasite, a plant that takes part of it’s nutrition from another plant. It seems to do this mostly when it is small, on the roots of a eucalyptus tree. When it is large it’s own green stems photosynthesise to produce nutrition. It seems that they also ‘give-back’ by returning nutrients and minerals to the soil below their canopy. Read more here: https://theconversation.com/native-cherries-are-a-bit-mysterious-and-possibly-inside-out-108760
I’ve always like them. Their fruit is tasty, they are pretty and different, and handy as marker trees, and a lot of wildlife can be seen around them. The soil underneath a cherry is dark and rich, and supports a ground layer of fruit-bearing native saltbushes, sweet young grasses and shade-lovers like kidney weed. Often a scrape in the soil on the south side will show where an Eastern Grey Kangaroo or Swamp Wallaby rests. Common Bronzewings are often seen under them, Common Ringtail Possums build their dreys in their branches, and a host of insects will be seen clinging to their trunks on hot days.
But my interest really grew when I realised that koalas like them, especially on hot days.
Koalas don’t roost in Cherry Ballarts a lot. But they use them consistently, usually in January, especially in hot, dry years. From 2016 to 2019, in the You Yangs in southern Victoria, we recorded koalas using Cherry Ballarts 80 times.
Over 4 years, an average of 1.6% of koala observations were in a Cherry Ballart. But the rate per year varied: in 2016 it was 1.3%, 2017 was 1.4%, 2018 was 1.1% and 2019 was 2.4%. 2019 was at the height of the 2017 to 2019 drought and Australia’s warmest year on record. Read about the weather profile of each year at the end of this article.
See all our records over this period here or below: Link to data table.
Why do koalas choose to sit in Cherry Ballarts?
Heat seems to be a factor, but there is some nuance to the choice. 37 of the 80 observations of koalas using Cherry Ballarts were on days with a maximum temperature over 30 degrees.
Another 12 records were on days when the temperature exceeded 30 C the day before. Another 2, when the temperature was over 30 C the day after. Yet another 2 records were on days the temperature exceeded 30 C on both the day before and after. Added together, that’s 53 of 80 observations that occurred in a spell of high temperatures.
But it is interesting that a few records occur in May and June. 3 of the 4 May records occurred in association with a high minimum temperature the day after. The single June record seemed to have no heat implications, and may have been random choice – I’m sure koalas sometimes climb any tree just to get off the ground.
I don’t know exactly why koalas choose Cherry Ballarts. Multiple studies have shown that koalas choose different trees in the daytime and nighttime (Crowther 2014, Ellis 2002, Melzer 2011), and daytime trees are primarily chosen for shelter, and this study (Crowther 2014) showed that as the temperature increased, koalas increasingly chose non-eucalypts over eucalypts. Certainly, Cherry Ballarts are shady, and this may be a factor. But there are other small, very shady trees in the area that don’t get used as much as cherries. Hakea laurina Pincushion Hakea is widely planted, similarly shady and was used 37 times over 4 years. Melaleuca armillaris Giant Honey Myrtle is shady and was used 29 times. Various large wattles: Acacia mearnsii, A dealbata, A implexa were used a total of 44 times.
Maybe cherries have particularly cool trunks. This study (Briscoe 2014) found that trees with cool trunks were preferred by koalas on hot days.
Is there a season for koalas using Cherry Ballarts?
Month/season could also be important. 30 (37.5%) of the records were in the month of January. The next most frequent use was 13 in February, and 10 in December.
That suggests to me that long hot spells have more of an impact on koala behaviour than single hot days.
Are some Cherry Ballarts preferred over others?
Koalas seem to have favourite Cherry Ballarts in their home ranges, and some Cherry Ballarts are popular with several koalas, over years and possibly generations. Large ballarts with a eucalypt growing right beside them, and large, multi-trunked ballarts, seem to be most popular.
CCB 1 – a big, thickly-foliaged Cherry Ballart with a River Red Gum growing through it – was used 18 times over 4 years. Mature male Clancy used it on 13 days in 2017 & 2018. Mature male Bundjalung used it 5 times in 2016. We have records of Bundjalung using this tree in 2015 too.
NCB 1 – another big, thickly-foliaged CB with a River Red Gum growing through it – was used on 6 days in 2019, by mature female Ngardang and mature male Gulkurguli. They shared this tree on one very hot day.
GCB 3 – a large solitary, multi-trunked cherry with no eucalypt neighbour -was used by mature male Gulkurguli on 3 days – twice in 2019 and once in 2018. The same tree was used once by Winberry in 2017.
PCB 2 – a multi-trunked cherry complex with no eucalypt neighbour, and a base surrounded by Fragrant Saltbush Rhagodia parabolica – was used by older female Pat on 4 days in 2019. On one of these days Pat was seen clinging to the base of the tree, completely covered by Rhagodia.
PCB 1 – a large solitary tree with no eucalyptus neighbour – as used 3 times by Pat twice in 2019, and Emma once in 2016.
Valley CB – a big thick cherry with a Blue Gum growing through it – was used three times by older female Fairy in 2016. This tree has a long history of use. It was very popular in the late Millenium Drought in 2007 to 2010. In 2007 it was used 21 times. On two occasions that year two koalas were using it on the same day.
Other cherries are used more rarely.
Do koalas learn to use cherry ballarts from their mother?
Maybe. Ngardang shared NCB 2 with her female joey Winjku in 2019. Winjku later returned to use that same cherry when she was independent, and was also seen using another, different Exocarpos cupressiformis. Two of Ngardang’s other joeys, male Bunyip and female Lakorra were also seen using native cherries as adults.
Do male koalas use Cherry Ballarts more than females?
