The first forests

Sometime around 500 million years ago, green algae floating in shallow coastal waters adapted to take advantage of the nearby dry land. Two major obstacles faced the early pioneers: the need to support their bodies and the threat of drying out. While some terrestrial plants (mosses, liverworts and hornworts) adapted to survive periods of dessication but remained dependent on water, others overcame these challenges by developing roots to anchor them, a waxy layer to prevent water loss and vascular tissues to transport water and provide structural support. Fossils like Cooksonia and Aglaophyton show how these features were developed and elaborated over tens of millions of years, adjusting the shape and structure of plants for a terrestrial life.

The plants of the early Devonian (420-390 million years ago) were relatively small; most were only a few centimeters tall, though Psilophyton could reach tens of centimeters in height. By the mid-Devonian (around 390 million years ago), tree-like ("arborescent") plants appear in the fossil record, rising to dominance through the Carboniferous (360-300 million years ago). The Carboniferous is named for these early forests, which became a rich supply of the coal that fuels our precarious civilization (carbo='coal' + ferre='to carry'), though coal is also found in beds from other geological eras. Carboniferous forests were composed of lycopsids (club mosses), sphenopsids (horse tails), and ferns; in the mid- to late-Carboniferous, progymnosperms and early seeds plants appeared at tropical latitudes as ferns overtook lycopsids and sphenopsids to become the dominant plants in the forests.

The earliest trees known in the fossil record are Wattieza and Archaeopteris (which shouldn't be confused with the similarly named Archaeopteryx). Around 10m tall, Archaeopteris was a progymnosperm, a group of trees that looked like modern gymnosperms but reproduced via spores instead of seeds. Archaeopteris is considered a likely ancestor of extant gymnosperms. Capped with large fronds, it had a woody stem with annual growth rings recording how it thickened over the course of its life; like gymnosperms, it sported an extensive root system. Unlike gymnosperms, however, Archaeopteris also reproduced via spores borne on special leaves, in the same manner as ferns do today.

The forests of the late Devonian and early Carboniferous saw the rise of the arborescent lycopsids ("giant club mosses") such as Lepidodendron and Sigillaria. Lepidodendron, which could grow up to 35 meters in height, was anchored in place by a root made up of several branching axes with spirally arranged rhizoids called stigmaria. The bark carried a distinctive pattern of diamond-shaped scars from the leaf bases and would have been green, since it was photosynthetically active.

The understory of the lycopsid forests was made up of arborescent sphenopsids — tree-like horsetails — like Calamites. The stems of Calamites consisted of a series of segments, similar to modern horsetails, with whorls of branches or needle-like leaves at the nodes between segments. Calamites were well-adapted to the wet and swampy conditions of the time and, though they were prominent in the first forests, probably didn't compete directly with the lycopsids, growing instead at watersides and in clearings. In addition to reproducing by spores, Calamites could also produce clones from its extensive rhizome network.

Tree ferns such as Psaronius were present in Devonian forests but only became common in the Carboniferous. Tree ferns have an apical crown of large fronds bearing spores on their underside; the stem is supported by a mantle of adventitious roots. Unlike arborescent lycopsids and sphenopsids, tree ferns didn't go extinct and can still be found in some parts of the world today.

While the rise of seed plants was still to come, two early seed plants also appear in the forests of the late Carboniferous: Medullosa and Cordaites. Medullosa, a pteridosperm (seed fern), looked similar to tree ferns, with an unbranched stem and a crown of large fronds. Cordaites was one of the largest trees of the Carboniferous, reaching 30 meters in height with upwards growing branches sprouting from the stem.

Vast though they were, these early forests would have been entirely unfamiliar to the modern eye. The Carboniferous and Devonian were warm and wet periods rich in carbon dioxide; towards the end of the Carboniferous, the climate became more cool and dry, glaciers spread over southern Gondwana, and the forests collapsed. Dominated by trees now long extinct, they gave way to gymnosperms in the Permian, with the more familiar conifers coming into their own in the Triassic. The remains of the first forests were buried under the soil and decayed into peat which compressed and heated over time, eventually becoming coal. It saddens me to think that we have burned so quickly through these early forests, the likes of which we will never see again.

Further reading
Willis, KJ and McElwain, JC The Evolution of Plants New York: Oxford University Press: 2002.

Image credits
Both reconstructions are from Wikimedia Commons.