The Elements of Evolution (16-1) Trees


The first tree-sized land plants evolved during the Devonian. Stalks in the Silurian were no more than a meter in height. The Carboniferous brought towering trees: woody and tough-leafed. These measures evolved as defensive gestures and included the production of discouraging toxins. Tree sap, resins, and gums help isolate and block infections, as well as quickly healing over wounds.

To animals, trees represented another opportunity to rob the copious cradle which plant life created: begetting beetles that could chew leaves and bore into wood. Another predator-prey evolutionary cycle had begun, one that has been unceasing. Insects and plants have been at war over 300 million years.

When plants could manage it, the interplay between plants and insects eventuated into a truce. Like the symbiotic relationship with nitrogen-fixing bacteria and fungi at their roots, plants managed to cajole insects to do their bidding: trading sweets for pollination services.

Cooperation between divergent species is not readily had. Among macroscopic life, plants have been the most successful in coaxing cooperative relationships with beings in other kingdoms.

Early in the Paleozoic era, most rivers in the world were shallow and wide. The coming of trees, with their tough root systems and woody debris, diverted water in ways beneficial to life. Trees and rivers coevolved. The rise of the forests created river channels and islands, deepening rivers, altering the landscape, and creating opportunities for animal speciation, as new niche habitats opened by both the change in flora and river flow. Trees carving rivers was an extension of work by smaller plants, which created and then employed mud to make their mark on water flows, fashioning favorable wetlands.

Mudrocks were rare before the appearance of plants and common thereafter. In addition to inhibiting erosion, plants also interact with river flows and promote the deposition of fine-grained sediment. This can help armor riverbanks and slow their lateral migration, aiding the preservation of muddy floodplain deposits. ~ American geologist Woodward Fischer

During most of Earth’s history, the decay of dead plants oxidized, producing CO2. The complete process is the carbon cycle. During the Carboniferous, the bulk of carbon was deposited in peat, oxygenating the atmosphere. As carbon dioxide is a greenhouse gas, its reduction meant more solar heat reflected into space. Consequently, with land covering the polar regions, the Carboniferous was a period of global cooling.