forestThe First Forests

Terrestrial vegetation of the Silurian and Early Devonian was typically short and restricted to a narrow band along the water's edge. But by the early Middle Devonian(Eifelian), taller, arborescent forms evolved independently within the cormose lycopsids, cladoxylopsids and progymnosperms.

Gilboa: A Hint of Things to Come

One of the more famous Middle Devonian sites is the Gilboa "Forest" in eastern New York State. The arrangement of mudstones, coarse sandstone and tree stumps suggest that this site was a natural levee subject to successive destructive floods interspersed with stable periods in which soils accumulated and the vegetation recovered. Plants at Gilboa included ground cover, shrubs and medium-sized trees. One aborescent lycopsid, Lepidosigillaria, may have reached 5 m in height. Eospermatopteris, a tree of uncertain affinity, may have been 9 m tall. A common cladoxylopsid, Pseudosporochnus, was about 3 m tall.

The vegetation at Gilboa may have reached considerable heights, but it didn't exhibit some important characteristics of modern forests. For one, the vegetation was apparently still restricted to a narrow band along the water's edge. This narrow band, combined with the absence of webbed leaves, indicate that there was relatively little shade and litter production. In addition the Gilboa vegetation lacked deeply penetrating root systems that would have stabilized stream banks and enhanced pedogenesis (soil formation). Nonetheless, Middle Devonian vegetation such as the Gilboa "Forest" probably moderated the physical environment of freshwater ecosystems and enriched them with organic matter.

The Arrival of Archaeopteris

Vegetation resembling modern forests first occurred with the appearance of Archaeopteris in the Late Devonian. This remarkable genus has been recorded from virtually all known Devonian land masses and ranged from tropical to sub-polar paleolatitudes. These large trees (20 m or more in height) possessed webbed leaves that produced relatively dense shade. In addition, they possessed a deeper and more extensive root system that greatly accelerated pedogenesis and probably allowed them to colonize drier parts of floodplains and coastal lowlands. The advent of shade and the greater spread of trees created an entirely new terrestrial habitat, the forest biome.

This new habitat was characterized by leaf canopy that moderated temperature and humidity regimes and shielded microbes and invertebrates from ultraviolet light. The seasonal duration of these moderated conditions, however, is unclear. We know Archaeopteris was deciduous, but not whether its leaves were shed once a year (e.g., at the onset of the dry season) or throughout the year. If they were shed throughout the year then the forest floor would have been continuously moderated. But if they were shed seasonally, then the forest floor would have been exposed for part of the year.

In either case, Archaeopteris forests produced what must have been an unprecedented amount of organic matter for microbial decomposers and invertebrate detritivores. However, evidence for any increase in the diversity or abundance of terrestrial detritivores is lacking, in large part because Late Devonian terrestrial invertebrates are poorly known.

While the impact of terrestrial plant production on invertebrate detritivores remains unclear, other consequences are evident in Late Devonian sediments. One of these is the first appearance of coals. No coal deposits have been found at Red Hill, possibly because of its seasonal climate, but similar age coals have been found elsewhere in the paleocontinent Euramerica (North America and part of Western Europe). Another consequence is wildfire. Red Hill is one of the two earliest localities for which we have direct evidence of wildfires. In this case, the primary "victim" appeared to have been Rhacophyton, an ancient "fern" that was co-dominant with Archaeopteris at Red Hill. These fires appeared to have been low intensity blazes that burned low-lying material (i.e., shrubs), but spared the trees.

The first forests probably also had profound influences on aquatic systems. They contributed to the moderation of flow regimes and stabilization of stream habitats. They also probably greatly enriched streams with substantial increases of organic matter, primarily in the form of detritus. Some of this would result from plant material falling directly into the water, but most of the organic matter created by the forests would probably enter via flooding.

The rise in forests is also associated with global changes of marine systems. The widespread deposition of black marine shales during the late Middle Devonian (Givetian) and Late Devonian has been attributed to a dramatic increase in the influx of terrestrial organic matter. In turn, these black shales are associated with the Late Devonian Mass Extinction.

