May 19. Devonian plants: the first forests

The Devonian saw the most fundamental change in the appearance of the land in several hundred million years. Plants, which had gotten started on land maybe by very late Ordovician time and for sure by the Silurian, began to spread across the landscape. For the first time, soils with organic matter began to form in abundance. You can visualize the development of soil as a sort of symbiotic relationship with plants – chemicals from plants, plus the mechanical action of their root systems, broke up rocks and changed them to the stuff we’d call soil. Soil in turn served as a reservoir of nutrients for future plants, as well as a substrate that was softer than hard rock, a place for plants to grow. The cycle of plant growth, death, and soils had begun.

Devonian forest

Early Devonian plants were still pretty primitive, but by the end of the period many diverse plants with true leaves and root systems were covering large areas. Many were relatives of modern ferns and horsetail rushes, but early varieties of other plants, such as pro-gymnosperms, spore-bearing plants that eventually gave rise to conifers, were also around. And the first true seed-bearing plants had evolved by the end of the Devonian.

You remember Cooksonia, from April 25, back in the Silurian? In contrast to those spindly stalks a couple inches tall, the Devonian saw the development of the first woody plants – trees – and the first real forests. The oldest known tree is called Wattieza, a fern-like tree from New York dating to about 385 million years ago, the Middle to Late Devonian. Some of these early trees were more than 30 feet tall. The oldest known forest, at Gilboa, New York, has upright stumps, roots and trunks that are interpreted as part of an extensive ecosystem that can only reasonably be called a forest. The stumps and trunks had been known since 1870, but it wasn’t until 2007 that the crowns of the trees were found and connected to show the geometry of the entire tree.

Apart from bragging rights for being the oldest forest, this discovery has huge consequences for the history of the earth. If large plants were widespread on the earth’s surface, it would have had a significant impact on the atmosphere – carbon dioxide in, oxygen out, and dying plants would be returning their elements – largely carbon, into that new product on the land, soil. All of this is part of the carbon cycle, the shifting of carbon around in the atmosphere, hydrosphere, and soils and rocks of the solid earth. That, in turn, has a great impact on the nature of climate and the kinds of life that can inhabit various ecological zones.

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May 19, 1871, was the birthdate of Reginald Aldworth Daly, at Napanee, Ontario. Daly worked as a geologist surveying the Canada-U.S. boundary for many years, leading to a massive report entitled North America Cordillera: Forty-Ninth Parallel. His work also resulted in a definitive book called Igneous Rocks and their Origins. Daly served as the head of the geology department at Harvard for 30 years. He worked on impact theory, and there are craters on the moon and Mars named for him.

Carl Beck, my graduate school major professor of mineralogy, was also born on this day in 1916. He put me on the convoluted path that took me from kidney stone mineralogy to geophysics in oil exploration.

—Richard I. Gibson

Devonian landscape painted by Eduard Riou, 1872 (public domain)

Link: Gilboa fossil forest