Dawn of Trees

Around 390 million years ago (Middle-Late Devonian), the Earth was relatively warm and stable, allowing for rapid evolution of plants.

Plants were increasing in diversity and size on the land, and progressively evolving to survive away from water. 

During this time, competition for light caused plants to grow taller and wider, eventually evolving leaves.

Competition for water and minerals, caused plants to grow deeper, eventually evolving elaborate roots.

Internally, plants evolved tissues to both support the increased height, and transport water and minerals from roots to leaves.

Throughout the landscape, the first tree-like plants begin to appear.

We see trees everyday, but rarely stop to think about what defines the term "tree".

• Some people may indicate that trees are large plants, but I'm sure you have seen a small tree in your life. 

A loose definition would describe a tree as a plant that has a single stem coming from the ground, which can attain larger heights using specialized cells. 

A stricter definition indicates that trees have special tissues, called wood, and they increase in girth over time.

By strict definition, pines and oaks are trees, because they possess wood, and increase in girth. On the other hand, palms or banana plants would not be considered trees, because they lack wood and the ability to increase in girth.

The earliest tree-like plants lacked wood, and did not increase in girth over their lifetime.

The earliest known tree-like plants are the Gilboa trees (Eospermatopteris), known from fossils dating back 385 million years ago (late Middle Devonian Period)

This plant was from an ancient and strange fern-like group, called the Cladoxylopsids

These plants attained heights of 25 feet, looking slightly like a modern trunk

• This increase in height is incredible given the fact that plants were about 6 feet tall during the early Middle Devonian (~390 million years ago). Plants were getting tall, quickly...

Cladoxylopsids had very complex anatomy for the time period, but did not possess wood like a pine or oak tree

• In fact, the details of how they grew is still a mystery given the limited fossil record of these plants

A strange feature of these trees was that they did not possess leaves! 

• They had a complex branching, which allowed stems to get smaller and smaller, but there is no evidence of leaf blades

• They probably had green twigs which did photosynthesis, and it was adequate for the time period

Since then, tree-like forms have evolved several times in history

• In fact, almost every plant group in evolution, including ferns, clubmosses, horsetails, and seed plants, have all evolved tree-like forms. 

The first recognized true trees (strict definition), which produced wood and increased the girth of the trunk was Archaeopteris.

This plant is from an extinct group called the progymnosperms. 

• Archaeopteris had a woody trunk and side branches similar to a conifer tree.

• This plant also had leaves, that looked like fern fronds with spores. It was not a seed plant!

Nothing like this plant exists on Earth today (i.e. Large woody plants that are spore-bearing)

This type of growth found in the progymnosperms set the stage for plants that would eventually reach heights of 350-400 feet (e.g Sequoia)

Sparse forests with large, tree-like plants appear by the Middle Devonian, about 385 million years ago

• Many of these earliest trees, like the Gilboa trees, would have been narrowly-constructed, and without leaves would provide little shading of the ground

• The Gilboa landscape was probably sparsely populated, with some larger vines growing underneath them

Also during the Middle Devonian, clubmosses evolved into larger tree-like forms, living in wetland environments forming the first swamps

By the Late Devonian, about 380 million years ago, true trees (Archaeopteris) created the first forests

• With large leaves and extensive root systems, they were able to survive in many different habitats

• Early ferns, such as Rhacophyton, are found in the understory of these forests, adapting to shady conditions created by these trees

Early seed plants also evolved in the understory of forests and swamps, and became taller later in geologic time. 

Increased diversity of plant life means that photosynthesis on Earth was happening at an amazing rate

• All of these plants were absorbing and removing large amounts of carbon dioxide from the atmosphere

• This creates a global cooling event as well, as a short, intense glaciation that occurred at the very end of the Devonian (~360 million years ago)

The abundance of plants may have also effected the life in the oceans

• The roots of these plants grew deeper and wider in the soil, loosing soil and minerals

• These nutrients were washed into rivers and oceans, along with large amounts of plant debris

Both the global cooling, and the massive amounts of nutrients, caused the mass extinctions of animals in the tropical oceans (Algeo et al. 1995)

This event known as the Late Devonian Extinction

• Half of known (aquatic) animals go extinct during this time; this event has little effect on plants