Diversity: More or Less

The Archaeopteris forests of the Late Devonian probably transformed terrestrial, freshwater and marine ecosystems. But another striking characteristic of these ancient forests is their apparent lack of diversity. Arborescent forms are overwhelmingly dominated by a single genus, Archaeopteris. Several leaf species of this climatically and geographically widespread genus are known, but it’s now clear whether they represent biological species or merely leaf-taxa of fewer true species. Arborescent forms are otherwise represented by a group of cormose lycopsids, which were apparently restricted to wetter sites.

Rhacophyton is the other dominant plant in most Archaeopteris forests as well as several Late Devonian wetlands. It’s not known outside of North America and Western Europe, but this may be a result of our ignorance of Gondwana macrofloral assemblages rather than a geographic restriction of this apparently prolific plant. Seed plants make their first appearance in the Famennian (late Late Devonian), but they appear to have been pioneer species that succeeded only in disturbed sites.

Another striking characteristic of the Late Devonian forests is the apparent disappearance of the forest biome following the extinction of Archaeopteris near the end of the Devonian. An increasingly diverse assemblage of seed plants, ancient "ferns" (zygopterids), sphenopsids (relatives of horsetails) and arborescent lycopsids occur in the earliest Carboniferous. Arborescent lycopsids (e.g., Cyclostigma and Lepidodendropsis) dominated many wetland sites, but floodplain and coastal lowland assemblages were characterized primarily by low-growing seed plants and zygopterids. Trees apparently didn’t become dominant again until much later in the Carboniferous.

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Scientific Papers:
Algeo, T.J., S.E. Scheckler and J. B. Maynard. 2000. "Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate." pp. 213-236. In: P.G. Gensel and D. Edwards (eds.). 2001 Plants Invade the Land: Evolutionary and Environmental Approaches. Columbia Univ. Press: New York.
Banks, H.P., J.D.Grierson, and P.M. Bonamo. 1985. "The flora of the Catskill clastic wedge." pp. 125-141 In: D. L. Woodrow and W. D. Sevon (eds.) The Catskill Delta. Special Paper Geol. Soc. Amer. 201.
Beerbower, J.R. 1985. "Early development of continental ecosystems." pp. 47-92. In: B.H. Tiffney (ed.), Geological Factors and the Evolution of Plants. New Haven: Yale Univ. Press.
Beerbower, J.R., J.A. Boy, W.A. DiMichele, R.A. Gastaldo, R. Hook, N. Hotton, III, T.L. Phillips, S.E. Scheckler, and W.A. Shear. 1992. "Paleozoic terrestrial ecosystems." pp. 205-235. In: A.K.Behrensmeyer, J.D. Damuth, W.A. DiMichele, R.Potts, H.-D. Sues and S.L. Wing (eds.) Terrestrial Ecosystems throught Time. Chicago: Univ. Chicago Press.
Cressler, W.L. III, 2001. "Evidence of Earliest Known Wildfires." Palaios 16: 171-174.
Cressler, W.L., 1999. "Site–analysis and floristics of the Late Devonian Red Hill locality, Pennsylvania, an Archaeopteris-dominated plant community and early tetrapod site." Unpublished Ph.D. Dissertation, Univ. Pennyslvania, Philadelphia, 156 p.
Daeschler, E.B. and W. Cressler. 1997. "Paleoecology of Red Hill: A Late Devonian tetrapod site in Pennsylvania" (abstract). J. Vert. Paleo, 17(3) Supplement: 41A.
Retallack, G.J. 1997. "Early forest soils and their role in Devonian global change." Science 276: 583-585.
Scheckler, S.E. 1986. "Floras of the Devonian-Mississippian transition." In: T.W. Broadhead (ed.) Land Plants: Notes for a short course. Paleontological Society.
Scheckler, S.E. 2001. "Afforestation - the first forest." pp 67-71. In: D.E.G. Briggs and P. Crowther, (eds.). Palaeobiology II. Oxford: Blackwell Science Ltd.
Image Credits:
The forest image is copyrighted © 2002, Dennis C. Murphy, (see Terms of Use).

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