What Were the First Trees? How Wood Changed the Planet

A plant on land faces a competition it cannot win by being wide. Light comes from above, and any plant that can raise its leaves above its neighbours takes the light and shades them out, so there is relentless pressure to grow tall. Height needs two things that early plants did not have: a structure rigid enough not to fall over, and a plumbing system able to lift water many metres against gravity. Wood provides both at once. It is made of cells with walls stiffened by lignin, a complex polymer that resists compression and rot, arranged into tubes that conduct water upward, and once a plant can add new layers of it each season it can keep growing for decades. The invention of lignin is arguably as consequential as any single biochemical innovation in the history of life, because it made everything above ankle height possible.

The oldest accepted tree fossils were a mystery for a century and a quarter. In 1870 a flood exposed hundreds of large fossil stumps in a quarry at Gilboa in New York state, each up to a metre across with a flared base, and they were named Eospermatopteris. What nobody had was the rest of the plant, so the reconstruction was guesswork, usually a trunk with fronds. The answer arrived in two stages: in 2005 a fossil crown was found at a nearby site, and in 2007 a complete trunk twenty-eight feet long. Together they showed that the stumps belonged to Wattieza, a relative of modern ferns and horsetails, which grew to eight metres with a slender unbranched trunk topped by a crown of frond-like branches that it shed as it grew, like a tree fern or a palm. It had no leaves in the modern sense and reproduced by spores, and at about 385 million years old it is the oldest thing that can properly be called a tree.

A few million years later came a plant that would look more familiar. Archaeopteris, unrelated to the bird of nearly the same name, grew up to thirty metres with a woody trunk that produced true wood of the kind conifers make, branched repeatedly, and carried flat green leaves rather than fronds. It formed the first forests in the ordinary sense, spreading across the world by the late Devonian and dominating for perhaps twenty million years. Several of its features had effects far beyond the plant:

• •Deep roots, which penetrated bedrock and broke it apart, creating genuine soil for the first time
• •Leaf litter, which fed a new decomposer community and put organic carbon into the ground
• •A branching crown that shaded the surface and created the first habitats with a canopy and an understorey
• •Wood that was slow to decay, since nothing yet had enzymes efficient at digesting lignin, so dead trunks accumulated instead of rotting
• •Roots that anchored river banks, changing rivers from broad shifting braids into the meandering channels with stable banks that dominate the landscape today

The late Devonian contains a protracted series of marine extinctions, among the five largest in the fossil record, and the leading explanation is the forests. Two mechanisms operate together. Deep roots and organic acids accelerated the chemical weathering of silicate rock, a reaction that consumes carbon dioxide, and the global drawdown was large enough to cool the climate substantially and contribute to glaciation. At the same time, roots and soils flushed enormous quantities of nutrients, particularly phosphorus, into rivers and then into shallow seas, where they fed algal blooms that stripped the water of oxygen when they decayed. The result was widespread anoxia in exactly the shallow marine environments where most animal diversity lived, and reef systems, armoured fish and many invertebrate groups suffered heavily. Life on land, in other words, nearly suffocated life in the water within a few tens of millions of years of standing up.

The consequences continued into the next period. Through the Carboniferous, vast swamp forests of giant clubmosses, horsetails and early seed plants grew, died and were buried without fully decomposing, and the accumulated carbon became the coal seams of Europe, North America and China that powered the industrial revolution. A long-standing explanation held that fungi had not yet evolved the enzymes to break down lignin, so wood simply piled up until they caught up, an idea supported by a 2012 genetic study dating the relevant fungal enzymes to around the end of the period. It has since been challenged by researchers arguing that the burial was mainly a matter of geography and climate, with vast low-lying wet basins in the tropics doing the work, and the question is open. Either way, the oxygen released by all that unrotted carbon reached perhaps 35 percent of the atmosphere, which is why the insects of that period grew to sizes impossible today.

Wood, made rigid and rot-resistant by lignin, let plants grow tall enough to shade their neighbours and lift water many metres, and the first true tree was Wattieza at about 385 million years ago, an eight-metre spore-bearing plant whose fossil stumps at Gilboa were known for 125 years before anyone found its crown. Archaeopteris followed with true wood, deep roots and flat leaves, forming the first real forests. Their roots made soil, stabilised rivers, drew down carbon dioxide and flushed nutrients into shallow seas, contributing to the late Devonian marine extinctions.