“Keep walking back with your kite. There you go. Now stop where you are. The distance between you and me right now is equivalent to about half the height of California redwoods—the tallest trees on earth. Can you imagine that?” This was my stab at an illustration of how tall trees can really get. But my eight year old son was having none of it. “Wind all that string around the reel Dad, and let’s go home!” Disappointed as I was with his response to my efforts, it was plainly obvious that he had to see something a lot more well-grounded than an unwound length of string tied onto a diamond-shaped piece of flyable canvas.
Once home he grabbed the Seasonal Forest installment of BBC’s mega-documentary Planet Earth out of the movie cabinet, slotted it into our DVD player and jumped into the couch with a bowl of ice cream in hand. As the acclaimed naturalist David Attenborough piled superlative upon superlative into his accompanying narration, I could see in my son’s gawping eyes that cogs were turning deep in the grey matter of his brain. He had finally grasped the enormity of these organisms. I kept quiet as he listened intently:
“One grove of redwoods in California contains three of the tallest trees on earth. Over 100 meters high: the size of a 30 story building. These forests were growing long before humans walked the earth. They were in their prime 20 million years ago and existed before the Swiss Alps or the Rocky Mountains were even raised. There is more living matter in a forest of giant conifers than in any tropical rain forest…..A giant Sequoia, a relative of the redwood, is the largest living thing on earth. Known as General Sherman, it’s the weight of ten blue whales. Higher up in the nearby mountains we find bristle cone pines- the oldest organisms on the planet. Some have been here for five thousand years. They were alive before the pyramids were built and were already three thousand years old when Christ was born” (1)
“Wow Dad, isn’t that just mind-blowing?” Not wanting to see my son’s fervor dwindle I nabbed a copy of popular science writer Len Fisher’s How To Dunk A Doughnut off the shelf, knowing full well that in its pages lay a weighty passage on the physics of redwood tree growth. Following Attenborough’s helpful words, I read the passage out loud verbatim:
“The leafy crown of the largest known specimen, the ‘General Sherman’, towers eighty-three meters above the tourists passing below. The water supply for the leaves is drawn up from the soil by capillary action. The menisci of these huge columns of water reside in the leaves, and a quick calculation shows that the capillary channels containing the menisci can be no more that 0.2 micrometers wide- about one-hundred-and-fiftieth of the diameter of a human hair. The pressure across such a tiny meniscus can support a continuous column of water, of which there are many in the bundles of tubes called the xylem, which runs up the trunk below the bark” (2)
“So what is the height limit of the redwood, Dad?” Six years ago, an adventurous group of botanists from Arizona and California published their estimate after making it up to the tree tops of the Humboldt Redwoods State Park in California using mechanical ascenders and dropping tape measures to the ground below (3,4). Scaling up tree trunks is certainly not a job for the acrophobic, me included. But the effort paid off. And the group got a Nature paper out of it. Three factors- (i) transpiration, (ii) water adhesion and (iii) surface tension- work in concert to draw water up the tree. The high tensile strength of water ensures that the long watery columns running up the trunk do not break (5).
As water evaporates from the leaves at the uppermost reaches of the redwood through transpiration, a negative pressure (low water potential) builds up. This is most marked at midday when transpiration ‘pull’ generates a pressure of approximately -18 atm. At such levels, cavitation (cold boiling/embolisms) risks along the water columns are at their greatest. Trees get around this threat by decreasing the aperture size of the stomatal pores on their leaves thereby reducing transpiration rates. The unfortunate byproduct of this is that further growth is limited- without additional photosynthetically-derived carbon being absorbed through stomata trees cannot increase in size (3).
A higher leaf mass:area ratio for leaves at greater heights also increases resistance to CO2 diffusion in the leaf once again reducing photosynthesis and carbohydrate availability for further growth. Low water potentials lead to reduced leaf cell turgidity which is why leaves at the upper reaches of Redwoods are considerably smaller than those in the lower crown (3,4). One of the physiological consequences of this is a reduced enzymatic discrimination between the Carbon-12 and Carbon-13 forms of CO2 (3). Indeed there is a close correlation between redwood tree height and the foliar abundance of 13C.
Across the 2,200 year life spans of the tallest redwoods, severe droughts are likely to have created water potentials low enough to produce cavitation (3). The evidence is overwhelming- most tall redwoods have multiple tree tops that have died and been repeatedly replaced (3). But these same specimens also enjoy protection from neighboring trees against buffeting winds that would otherwise damage the tall canopy. Having traipsed through forests aplenty in our home state of Wisconsin, wind protection is certainly one benefit that my son can readily grasp.
There is of course a question that awaits its time: “Dad can we climb a redwood?” When the moment comes to define my boundaries, I will have my scripted answer at the ready: “Son, I agree that those trees are towering giants of meniscal beauty. But the angel-holding knurl at the top of our Christmas Douglas fir is about as high as I dare go. Let’s just leave it at that”. I rest confident that my retort will be graciously accepted.
- Planet Earth: The Complete Series, A BBC/Discovery Channel/NHK Co-Production (2007)
- Len Fisher (2002) How To Dunk A Doughnut: The Science Of Everyday Life, 1st Ed, Weidenfeld & Nicolson, London, p.12
- G.W.Koch, S.C.Sillett, G.M.Jennings, S.D.Davis (2004) The limits to tree height, Nature Volume 428, pp. 851 – 854
- Michael Hopkin (2004) Height limit predicted for tallest trees, See http://www.k8science.org/news/news.cfm?art=919
- Don Garlick (2007) How Can Redwoods Grow So Tall? The Journal, Humboldt County, October 11th, 2007, See http://www.northcoastjournal.com/outdoors/2007/10/11/how-can-redwoods-grow-so-tall/