A small raptor alighted for a few minutes yesterday on the top of a tree in my yard. The bird perched up in the crown, on branches that have been bare for a few weeks. The tree is a pin oak, whose health was questionable when we moved to this house in the spring, yet which still has most of its leaves, now crisp and a warm orange-brown. The bird, the tree, the angle of the low November sun: it all made for a fine little fall tableau. All I could think about were killer asteroids.
The bird and the tree are both survivors. They are descended from beings who lived through the darkest hours in the history of Earth, one summer day 65 million years ago when a 9-mile-wide space rock slammed into the the Yucatán peninsula in the Gulf of Mexico. There is much to learn from their survival, especially now, as trees drop their leaves (sometimes all at once) in a seasonal display that many people find beautiful, and that I find inalterably depressing.
Apart from humans, maybe, trees are the best form of life on this planet. Trees remain in one place, but reach elsewhere always. They stretch down into the ground, and they constantly strain toward the sun. They are the embodiment of our shared presence on a rocky planet that orbits a star. Hedgehogs and helminths may be interesting, but they don’t constantly remind us, simply by existing, that we are in a solar system.
Trees are also hosts for every other form of life. Their roots chew up the very crust, a process aided by microbes, and in doing so, trees remake continents. Their bark harbors fungus and lichen. Their branches and leaves shelter and feed insects, birds and mammals. Even now, when humans are capable of building machines that fly to Mars, we still use trees for shelter. We also cut them down and burn them for warmth. Some of us cut them down and bring them inside and festoon them, for a form of psychic warmth that lasts a few weeks and is the only reason I can tolerate December.
Trees are strivers. My pin oak is one of the fastest-growing species of hardwood trees, according to the Arbor Day Foundation. It can grow two feet per year. Trees also bide their time. The oldest living thing on Earth is a tree in California**, a bristlecone pine that sprouted from a seed a few years before the invention of writing, in 3200 BCE.
Trees live longer than civilization, but they also die a thousand times before their death. Deciduous trees, which are found all over Earth, self-destruct every fall. Their leaves spend the warm months collecting sunlight and converting it into carbohydrates, and when temperatures and light levels drop, they stop. The tree severs its ties to the leaves, and the wind takes them away. Goodbye, leaves.
I think about this death every fall. As the northern hemisphere tilts away from our star, life in these parts shuts down and hibernates — if it’s lucky. We rake up the leaves and pulverize them. The tree goes quiet until the Earth tilts back. It bums me out.
But yesterday, the bird in the tree and the light on them both reminded me of something. All summer, I could not stop thinking about a relatively short line in a book by my friend. Deciduous trees might have proliferated because of this perennial loss, he wrote. They might have lived because they die.
When that 9-mile-wide bolide careened through the atmosphere, it ignited a global firestorm. The impactor itself caused a shockwave and fireball that would have caused hurricane-force gusts 1,800 miles away from the crater. It also carved a gash down to Earth’s mantle, loosing a mountain of molten rock that went briefly into orbit before raining down again. During this deluge of fire and brimstone, Earth would have broiled for 20 minutes, according to my friend’s book. Wet, papery leaves on trees would have vaporized, leaving only the trunks behind. But the trunks knew what to do.
“Deciduous trees are better able to withstand months of cold, dark privation,” he writes, “and have enjoyed the advantage of this global pruning ever since.”
The scientific literature further explains that after the impact, fast-growing, deciduous angiosperms — forebears of my pin oak — had replaced their slower-growing evergreen brethren. “The Chicxulub bolide impact appears to have led to the selective extinction of plant species with ‘slow’ leaf ecological strategies,” wrote Benjamin Blonder and colleagues in a 2014 paper.
The pin oak lives because its ancestors died, every fall, and it was adapted to withstand a period of hunkering down. This is appealing. I can work with this. Leaves falling in the autumn is not a symbol of giving up, but a sign of waiting out. It is not a symbol of death, but a sign of life.
Trees can withstand a lot on this planet. Maybe they can do the same on other planets, too. When we find other worlds that harbor life*, trees may be the first thing to greet us.
Sometime in the next decade, we will start peering into the atmospheres of planets around distant stars, and we’ll be able to tell if anything lives and breathes there. This will be much easier, or at least more likely, than finding a radio beacon from some form of intelligence. But it will be no less profound. The first extraterrestrials we encounter are not likely to be personlike, or even aware, at least according to our definition of awareness. But they will breathe just as surely as we do. They might reach down, anchoring themselves in a lithosphere of some kind. They will reach up, maybe with canopies of red leaves whose chlorophyll is adapted to the wavelengths of light emitted by a red dwarf star. They might harbor other forms of life we can only imagine. Maybe they will also shed their leaves, having adapted to asteroid or comet impacts of their own, or to ongoing stellar radiation. Maybe they will be survivors too.
* I have stopped writing this sentence with conditional language.
Photo of a bristlecone pine by Flickr user Rick Goldwaser, from Wikimedia Commons under CC-BY-2.0.
** I originally wrote Arizona, but this is not correct.