What Is a Fossil Forest? Trees Preserved Where They Grew

Most fossil wood is transported, meaning trunks carried by water and deposited somewhere else, which tells you the trees existed and almost nothing about where or how they grew. A fossil forest is different because the trees are preserved in position, with stumps upright, roots extending into the layer beneath and that layer being a genuine fossil soil with the structure and chemistry a soil has. That combination allows the spacing of the trees to be measured, the composition of the stand to be determined and the environment to be reconstructed from the soil itself, which is why such sites are studied out of all proportion to their number. Preservation in position requires burial fast enough to bury standing trunks, which is why the mechanism is usually volcanic ash, a flood of sediment or rapid subsidence.

The routes to preservation are few and each leaves different evidence:

• •Burial in volcanic ash, which can bury a standing forest in hours and preserves fine detail
• •Rapid subsidence with sediment burial, which is how many coal-forming forests were preserved
• •Permineralisation, where dissolved minerals fill the cell spaces and preserve the internal anatomy
• •Casting, where the trunk rots away and sediment fills the cavity, giving shape without structure
• •Charring by fire, which converts wood to charcoal that resists decay and preserves cell structure
• •Waterlogging in acid conditions, which excludes the organisms that would otherwise break the wood down

The information extracted from these sites is unusually rich. Growth rings in the wood record the seasons the trees experienced, indicating whether the climate had a marked dry or cold period and how variable conditions were from year to year, which is direct evidence about ancient climate from the organism itself. Tree spacing and trunk diameter give an estimate of the density and biomass of the stand. The soil beneath records drainage, chemistry and the activity of soil organisms. Insect damage in the wood documents relationships between plants and animals. And the composition of the flora, including the understorey preserved alongside, shows which plants grew together, which cannot be determined from isolated specimens. Several sites have produced forests from periods when the relevant plants are known almost entirely from fragments.

The plants forming the oldest of these forests were not related to anything that makes wood today, which is easy to miss in a photograph of a stump. The great Carboniferous coal-swamp trees were club mosses, relatives of small creeping plants that now grow ankle high, and they reached forty metres with trunks supported largely by a thick bark-like outer layer rather than by the dense internal wood of a modern tree. Horsetails, now knee-high weeds, formed stands of comparable height. Early seed plants and tree ferns filled other roles. These growth forms have no modern equivalent, which means reconstructing how the forest worked cannot rely on analogy and depends on the anatomy preserved in the fossils themselves. Their trunks were also structurally different enough that they fell and decayed differently, which affects how the deposits formed.

The well-known sites are scattered and each preserves a different world. Upright stumps of the enormous club-moss trees that formed Carboniferous coal swamps are exposed in cliffs and quarries in Britain and North America, with one famous locality preserving animals inside hollow trunks. A Devonian site in New York State preserves the roots and stumps of some of the earliest known trees and has been used to argue about how the spread of forests changed the atmosphere. Petrified forests in the American southwest preserve Triassic trunks in brilliantly coloured mineralised wood. Forests buried by volcanic activity are known from many periods, including one in Antarctica that grew at high latitude in a warm world and experienced months of darkness each year. Each site is effectively a single moment preserved rather than a sequence.

The distinction is preservation in position, with stumps upright, roots in the layer beneath and that layer being a genuine fossil soil, which allows spacing and environment to be reconstructed. Volcanic ash, rapid subsidence and mineral infilling are the main routes to preservation. Growth rings in the wood record the seasons the trees lived through, which is direct evidence about ancient climate.