Yes, it seems that male koalas are more likely to be seen in Cherry Ballarts than females. 50 out of 80 records are male koalas using Exocarpos cupressiformis. Furthermore, 40 of those 50 records were of high status males, ie long-term, territory-holding resident males with no rivals in their home range.
Cherry Ballarts as climate refugia
There’s a very serious side to this story. Cherry Ballarts are koala life-savers.
Two of the records discussed above were on a day over 45 C. Another day was 42.8, others on days of 41.8 & 41.6 and another was during a 3 day heatwave of 40-39-38. Koalas don’t live through days like that without making smart choices.
Heatwaves are known killers of koalas: a quarter of the Gunnedah koala population perished in late 2009 due to heatwaves near the end of the Millenium drought; and at the same time in Victoria, one-third of the You Yangs koalas died in late 2009/early 2010 for the same reason.
This study (Crowther 2014) says:
“Habitat selection at the scale of individual trees is the only recourse for a heat-stressed koala. Our findings show the importance of shelter trees in tree selection by koalas and identify that these shelter trees critically affect their capacity to withstand climate change, particularly in hot years.”
If we are serious about providing climate refugia for this threatened species, it will have to include Cherry Ballarts.
There could be another advantage of Cherry Ballarts.
In January 2006 a bushfire roared through a population of koalas in the Brisbane Ranges NP. I was one of the last to leave, and I noted male koala “Bear” in a big Cherry Ballart. I mentioned this to the fire services as they went past.
When I returned a few days later, Bear was one of just three, out of a population of 30, that had survived the blaze. The ground under the Cherry Ballart was barely burnt, and the tree still had green foliage.
Conclusions and recommendations
I don’t know exactly what it is about Cherry Ballarts that is so important to koalas, but it is clear that koalas need them to survive the heatwaves and droughts that are coming with climate change. Possibly the fires too. We can’t propogate them in a nursery – they have defied all efforts at propogation so far. So we must preserve and protect them where they naturally occur.
Cherry Ballarts are sending us a message. We can revegetate, but we can’t restore lost ecosystems. Only nature can do that. Even with great advances in science, we still don’t have the knowledge to restore every plant and organism. Better then to protect the forests we have, every single one.
- note: Cherry Ballarts are also native to Tasmania, where koalas do not live.
Weather profile by year
2016
In 2016 we made 1267 observations of koalas, and 16 of those were in Exocarpos cupressiformis (1.3%). 2016 was Australia’s 4th warmest year on record, with mean temperatures in our region Very Much Above Average. Rainfall in our area was Average. January, March & December maximum temperatures in our area were Above Average, February, October & November were Average. Minimum temperatures were mostly Above Average or Very Much Above Average as well, except for October & November which were Average. http://www.bom.gov.au/climate/current/annual/aus/2016/
8 of the records were in January, 2 in February, 4 in March, 1 each in May, October and November.
2017
In 2017 we made 1116 observations of koalas, and 16 of those were in Exocarpos cupressiformis (1.4%). 2017 was Australia’s 3rd warmest year on record, with mean temperatures in our region Very Much Above Average. Rainfall in our area was Average. January & December maximum temperatures in our area were Above Average, February was Average, March, October & November were Very Much Above Average. Minimum temperatures were mostly Very Much Above Average as well. http://www.bom.gov.au/climate/current/annual/aus/2017/
5 of the records were in January, 5 in February, 1 I April, 3 in December.
2018
In 2018 we made 1294 observations of koalas, and 15 of those were in Exocarpos cupressiformis (1.1%). 2018 was Australia’s 3rd warmest year on record, with mean temperatures in our region Very Much Above Average. Rainfall in our area was Below Average. We were officially in drought that would continue until Black Summer. January, October & December maximum temperatures in our area were Very Much Above Average, February, March & November were Above Average. Minimum temperatures were mostly Above Average, and the December minimums were Highest On Record. http://www.bom.gov.au/climate/current/annual/aus/2018/
5 of the records were in January, 4 in February, 2 in March, 3 in May and 1 in November.
2019
In 2019 we made 1423 observations, and 34 of those were in Exocarpos cupressiformis (2.4%). 2019 was the warmest year on record, with mean temperatures in our region Very Much Above Average. It was also the driest year on record, with annual rainfall in our area Below Average. In January maximum temperatures in our area were the highest on record, Above Average for February, Very Much Above Average for March, October and December. 12 of the CB records were in January, 7 in November and 7 in December of that year. It is interesting that November was average for both max and mean temperature and rainfall, and below average for min temperature. http://www.bom.gov.au/climate/current/annual/aus/2019/
12 of the records in this year were in January, 2 each in February, March & April, 1 each in June & October, 7 each in November & December.
Notes & references
Briscoe, N.J., Handasyde, K.A., Griffiths, S.R., Porter, W.P., Krockenberger, A. and Kearney, M.R., 2014. Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals. Biology letters, 10(6), p.20140235. https://doi.org/10.1098/rsbl.2014.0235
Crowther, M.S., Lunney, D., Lemon, J., Stalenberg, E., Wheeler, R., Madani, G., Ross, K.A. and Ellis, M., 2014. Climate‐mediated habitat selection in an arboreal folivore. Ecography, 37(4), pp.336-343. https://doi.org/10.1111/j.1600-0587.2013.00413.x
Ellis, W.A.H., Melzer, A., Carrick, F.N. and Hasegawa, M., 2002. Tree use, diet and home range of the koala (Phascolarctos cinereus) at Blair Athol, central Queensland. Wildlife Research, 29(3), pp.303-311. https://doi.org/10.1071/WR00111
Melzer, A., Baudry, C., Kadiri, M. and Ellis, W., 2011. Tree use, feeding activity and diet of koalas on St Bees Island, Queensland. Australian Zoologist, 35(3), pp.870-875. https://doi.org/10.7882/AZ.2011.040
