Why don't trees grow above a particular altitude?
Why is it that after one billion years of evolution trees still do not grow above the 3,000 metre level? Have they shifted with time?
Why is it that after one billion years of evolution trees still do not grow above the 3,000 metre level? It seems weird that they haven't adapted to lower temperatures or less oxygen — or whatever — after so many generations—Kym
The beautiful snow gum dominates Australia's tree line.
Separate species of the small eucalypt, known for its spectacular twisted trunk, grow in Australia's alpine regions on the mainland and Tasmania.
The mainland snow gum (Eucalyptus pauciflora subsp niphophila ) can be found up to 2,000 metres in the Snowy Mountains, whereas the Tasmanian snow gum (E. coccifera) forms the tree line in Tasmania up to 1,200 metres.
Elsewhere in the world the tree line is much higher — up to 4,800 metres in Bolivia — and much lower descending to sea level and merging with the Arctic and Antarctic tree line in Alaska, Siberia and Chile.
But it's not altitude that forms the tree line, it's temperature, says plant scientist Associate Professor Mark Hovenden from the University of Tasmania.
"In Papua New Guinea the tree line is on the equator, so it's at a much higher altitude than it is in northern Sweden, or somewhere where it's very cold at even very low altitude," says Hovenden.
In the past, it was thought that plants were simply unable to produce enough energy to build big stems at low temperatures.
But the mechanism is more complex, says Hovenden.
It appears that plants — irrespective of species wherever they are across the world — can't effectively build cells once the temperature drops below 7°C.
"It's not a shortage of energy, it's not a shortage of carbon, it's more a shortage of the number of days that are warm enough to actually build the cells."
And it is unlikely that temperature limit has changed over time, says Hovenden.
"You've got to think of it not in terms of evolution — plants have evolved to be very tolerant of low temperature conditions — it's just that it's a physiological constraint on building cells that can't be overcome."
In fact, trees have become a victim of their own success.
While smaller tree-like shrubs grow above the tree line at much colder spots, trees are limited by their canopy, which shades the ground and makes it colder.
"When you get above the tree line the actual local conditions for plant growth are warmer because you don't have the shading effect of the trees."
"Trees could grow at higher altitudes if they didn't shade themselves."
The other thing is that trees by their very nature stand taller than surrounding vegetation. This exposes them to chilling winds, which make it colder and can damage the tender growing buds.
Local conditions such as topography and aspect also affect the height of the tree line.
"We've done work here in Tasmania on Mount Wellington just to the west of Hobart.
"The tree line is at a different altitude on the north side of the mountain than what it is on the south side of the mountain by up to 150 metres simply because the south side of the mountain is cooler."
Here, aspect also affects the species of trees.
"It goes from being a Tasmanian snow gum on most of the mountain, to where you go around to the south-west where it's more sheltered and cooler and wetter where you go to myrtle beech (Nothofagus cunninghamii).
Shifting tree lines
Although the physiological temperature constraint may not have moved over time, the height of the tree line has changed throughout history due to climate change.
Today, as temperatures rise, there is "very good evidence" tree lines are moving up in altitude in many parts of the world such as New Zealand and Europe, says Hovenden.
But the picture is very different in Australia.
"There's no evidence of seedlings establishing above the tree line anywhere in Australia," says Hovenden.
"One of the reasons for that is the type of trees we have at our tree line tend to be eucalypt trees — and eucalypt seedlings mostly establish following fire."
There's no movement in the tree line in Tasmania, but if it were to burn it is possible that the new tree line would establish 50 metres or 100 metres higher in altitude than what it is at the moment."
On the mainland, modelling of the Snowy Mountains shows that temperature conditions by which trees could grow now extend about 100 metres above the current tree line, says Dr Ken Green, principal research scientist with NSW Parks and Wildlife Service.
But the tree line is not moving uphill in this area, despite exposure to fire in 2003 that produced ideal conditions for seedling establishment with an ashbed where soil was disturbed, nutrients were available, competition was removed and good rains followed the fire.
"There are plenty of seeds, conditions are right, but it just wasn't moving uphill.
In fact here, the trees are moving downhill as seedlings normally suppressed by snowdrifts backfill in below historic ribbons of trees that established above the tree line as snowfall in the area decreases.
Unlike conifers, which dominate colder regions in the northern hemisphere, eucalypts have poor seed dispersal mechanisms.
"Conifers have wings on the seed and therefore they can blow up hill whereas snow gums just shed their seeds by gravity so it's more likely to go downhill than uphill," explains Green.
"That's probably the main reason we're having these slower movements here than elsewhere."
Associate Professor Mark Hovenden is the deputy head of the School of Plant Science at the University of Tasmania. Dr Ken Green is the principal research scientist at the NSW Parks and Wildlife Service. They spoke to Genelle Weule.
Published 16 September 2013[an error occurred while processing this directive]