Page last updated at 15:03 GMT, Wednesday, 13 August 2008 16:03 UK

Water's the limit for tall trees

By Mark Kinver
Science and nature reporter, BBC News

Researcher climbing a Douglas-fir
The tallest living Douglas-fir tree measures 100 metres

Douglas firs, one of the tallest tree species, are unlikely to surpass 138m (453ft) in height, a study suggests.

A team of US researchers found that there was a limit on how high the giant trees were able to pull water up their trunks to supply upper branches.

As the firs reached their physical limit, the upper foliage experienced "drought stress", struggled to gain enough water and died back, they said.

The findings appear in the Proceedings of the National Academy of Sciences.

"The trees are moving water purely as a result of physics, " explained co-author Barb Lachenbruch, a professor of wood science at Oregon State University.

"The wood has to be designed to be safe at the top of the trees, which means preventing air bubbles getting into the columns that transport the water."

Tall trees face a greater risk from air bubbles blocking their water supply, known as xylem embolism, because the tension in the columns increases with height as a result of conflicting forces such as gravity.

"If you had a straw that was three feet long and sucked up water before attaching it to the bottom of your tongue, your tongue would be pulled into the straw solely because of the weight of the water in the straw," Professor Lachenbruch told BBC News.

I think it is really remarkable that wood cells, which are about the size of an eyelash but a little bit fatter, can tell us how tall a tree can get
Professor Barb Lachenbruch,
Oregon State University

"If you do that with a 300-foot column of water, it would pull incredibly hard; that's what the force is inside the trees' cell walls, and that's why air bubbles can get in.

"The wood has to be designed to be really safe to ensure the air bubbles do not get in. But this also affects the development of the foliage."

Water-tight valves

The team sampled 16 Douglas firs (Pseudotsuga menzieii) at five different sites, at heights ranging from six metres to 85.5m (20-280ft).

They found that the cellular structure of the wood changed as the height increased in order to prevent air bubbles entering the wood.

Man walking through a redwood (Image: BBC)
Coastal redwoods can reach heights in excess of 110m

"As tree height increases," they wrote, "the structural modifications needed to satisfy safety requirements eventually will reduce water transport virtually to zero."

The firs' wood is mainly made up from dead cells called "tracheids", which have pits on their sides that act as valves, allowing water to pass from one cell to the next.

The team found that the cells' pits became increasingly smaller in relation to an increase in height, resulting in less water being transported to the upper reaches of the trunk and branches.

The point where the water supply became non-existent determined the maximum height of the tree, the researchers added.

Professor Lachenbruch said the team calculated this point as being 138m, but added that it could be between 131m (430ft) and 145m (476ft) once a margin of error was taken into account.

She observed: "I think it is really remarkable that wood cells, which are about the size of an eyelash but a little bit fatter, with holes on the side, can tell us something about how tall a tree can get."

The tallest Douglas fir standing today, measuring 100m (329ft), can be found in Coos County, Oregon.

However, the highest living tree is a coastal redwood (Sequoia sempervirens), which is more than 112m (367ft) tall.

Professor Lachenbruch said their modelling only applied to Douglas firs, but added that it was possible that the same mechanism could determine the maximum height of other tall species, such as the redwoods.

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