A very British liverwort mat. Image via Wikipedia
Evidence for the earliest colonisation of the continents by plants is in the form of spores and body fragments from terrestrial sediments of Middle Ordovician age (~470 Ma) (Rubinstein, C. et al. 2010. Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana). New Phytologist, v. 188, p. 365-369)suggest that the first vegetation cover involved simple ground-hugging plants that lacked stems of roots, very like the liverworts that I struggle to deter from my gravel drive. Vinegar is the only solution, preferably boiling, but that does not harm their spores and inevitably they re-emerge. Rearranging the gravel, of a pale pink limestone, is one of a very few means of keeping fit that I can bear, and I suppose the liverworts spice that up a little: but I do detest them. Part of their irritation is that they form an impermeable coating to what once was a passable if minor aquifer that channelled rainfall that would otherwise repeat the house-flooding that greeted me within a day of my moving in. So it was with some solemnity that I read a paper on how these damnable organisms transformed the Ordovician continental surface and the geomorphological processes that shaped it (Gibling, M.R. & Davies, N.S 2012. Palaeozoic landscapes shaped by plant evolution. Nature Geocience, v. 5, p. 99-105).
Sedimentologists have shown that rivers of earlier times formed wide tracts of ephemeral braided channels that transported and reworked sands and gravels that were not hampered by any vegetable binding agent. Floods merely accelerated the braiding and spread coarse sediment across valley floors, repeated spates washing out almost of the fines to take them ultimately to the continental shelves: there are few if any relics of Cambrian and older muddy floodplains. Moreover, untrammelled by vegetation any remaining fine material would be picked up by wind, even in humid climates, to meet the same marine fate. Overbank deposits of silts and clays, unsurprisingly, demand banks over or through which floodwater escapes from defined channels and is then delayed by low gradients away from the main flow, so to deposit the fines carried by its sluggish speed. Except in arid terrains where braided channels are still the rule, in succeeding geological time evidence grows for nowadays familiar channels, meanders with point bars and eroded opposite banks, levées and floodplains on every conceivable scale. Apparently, they became conspicuous in Silurian times and then forming 30% of all fluvial sediments by the Devonian.
Meanwhile, plants were diversifying though evolution of vascular systems that transport sap up supporting structures that emerged in parallel eventually to form trunks and branches. The consequent rise in volume and in area exposed to sunlight and photosynthesis of a plant’s tissues increased the potential to draw CO2 from the air, witnessed by changes in carbon isotopes that show carbon burial rising shortly after the mid-Ordovician from far lower values in earlier times. (Incidentally, it seems likely that such meagre colonisers as early liverworts thrived sufficiently to contribute to the cooling in the Upper Ordovician that led to sporadic glacial episodes). Preservation of wood in peats – liverworts are not implicated in any kind of fossil-fuel production – helped to maximise carbon burial by the end of the Palaeozoic Era. But trees make logs and, carried by rivers, logjams. By the Upper Carboniferous effects of damming become common in fluvial sediments, which seemed to serve the formation of islands within wide river channels.
By the present day, vegetation has come to dominate all but the most arid river systems. Even in central Australia sturdy gums able only to get water from below ephemeral river beds end up defining the flow regime and stabilising it on low relief plains that would otherwise be ravaged by sheet floods every rainy season. The authors support stratigraphic observations through the use of scaled down models of channels in vegetated areas by the cunning use of alfalfa seeded to sprout during simulated dry conditions then resuming channel flow in a flume tank.
Fossils tree stumps from Gilboa, New York (Photo credit: Dougtone)
The earliest substantial trees, represented by wood fragments rarely assignable to any particular structure, occur in the Middle Devonian (385-400 Ma). Although some groups can be differentiated, how their encompassing woodland ecosystems looked has been a mystery until recently . Being ‘priitive’ it has been assumed to be very simple, unlike the well-documented forests of the Carboniferous coal swamps. But, once in a while, a site of exceptional preservation is unearthed, one such being a palaeosol that clearly formed on the floor of a Middle Devonian woodland exposed by quarrying in New York state, USA (Stein, W.E. et al. 2012. Surprisingly complex community found in the mid-Devonian fossil forest at Gilboa. Nature, v. 483, p. 78-81). Once backfill accumulated during the quarry’s active life was removed it became possible to plot the arrangement of roots systems of the last trees to live at the site before inundation and preservation. Together with other plant material found in the ancient soil, the growing sites have been reconstructed to assess the full ecosystem involved. It was a great deal more complex than previously thought possible, with a series of tiers formed by three large tree types: tall, lollipop-like Eospermatopteris; smaller lycopsid-like trees and subsurface propagators related to gymnosperms that sprouted to form an understorey that may have climbed the larger trees in the manner of vines. Its setting was akin to that of modern mangrove swamps – by the sea – subject to sea-level change that inundated, killed and preserved the coastal woodland.