Cumbungi, dams and dementia

Cumbungi - might it be useful? Image used under Creative Commons licence.
Cumbungi – might it be useful? Image used under Creative Commons licence.

Cumbungi or bullrush is a weed that commonly infests Tasmanian dams.

It is considered a pest, but could it be beneficial in some circumstances?

There is a global trend for even historically clean waterbodies to turn green, the result of warmer temperatures, more sunlight, lower water levels and added nutrients.

The colour is caused by blue-green algae, which is surprisingly nasty stuff.

Blue-green algae contamination of drinking water and food has been linked to fatal motor neurone disease and dementia, the suggested cause being a combination of exposure to the neurotoxic amino acid BMAA, along with genetic susceptibility in some individuals.

Might cumbungi be useful?

Cumbungi and silt had built up in my dams so I had them dug out with an excavator.

When cumbungi was present the dam water was clear most of the time, and without cumbungi the water went bright green in summer.

Cumbungi, being thick and tall, shielded much of the water from sunlight, which presumably lessened algal growth.

I expect cumbungi also pulled nutrients from the water, such as those from washed-in animal poo.

There was also life in the cumbungi – frogs and birds liked it. They went away with the cumbungi removal.

Cumbungi is deemed a problem because it “clogs dams, reduces capacity and may pollute water when it dies off in autumn/winter”.

Given that many farm dams only need to retain enough water for stock to drink in summer, reduced capacity from cumbungi may not be an issue.

As a livestock farmer, water quality is more important to me than water quantity, as long as it doesn’t run out.

Cumbungi may use water, but it may also reduce wind and light-driven evaporation.

There is evidence that using blue-green algae contaminated water in agriculture may introduce toxic BMAA into the food supply, something to consider if you are using green water to grow vegetables and feed livestock.

So, given the risks of BMAA exposure, is summer cumbungi growth potentially useful?

I have not noticed water quality problems when the cumbungi dies off in autumn/winter, but that does not mean all dams may be unaffected.

Here’s a few links meanwhile …

Tasmania’s DPI cumbungi page
BMAA and Parkinson’s disease
BMAA in Australian waterbodies
How farms make BMAA

Late edit: BMAA from hot waterholes may be to blame in the deaths of dozens of elephants in Africa.

12 years of growth

She Oak Hill tree coverage 12 years apart ... images from Google Earth
She Oak Hill tree coverage 12 years apart … images from Google Earth

She Oak Hill is located on the upper settled part of the Huon Valley in Judbury.

These two satellite images are from the north face, which is the drier, warmer side of the hill.

The zoom level and location are roughly the same.

It appears the hill was cleared some time before 2005, perhaps by fire, or for farming – I do not know which.

She Oak Hill was presumably named after its Tasmanian casuarina cover.

Today, the she oak canopy is extremely thick.

Many of the large eucalypts that poke above the she oak “blanket” were damaged or killed in a fire some time around 2009/2010, and stand as skeletons.

Locals have told me that “back in the day” they would have burned such a dry hill before the undergrowth got so thick.

“Why not burn it now?” I ask.

Apparently organising such things is no longer as simple as it was.

The sheep like eating the casuarina, probably an important addition to their diet.

The wood burns well in a heater, although it seems wasteful to burn such an attractive, workable timber … see it here.

The wind grows stronger

Trends in wind speeds
Trends in global wind speeds … graph adapted from study by Zhenzong Zend et al 2019

We are told constantly the planet is getting warmer, but wind speed trends are rarely mentioned.

The graph above shows wind speed trends over land.

There was a downward trend until 2010, then it started getting windier.

Wind speeds over the sea have been going up for decades, with a study showing the fastest increase in the Southern Ocean.

Young and Ribal [2019] assessed trends in oceanic wind speed and wave height from 1985 to 2018 using satellite altimeters, radiometers and scatterometers.

“The largest increases occur in the Southern Ocean. Confidence in the results is strengthened because the trends were confirmed by all three satellite systems,” they said.

It is a mystery why land wind speeds went down, while sea wind speeds increased.

But make no mistake, land and sea wind speeds are now both going up and will shape Tasmania’s future.

The state is already a windy place, being located in the Roaring 40s wind belt.

Gaining access to local historic wind speed data is unfortunately not as easy as obtaining rain and temperature records.

We know it is getting warmer.

Rainfall trends are harder to assess, because annual rainfall totals only tell part of the story.

The Bureau of Meteorology acknowledges that the timing of annual rainfall has changed in Tasmania.

This is where I fall back on local anecdotes.

What I have seen in 10 years on a Huon rural property is a drying trend.

There is less drizzle, but more short, heavy deluges.

There seems to be more wind, and warmer summer temperatures.

Normally I would put this down to decadal variation, but what I see matches current observations elsewhere.

What’s it all mean?

Where wind, warmth and rain intersect is in evaporation.

More wind and warmth creates dry soil and dry forests, even if rainfall remains static.

Dry forests burn, especially when it is windy.

We are seeing a recipe brewing for repeated epic bushfire seasons in Tasmania.

It is hard to imagine a bushfire as I look outside today at the lush green hills and thick fog over the river.

But the reliable spring and summer gales, the cracked soil around the dams, and the hundreds of dead trees on the hills are a huge hint.

Hopefully our community managers are taking notice of the brewing recipe.

It’s not hand-wringing time, but it is time to be prepared.

For rural dwellers, fire pumps and fuel removal strategies are essential, and you should have ember-intrusion prevention measures.

If you want to know more about wind trends, look up Australia’s wind and hydrology expert at CSIRO, Tim McVicar.

Meanwhile, here’s a recent story about the latest Australian weather trends.

Tasmanian Life