With spring in the Northern Hemisphere in full bloom, forests are again beckoning hikers onto paths that wend their way through thousands of acres of old growth trees. Such forests not only offer salve for the soul, but their role in Earth’s ecology is arguably crucial to the health of our planet.
But can we expect anything resembling trees on an extrasolar Earth?
It seems really probable that some form of photosynthesis —- harnessing light energy into storable chemical energy —- would evolve in any life system that is orbiting a star like our Sun, Erika Edwards, an evolutionary biologist at Yale University, told me via email. If photosynthesis moves out of water and onto land on an Earth 2.0, as it did on our current Earth about 500 million years ago, I’d predict that some sort of tree-like form would evolve, says Edwards.
An Earth Without Trees
But Earth has been without trees for most of its history. In fact, they’ve only been in existence for some 400 million years, about a tenth of Earth’s history. There are even species of sharks older than the oldest tree species.
There were no trees here for most of the history of life on Earth and, indeed, for most of the history of photosynthesis, Edward Schwieterman, an astrobiologist at the University of California, Riverside, told me via email. Geologically speaking, trees are a relatively recent development, arising in the Devonian period about 385 million years ago, he says. Yet planets with trees are more likely to host robust and diverse land-based ecosystems, says Schwieterman.
One of the oldest trees on Earth, a spruce in Sweden clocks in at more than 9,500 years. Would evolution select species with that kind of longevity on an exoearth?
Most tree species’ mortality occurs in the first stages of life; it all depends on environmental stability, Milton Mendonca, an ecologist at Brazil’s Federal University of Rio Grande do Sul, told me via email. On planets with very unstable or unpredictable climates and conditions it might pay off to have alternative strategies, like reproducing all you can and leaving hardy spores, he says. That could occur early in the existence of unstable extrasolar systems, or on an exomoon with complex orbital dynamics, says Mendonca.
Will tree-like organisms that harness photosynthesis evolve on exoearths?
Any complicated ecosystem will need primary producers —- the harnessers of energy into the life system, says Edwards. On Earth, our primary producers are photosynthetic organisms, so, it’s hard to see beyond that bias, she says.
Trees Are Climate Changers
Individually, they alter local temperature, humidity, and wind flow, says Mendonca. And when together in forests, they can change regional climates, he says. Mendonca notes that what’s left of our planet’s long remaining stretches of tropical forests have a very discernable impact on our planet’s biosphere.
These forests are also ecological substrates for other forms of life —- on Earth, plants, animals of all sorts, fungi and bacteria all live on their bark, shoots, trunks, leaves, roots, flowers, and fruits, says Mendonca.
As to how forests on an exoplanet might differ from what we find on Earth?
Most likely, exoplanets with tree-like lifeforms would be quite different from exoplanets without them —- probably, more quiet, less windy places, but on the other hand more buzzing with life, says Mendonca.
Edwards wonders what trees here would look like today if plants had evolved some other system for acquiring and distributing water and nutrients throughout their bodies.
A lot of the potential ecological constraints on how trees function here come back to an ancient architecture that evolution has elaborated on for hundreds of millions of years, says Edwards. Tree height, productivity, tolerance of drought and cold, etc., in large part, are all dictated by the vascular system that plants here on Earth created, she says.
Are trees truly important for staving off anthropogenic climate change?
While trees certainly play an important role in acting as carbon sinks, due to their intake of carbon emissions from anthropogenic climate change, the timescale and volume of their interaction with the carbon cycle is much smaller than the role of rock and planetary interiors, Michaela Leung, a planetary astrobiologist at the University of California, Riverside, told me via email.
These abiotic “thermostat” processes were stabilizing the climate of Earth long before trees arose. So a planet without trees could maintain climate conditions over long periods of life, she says.
Many species of trees on Earth act as environmental filters of our atmosphere. Would exotrees evolutionarily select for the same traits?
To the degree that trees’ filtration processes improve conditions for themselves, I would think that this trait could also arise, says Leung. Control of local environments through processes such as detoxification may be a key trait for life, she says.
Will there be a way to determine if trees are present on some future newly-discovered exoplanet?
If large scale plants like trees did evolve and covered a large fraction of the surface, they may be detectable via telescopic observations of the surface, says Leung. Detecting an exo-forest could potentially be possible in the future, she says.
And if we will soon be able to detect extrasolar forests, if they are out there on some far-flung exoplanet, alien astronomers within a thousand light years of our Sun will probably be able to do it too.
In this way, forests have contributed to the Earth’s own remotely detectable signals, says Leung.
Exotrees Would Likely Have A Global Effect
If they only are common enough, I have no doubt that exotrees would have a deep and permanent impact on the ecology of the planet they occupy, says Mendonca.
The lesson here is that even though trees came late to the evolutionary party here on our own planet, life on earth would be pretty unimaginable without our remaining majestic forests.
An Earth without forests is a very grim picture, says Edwards. Forests are the most species-diverse places on Earth; trees facilitate diversification and co-existence of other groups of organisms, she says. If plants hadn’t figured out how to become trees, Earth would be a far less interesting place and we (humans) probably wouldn’t even be here, says Edwards.
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