Where is the biggest tree in Australia, and just how big is it?

That's what Derek McIntosh is hoping to find out through his aptly named National Register of Big Trees.

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Mr McIntosh wants to locate and document all our largest trees, and he's calling on the public to help by nominating trees based on their circumference, height, and crown spread.

Those taking part might even discover the tallest tree in the world. The title is much disputed, particularly for historical specimens.

At least with a living tree you can measure its height and girth, but tales abound of giant trees that are no longer standing, and there is simmering debate about whether Australia or the United States holds the record for height.

Giants of North America

In 2005 the world's tallest measured living tree was a 112.7-metre-high coast redwood growing in Humboldt Redwoods National Park, California.

It is still considered the tallest — presumably plus or minus a metre or two now — and is a relative of the giant redwood, which grows just a few hundred kilometres to the north.

Trunk circumference is not the only thing being measured, but it's important. ( Supplied: Tim Entwisle )

For the botanical record, the coast or Californian redwood (Sequoia sempervirens) and the giant redwood (Sequoiadendron giganteum) are each the only species in their genera.

Despite its name, the giant redwood doesn't grow quite as tall as the coast redwood, but it achieves more bulk — the volume of its trunk is the largest in the world.

The Montezuma cypress (Taxodium mucronatum) in Mexico actually has the largest girth at over 15 metres in diameter. The tallest tree in the Royal Botanic Gardens, Victoria, is one of these.

And even though both the redwood species frequently top 100 metres in height, the tallest conifer ever recorded was a 126-metre Douglas fir (Pseudotsuga menziesii).

Gums can grow tall too

All these record holders are what we call conifers; that is, they produce cones. Of course, flowering trees like our gums also grow very tall.

Although there are claims of mountain ash (Eucalyptus regnans) in southern Australia growing to over 120 metres, the tallest ever officially measured was 107 metres.

Today the tallest living known specimen is a 99.8-metre tree called Centurion in the Arve Valley, Tasmania. It was found and measured in 2008, replacing the previous record holder, the 97-metre-high Icarus Dream in the Styx Valley.

If we look after the surrounding forests, Centurion and Icarus Dream could both exceed the 100 metre mark in the years to come.

But no matter how long they live, we don't expect any tree — conifer or gum — to reach 130 metres.

The limit on their height comes down to a plumbing problem: how do you get water from the ground to a leaf fluttering 100 or so metres above?

The battle against gravity

Water is drawn up a tree — what we call transpiration — in a continuous column as it evaporates from leaves into the atmosphere.

A few years ago, scientists from Northern Arizona University climbed the world's tallest trees to measure their water potential and photosynthesis in the highest branches, and analysed leaves taken back to the laboratory.

They found that gravity starts to win out against water cohesion at around 110 metres.

Australia's gum trees can grow very tall: one has been measured at 107m. ( Flickr.com: Alpha, CC-BY-2.0 )

The leaves most distant from the base of a gigantic redwood are under extreme water stress, and their small size and low photosynthetic rates may be due to the plant closing some of its breathing pores, known as stomata.

This would not only retain precious water, but also slow down the rate of water transport through the plant, reducing the possibility of air bubbles being formed, which would mean death for a lofty limb.

They also found that to keep one of these big trees alive and transpiring healthily, the surrounding forest must remain intact, which maintains high moisture levels and buffers the trees against storm damage.

So if we want to see tall trees in Australia, we've got to look after the forests that surround them.

Recent droughts have put some of these big trees under additional stress, so doing what we can to reduce the severity and impact of climate change will also help our botanical giants survive.

A point system for the biggest and bulkiest

Mr McIntosh's citizen science project is an endeavour to get more Australians interested in and caring for our trees.

The idea came from a similar project he encountered in his time living in the US during the 1970s.

Though there are some lists of important trees in Australia already, he wants to tap into the excitement and competitive element that comes with trying to find the biggest of anything.

Contributors to his National Register of Big Trees use a points system originally used in the United States, which totals up the circumference of the trunk (in inches), the height (in feet) and the average crown spread (also in feet — although the register records both metric and imperial statistics).

If we want to see tall trees, we've got to look after the forests that surround them. ( Flickr.com: Ryan, CC-BY-2.0 )

The largest tree in the register is a mountain ash called Kermandie Queen, growing near Geeveston, Tasmania.

At 77 metres high, with a trunk that is 22 metres in circumference and a crown spreading some 25 metres, it scored a total of 1,125 points.

Brett Mifsud, the nominator of this mighty specimen, estimates it was 500 years old when he took the measurements in 2013.

Assuming we care for the forest around it, the Kermandie Queen has the potential to improve on all its vital measurements.

Who knows, it may even become the tallest tree in Australia, or even the world.