Why do some trees get so tall?

published 12/31/2019 in the York Weekly, Foster’s Daily, the Portsmouth Herald and online on seacoastonline.com

I was just out visiting my son who lives on the coast of Northern California, the land of tall trees, the coast redwoods.  Coast redwoods (these are the state tree of California) are the tallest trees on Earth. I was thinking about our New England tall tree, the white pine (the state tree of Maine), and wondering whether they ever had a chance of growing as tall as the coast redwood.

Navarro River Redwood State Park near Mendocino California

There are a large number of factors that determine how and why a tree grows tall: genetics, environment, age.  The primary reason for a tree to grow as tall as it can is to beat surrounding trees to the sun. White pines and coast redwoods are great at this and will dominate a forest if they can get to the sun first.  White pines typically grow to about 150 feet tall (however pre-colonial white pines have been estimated to reach heights of at least 230 feet) while coast redwoods grow twice as tall-300 feet and taller (the tallest tree in the world is a 379 foot tall coast redwood named Hyperion).  How do they do it?

The two problems that have to be overcome to grow super tall are water and wind.  Coast redwoods tend to occupy sheltered valleys where they are protected from wind.  Like all trees they need to somehow distribute water to all parts of the tree. The long-held dogma of how this is accomplished is that groundwater enters the roots and is pulled upward through the tree by water evaporating from pores in the leaves.  Water is sticky, so as one water molecule evaporates from a pore in a leaf it pulls another water molecule up after it. The taller a tree grows the more difficult it is to get water up to the topmost leaves-gravity becomes a major drag. If those leaves can’t get enough water photosynthesis and growth start to slow.   

However this upward flow of water  isn’t the whole story. It turns out many trees can also absorb water into their leaves and move it downward towards the roots. There has to be water vapor in the atmosphere for this to be a useful adaptation.  Coast redwoods live in the fog belt, their needle-like leaves are almost constantly bathed in fog. Studies by the National Park Service found that some coast redwoods obtain 40% of their water from fog, not their roots!   And, according to a study on coast redwoods published in Functional Ecology (“Pushing the limits to tree height: could foliar water storage compensate for hydraulic constraints in Sequoia sempervirens?” published 2014 by H. Roaki et al) the pinnacle (topmost) leaves of the tallest growing coast redwoods stored water better than trees that didn’t grow as tall. So, tall-growing coast redwoods can take advantage of all that ambient water in the surrounding fog and both store and use it for growth.

Another factor in height is age-it takes time to acquire all of that biomass.  White pines easily live a couple hundred years-the oldest known specimens are over 400 years old.  Coast redwoods, on the other hand, are among the oldest living things on Earth-they can live for more than 2,000 years (the oldest living coast redwood is 2,200 years old).   Hyperion, the tallest tree on Earth, a baby by coast redwood standards at somewhere between 600 and 800 years old has still had a remarkably long life.

Redwood stumps provide nutrients for new growth. Here some sorrel grows at the base of a stump

So, while our East Coast white pines probably couldn’t ever grow as tall as coast redwoods, -conditions and adaptations aren’t quite right-one similarity between these two species is how it feels to walk into a white pine or a coast redwood forest.  There is a cathedral like quality to these forests, they are quiet and dark. The filtered sunlight adds to their majesty, illuminating the mist in much the same way that light falls through the stained glass of a human-made cathedral and illuminates the dust motes floating through the air.   These forests we walk through today are largely young trees. Just imagine the grandeur of a pre-colonial forest, fully mature white pines or coast redwoods reaching towards the stars.