Tuesday, March 3, 2015

You Try Carrying Water Up 115 Meters!

Written By: Lindsey Whitaker

The towering tree pictured above is Hyperion, the Coast Redwood tree. Yes, this tree is so amazing it has it’s own name. Hyperion is the tallest tree in the world at 115.76 meters and is discretely hidden in California’s Humboldt Redwood National Park. This tree is taller than both the Statue of Liberty and Big Ben. Naturalists, Chris Atkins and Michael Taylor found Hyperion in 2006. Steve Sillet and his team of researchers from Humboldt University climbed to the very top to accurately measure this tree (Sillet, unpubished). Steve has climbed many of the world’s tallest trees, which might be the coolest job ever (see video at the bottom).

Coast Redwood Range

Coast Redwoods (Sequoia sempervirens) are known for being very tall and are one of the only species known to have trees taller than 100 meters (Ishii 2014). Coast Redwoods are found in a narrow band along the California Coast from near the Oregon border to the Santa Lucia Mountains in southern Monterey County. Throughout the northern portion of the Western United States, redwoods can be found in small groves near the ocean or in foggy canyons, around creeks, and in other areas of cool temperatures and sufficient rainfall (Los Padres Forest Watch 2014).

The botanist in me can’t help but think about Hyperion and ponder… How does this work?

Let me introduce Norman the normal, average height tree. While Norman is not Hyperion, he still does amazing things for us humans and deserves his own name. If you enjoy breathing, laying in the shade or living in your wood house, thank Norman (Thank you Norman). Normal trees get their nutrients and water from their roots. Those roots are able to transport water and nutrients to the xylem at the center of the tree. Once in the xylem, water-conducting cells, called vessel elements and trachieds pass the glorious water and nutrients to the leaves (Taiz and Zeiger 2010). 

This may seem simple but of course there is that pesky thing called gravity, always pushing us down. What a bully… The theory of how xylem works is called the cohesion-tension theory. While I will not go into all of the details, this theory says it is a combination of the pressure and tension created by transpiration in the leaves and the properties of water (cohesion and adhesion) that carries water through out the plant. Norman and his trees friends are able to pressurize their xylem to get water up to their leaves. This in itself is amazing and there must be a limit to how much pressure can be created within the xylem (Taiz andd Zeiger 2010).

This is where Coast Redwood trees throw everyone for a loop. How do they get so tall and still get water to their leaves? Botanists from all over the world have been studying the same question.
It was originally thought Coast Redwood trees grew so tall because of the high competition for light (King 1990). The habitat in which coast redwoods grow is usually foggy and dense with trees all competing for a limited light source. More recently, researchers have found it is not competition for limited light, but the pressure created in the xylem that is controlling growth (Oldham 2010). Coast Redwood trees have an increased percentage of water transporting tissue (xylem tissue) in their leaves. The total number of xylem tissue in the upper crown leaves more than doubles that of leaves from the lower crown (Jennings 2002). By increasing the amount of xylem tissue in the upper crown leaves, Coast Redwood trees are able to bring water to their leaves faster and more efficiently. 

Transfusional tissue holds water. Taller the tree, the more tissue available to store water (Oldham 2010).
That is not all! Coast Redwoods can also store water collected from the fog in their leaves. Water is taken in through the leaves and stored internally to reduce reliance on water transported from the roots. Leaves alone can store more than five times the daily transpirational demand at the treetop (Ishii 2014). 
Coast Redwoods were found to be able to holder more water in their leaves, the taller the tree was (Ishhi 2014).
Coast Redwood trees are truly astonishing. They are able to become enormously tall and remain extremely strong. This is all due to adaptations they have made in their leaves. They are able to change the amount of xylem in their upper crown leaves to quickly move water to the leaves. They have also adapted to bring water into their leaves at astonishing rates to lessen the pressure on the roots to bring water to such great heights.

Researchers have predicted 115 meters is not the tallest capabilities of Coast Redwoods. They predict a Coast Redwood tree could grow up to 130 meters in the correct conditions and still be able to get water to its leaves (Koch 2004). Pretty damn impressive. 

Ishii, H.R., W. Azuma, K. Kuroda, and S.S. Sillett. 2014. Pushing the limits to tree height:could foliar water storage compensate for hydraulic constraints in Sequoia sempervirens? Functional Ecology. 28:1087-1093. 

Jennings, G.M. 2002. Vertical hydrualic gradients and the cause of foliar variation in tall Redwood trees. Thesis, unpublished. 

Koch, G.W., S.C. Sillet, G.M. Jennings, and S.D. Davis. 2004. The limits to tree height. Nature. 428:851-854.

Oldham, A.R., S.C. Sillett, A.M.F. Tomescu, and G.W. Koch. 2010. The hydrostatic gradient, not light availability, drives height-related variation in Sequoia sempervirens (Cupressaceae) leaf anatomy.  American Journal of Botany. 97(7): 1087-1097. 

Taiz, L., and E. Zeiger. 2010. Plant Physiology, 5th Edition. 

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