Overberg Geoscientists Group
470 Mya
First Land Plants
All plant life started in the oceans. It took the development of a number of other aspects in order for the green algae from which they are derived, to be able to survive away from the water.
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These algae that developed into the land plants had to overcome environmental issues before they became established on land.
The first was the issue of desiccation.
More evolved plants control the hydration of the plant by making the outer tissue layers able to absorb moisture or prevent drying out. It is likely that until these mechanisms evolved, the plants were confined to moist environments.
These were mosses and liverworts
The exposure of land plants to a gaseous rather than aquatic environment implied that they were not supported by the surrounding water but had to support themselves. This meant the development of more physically stronger cells to enable them to grow higher.
The final major development was in their reproduction. Various mechanisms were available in the aquatic environment, but not on land.
The first of these of which we have evidence is the development of spores that were able to survive out of water and were decay resistant thus could generate more spore-producing plants.
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These spores are found fossilized in rock around 470mya.
There was also, of course, the effect of the vegetation on the land.
There had been for a long time previously biomats of bacterial stromatolites and algae existing on the land, usually along sea margins and other limited environments. Now we have plants that are increasingly covering land throughout the continents.
What was the impact of this on the Hydrological Cycle of the Earth?
Land plants affect the climate and oxygenation state of the Earth in multiple ways.
Plants accelerate the hydrological cycle through evapotranspiration and stimulate rainfall. Plants also enhance chemical weathering processes and clay formation in soils, which ultimately represents long-terms sinks.
Therefore, vegetation also affects the energy budget of the Earth, via cooling through the greenhouse gasses (drawdown CO2) both by increasing the amount of organic matter in the soils and by the use of CO2 to build cells. This cooling effect may have given rise to a longer-term global drop in temperature.
By evapotranspiration, land plants pump water from the soil into the atmosphere and affect the global hydrological cycle. Today, this recycling of moisture constitutes approximately 2/3 of the precipitation on land. As a consequence, large scale removal of tropical forests today will probably decrease global precipitation.
Both non-vascular and vascular land plants stabilize soils. Processes arising from the evolution of land plants increase surface area of crustal rocks and prolong the exposure of mineral surface to weathering.
The colonization of land by plants encompasses transitions that all may have affected the Earth system in distinct ways. The main predicted consequences of plant colonization are:
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Atmospheric CO2 decline and climatic cooling (permanent transition)
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Atmospheric O2 rise and ocean oxygenation (potentially permanent transition)
Further Reading:
Tais W. Dahl, Susanne K.M. Arens The impacts of land plant evolution on Earth's climate and oxygenation state – An interdisciplinary review. Chemical Geology
Volume 547, 5 August 2020
T.J. Algeo, S.E. Scheckler, J.B. Maynard
Effects of the Middle to late Devonian spread of vascular land plants on weathering regimes, marine biotas, and global climate
P.G. Gensel, D. Edwards (Eds.), Plants Invade the Land, 12, Columbia University Press, New York Chichester, West Sussex (2001),
Steemans, P.; Herisse, L.; Melvin, J.; Miller, A.; Paris, F.; Verniers, J.; Wellman, H. (Apr 2009). "Origin and Radiation of the Earliest Vascular Land Plants". Science. 324
C.K. Boyce, J.-E. Lee
Plant evolution and climate over geological timescales