Water’s the limit for Douglas firs

Scientists are betting that you will never find a Douglas fir taller than 138 metres.

Writing in the scientific journal Proceeding of the National Academy of Sciences, a team of researchers found that there was a limit on how high the giant trees were able to pull water up its truck to supply its upper branches.

As the firs reached their physical limit, the upper foliage experienced “drought stress”, struggled to enough water and died back, they explained.

“The trees are moving water purely as a result of physics, ” co-author Barb Lachenbruch, a professor of wood science at Oregon State University, told BBC News.

“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 environment reporter Mark Kinver.

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

“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, they added.

Professor Lachenbruch said the team calculated this point as being 138m, but added that it could be between 131m and 145m 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.”

Source: PNAS/BBC News website

Date: 11/08/2008

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One Response

  1. 131 – 145 m is extremely tall, but tallies with historic trees — The tallest Douglas firs on record have been measured at heights of 100 to 127 meters– close to the projected maximum.

    A 116 m tree measured in 1900 at Nisqually River Wa., 120 m fir measured near Mineral, Wa in 1925. Trees in British Columbia were occasionally over 120 meters: A 120 m tree was felled in south Vancouver in 1896, a 127 m fir was felled in Lynn Valley, North Vancouver in 1902.

    The tallest Redwoods today are 100 – 115 m, but historical records indicate a few trees as tall as 115 – 130 m.

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