What Were the First Forests? Trees Invented Themselves Several Times

A quarry at Gilboa in New York State exposed fossil stumps in the nineteenth century that were recognised much later as the remains of a Devonian forest, and excavation of the site and of a nearby location at Cairo has revealed extensive root systems preserved in place, mapping the layout of a standing forest rather than a collection of transported fragments. The dominant tree was Eospermatopteris, known from a fossil crown called Wattieza, which grew several metres tall with a trunk that was not wood in the modern sense but a column of tissue, topped by frond-like branches and reproducing by spores rather than seeds. Alongside it grew Archaeopteris, which is much closer to a modern tree, with true wood, a branching trunk and flattened leaves, and which spread globally and dominated late Devonian forests. Root traces at the Cairo site extend metres outward, showing a substantial root system, and a third type of root trace found there belongs to a lycopsid, indicating that the forest contained several unrelated tree-like plants at once.

The consequences of tall deep-rooted plants extended far beyond the plants themselves:

• •Soil formation accelerated enormously, since roots break rock physically and secrete acids that break it chemically, producing deep soils where thin crusts had been
• •Atmospheric carbon dioxide fell substantially as weathering consumed it and as carbon was buried in accumulating plant material, contributing to cooling and eventually to glaciation
• •Oxygen rose, since buried organic carbon is oxygen that was released and not consumed, which permitted larger animals including the giant arthropods of the following period
• •River systems changed shape, since banks stabilised by roots allowed meandering channels to form where braided sheets of shifting sediment had dominated
• •Nutrient runoff into the oceans increased, which has been linked to algal blooms, oxygen depletion and a series of marine extinctions in the late Devonian
• •Terrestrial habitat structure appeared for the first time, with a canopy, shade, leaf litter and standing dead wood, which created ecological niches that land animals subsequently occupied

Wood is mostly cellulose and lignin, and lignin is difficult to break down, which created a temporary imbalance with lasting consequences. An influential hypothesis held that for a period during the Carboniferous, fungi capable of digesting lignin had not yet evolved, so dead trees accumulated without decaying and were buried in enormous quantities, producing the coal deposits that the period is named for and that powered industrialisation. That explanation has been substantially revised, with subsequent work arguing that lignin-degrading fungi appeared earlier than the timing required, and that the dominant factor was instead tectonic and climatic, since extensive low-lying tropical wetlands with high water tables and rapid subsidence provided ideal conditions for burying plant material before it could decay. The current position gives weight to both, with the evolution of decomposers mattering and the geography of the coal swamps mattering more, and it is a useful case of a clean elegant explanation being complicated by better data.

The tree is a growth form rather than a group, and it has evolved independently many times in unrelated lineages, which is visible in how differently early trees were built. Lycopsids of the Carboniferous coal forests grew to great heights with trunks supported mainly by a thick outer bark layer rather than by wood, which is structurally unlike any modern tree, and they were closely related to small creeping clubmosses that survive today. Horsetails reached tree size in the same forests and their surviving relatives are knee high. Tree ferns achieved height with a trunk of matted roots rather than wood. Seed plants took over subsequently, with conifers and later flowering plants producing the wood-based construction that dominates now. Palms are flowering plants that build a trunk in an entirely different way from a broadleaf tree. The repeated independent arrival at the same solution reflects the overwhelming advantage of height in competing for light, which is a selection pressure that applies wherever plants grow close together.

Standing forests appear around 390 million years ago, and root systems preserved in place at sites in New York map their layout. Deep roots broke rock into soil, pulled down atmospheric carbon dioxide, raised oxygen and stabilised riverbanks enough to create meandering channels. Nutrient runoff has been linked to marine extinctions. The tree is a growth form that unrelated lineages invented repeatedly, built in quite different ways.