Shafts of light pierce the mist suspended among monolithic tree trunks while your feet squish on damp needles, and waist-high sword ferns yield to your passing even while they drench your pants with dew. A Hermit Thrush plays a melody on its metallic flute to a chorus of squirrel chatter, and banana slugs the length of your arm (or so it seems) weave mercury trails across your path. The oldest forests of the Pacific Northwest are hallowed magical places and the realm of giants. If you have never been, then I beg you to go. Try Mount Rainier or Olympic National Parks in Washington, or Jedediah Smith State Park in California. A visit to the old trees will shift your perspective on yourself and mortality forever.

If you haven’t been to see the old trees, then nothing I can write will prepare you for their sheer size. Douglas-fir, Sitka spruce, and redwood trees can be 300 feet tall (100 meters, or as tall as a football field is long) and are in fact the tallest living things on the planet. Despite all being giants among trees, these three species have different strategies for growing so large. That is what makes them unique, and it’s the topic of this blog. Scientists like Russell Kramer, Steve Sillett, and Bob Van Pelt are the ones unlocking the mysteries of the trees, and it’s their work I’ll summarize for you, especially Russell Kramer’s.

First, for comparison, let’s look at some numbers. The oldest known Sitka spruce, Douglas-fir, and coast redwood trees are 480, 1,000 and 2500 years old. In those times, the biggest trees of each species reach diameters of 16 feet (spruce), 15 feet (Douglas-fir), and 22 feet (redwood). In other words, redwoods flat outlive their rivals, and their amazing size makes their competitors look like shrimps.

Don’t think the story ends there. Trees are cutthroats. Competition for limited sunlight spells life or death in the deep shade under the canopy of old forests. In these conditions every advantage counts, and in the arena of how-fast-can-you-grow, Sitka spruce is the undisputed champion. Although Sitka spruce only lives 1/5th as long and only attains 30% of the mass of the largest redwoods, it has nearly 80% of the leaf mass. The high leaf investment of Sika spruce enables astounding growth that outpaces even coast redwood. By 300 years spruce acquires 1.5 times more mass and by the end of its life around 500 years will be nearly 3 times the mass of a redwood the same age.

Why does the spruce grow so many leaves? Leaves produce energy, and energy begets growth. A little sapling that sprouts in a forest must grow fast or die before large neighboring trees overwhelm it with shade. When it comes to growing fast, no conifer beats the Sitka spruce.

There are other important differences between these tree species. The wood of redwoods is famous because it lasts forever. In fact, the species name Sequoia sempervirens means ever-living sequoia. Trees that persist for over 2000 years in a forest deluged with six feet of rain a year can only do so by producing chemicals to battle wood decaying fungi. Redwoods also have thick bark to survive fires, something they will encounter in two millennia. Sitka spruce forego all that; they’re stingy and smart. It takes energy to produce chemicals and grow bark. Spruce don’t live as long as redwoods, because they succumb to fungi that eventually kill them. Instead, spruce are all about growing leaves and growing tall. It’s a dog-eat-dog world in the old forests, and Sitka spruce survive because they Grow Fast and Die Young.

Chapter 2. How do we know all this?

Now for the cool part. How do scientists know how tall are the tallest trees, how dense the wood is, or how much the leaves weigh? Easy: they climb them. Scientists like Kramer, Sillett, and Van Pelt climb over every foot of every tree, from the base to the highest leaf, and out to the ends of every branch, and they measure everything. Of course this is time consuming. Kramer climbed and measured 36 spruce trees for his research over two field seasons lasting eight months. His largest spruce required approximately 12,000 measurements to describe, not including measuring growth-ring widths. All the data they collect are then used to develop equations to explain tree size and growth rates. It requires amazing, rope-based tree climbing methods developed over the last 20 years, and genius ability in mathematics. That, my friends, is how science is done.

Let’s reflect on scientific tree climbing for a moment. In the old days when someone wanted to measure a tree they just cut it down. By some accounts, the extent of old forests in the USA is now less than 5% of what it used to be. Cutting old trees is unethical for any reason. And so is careless climbing. Large branches support soils, mosses, ferns, and woody shrubs, and even other trees in the canopy. It takes more than 150 years before spruce is tall enough to grow branches above the height they would be killed by shade from neighbors, and another 150 before they support rich communities of other plants. Kramer, Sillett, and others use the most advanced tree climbing techniques in existence, combined with an emotional, almost religious, appreciation for trees, to climb them without damaging the forest canopy ecosystem. You should also know that national parks and California state parks prohibit tree climbing without previously approved scientific permits and a climbing plan, precisely to protect this fragile world.

References used in this paper

Kramer, R. D., S. C. Sillett, and R. Van Pelt. 2018. Quantifying aboveground components of Picea sitchensis for allometric comparisons among tall conifers in North American rainforests. Forest Ecology and Management 430:59-77.

Sillett, S. C., and R. Van Pelt. 2007. Trunk reiteration promotes epiphytes and water storage in an old-growth redwood forest canopy. Ecological Monographs 77:335-359.

Sillett, S. C., R. Van Pelt, J. A. Freund, J. Campbell-Spickler, A. L. Carroll, and R. D. Kramer. 2018. Development and dominance of Douglas-fir in North American rainforests. Forest Ecology and Management 429:93-114.

Footnote

The wood of Sitka spruce is special in its own right. It has the highest strength-to-weight ratio of any wood. Sitka spruce wood was the #1 material used in airplane construction during WWI, and the United States employed 20,000 men during the war to harvest it.