Changing the Face of the Planet
Published by The EarthSphere Blog. Feature Image: Rising Trees (©ArcheanArt)
Prologue:
We are midway through the Devonian as we trace life’s journey through time in the Forgotten Origins series. Trees arose in the middle Devonian, forever changing the face of planet Earth.
A Revolution of Vascular Plants
Trees, structured like the ones we know today, didn’t appear on the scene until about 385 million years ago during the middle Devonian — deep roots, a stout bark-covered trunk, and branches supporting a thick cover of green leaves. They probably didn’t look much different from the trees in your backyard or a city park.
The rise of vascular plants in the preceding Silurian Period was an evolutionary breakthrough, and it formed the beginning of a revolution on land. The ability to effectively transport fluids and nutrients from the ground to a plant’s distant extremities created a rich fabric of opportunity, allowing these vascular plants to exploit new niches in Earth’s emerging land-based ecosystems.
Trees evolved during a time when the climate was warm and relatively stable. Plant evolution experimented with various strategies for surviving away from bogs and open water. Competition for resources sparked new approaches, simultaneously sending plants higher above ground and deeper below the surface.
Competition to grow tall and wide was fierce because access to direct sunlight determined who captured the most energy. But extra height induced structural instability, making it difficult for the plants to stay upright. This problem with the physics of gravity facilitated growing both upward and downward, developing deep root systems to keep the growing mass above the ground from toppling over. These deep roots allowed trees to excel at capturing water and nutrients deep below the surface.
Trees
The earliest trees developed survival strategies that were so successful they are still in use 385 million years later. The rise of trees in the middle Devonian was a turning point in Earth’s history. Terrestrial ecology was forever changed, along with climate and geochemical cycles — trees became a major force shaping the surface of our planet.
Investigations into paleosol (fossil soil) in the New York Catskill mountains recently uncovered definitive evidence of fossilized root systems from the genus Archaeopteris, Earth’s earliest-known true tree. The preserved evidence shows a highly evolved root system similar to modern-day trees.
Anyone who has flown over the vast boreal forests of the Northern Hemisphere, hiked the Appalachian Trail, visited the Brazilian Rainforest, or driven through the coastal range of Oregon knows trees are the backbone of woodland ecosystems. These types of forests dominate large areas of our planet.
Trees are the anchor species around which entire ecosystems survive. We have learned the hard way; destroy the trees, and you destroy the ecosystem. The trees of ancient Devonian forests paved the way for rich and diverse terrestrial ecosystems.
Trees alter the environment by digging into the ground and breaking down rock into soil. During the process, nutrients are released for a variety of living organisms to utilize. Water and erosion carry these nutrients to the oceans, where they boost marine productivity. Trees remove organic carbon from the air, storing it in their biomass and in the soil below. This process allows the development of humus-rich soils — a perfect setting for plants, bacteria, and animals to flourish.
Earth’s new Devonian forests altered the climate and landscape, creating new dynamics for the world’s food webs and providing vast new ecological niches for evolution to fill.
The Tree Effect
The impact of trees on Earth’s history speaks to how they interact with rock, soil, and air. Deep roots allow trees to physically and chemically alter Earth’s surface. As the roots break down solid rock, they expose more surface area to weathering. Weathering involves both physical and chemical processes. The physical process produces smaller particles and allows for increased erosion as rainwater carries these particles away into the rivers and streams. Chemical weathering breaks down the mineral components of the rock, releasing nutrients. Importantly, chemical weathering also absorbs carbon dioxide from the atmosphere, altering the balance of greenhouse gases. Less carbon dioxide in the atmosphere leads to a cooler climate.
Another aspect of a more forested Earth is the effect of trees on marine food webs. A slow, steady feed of nutrients into coastal oceans allows for higher marine productivity and more robust ocean ecosystems. But if forestation results in too many nutrients entering into the waterways, then algae blooms form in the coastal oceans. Massive algae blooms release toxic compounds and cause anoxia by using all available oxygen in the water during periods of hyper-productivity.
Rapid forestation of Earth’s continents during the Devonian came at a price. It disturbed the existing balance between the biosphere, atmosphere, geosphere, and hydrosphere. The final effects of these changes heralded the end of the Devonian.
(The Forgotten Origins series is also available on ArcheanWeb)
Sources:
The 385 Million-Year-Old Tree Roots That Rewrite History (by Richard Milner; Grunge)
Rise of Trees (Source: Plant Evolution & Paleobotany)
Mid-Devonian Archaeopteris Roots Signal Revolutionary Change in Earliest Fossil Forests (by William E. Stein, Christopher M. Berry, Jennifer L. Morris, Charles H. Wellman, David J. Beerling, and Jonathan R. Leake; Current Biology)
