Emergence of the Rainforest in Absence of the Dinosaurs

Recently published in Science, research focusing on the plants, rather than the usual star of the show, dinosaurs, reveals new information about the evolution of rainforests. But don’t worry we will talk about dinosaurs later. In the field of paleobotany, the study of fossilized plants, studying rainforests was once thought to be impossible. The high amounts of decomposition aided by high biodiversity were thought to prevent fossilization. However, that isn’t quite true. The fossils exist, they are just super hard to find. The results of over fifteen years of work in present-day Colombia reveal the evolution of modern-day rainforest by studying the pollen and leaves from 72 to 58 million years ago.

Although millions of years have passed, the climate of Colombia now and before the end-Cretaceous eruption was largely the same. Before the extinction, researchers found pollen and spores from ferns and flowering plants with conifers towering over the rainforests. The canopies were sparser than present-day and let more light into the understory.
After the extinction, the climate change due to the extinction caused many of the conifers to die off leaving flowering plants and legume trees to dominate the forests. Today, rainforests boast thick canopies which do not let light into the understory leading to layers of canopy getting different amounts of light and many new niches. In addition, the thick canopies lead to increased evapotranspiration or the transport of water from the soil to the air through the leaves. This created high humidity which facilitates the many epiphytic species such as orchids (Read more about orchids here).

In the modern-day, Colombian rainforests show approximately 10% of the world’s biodiversity. The level of biodiversity was decimated by the extinction, which killed off 75% of all life on the plant, and it took approximately 6 million years to reach similar forests to what we see today. What factors lead to the elimination of the conifers and densification of the rainforests? Perhaps, the lack of dinosaurs. The lack of large herbivores to eat away the canopy may have allowed it to grow into the beautiful diverse habitat it is today. Another explanation is that the ash spread from the impact may have enriched the soil with valuable nutrients. Or the extinction of conifers decreased competition for flowering plants to take over.