Episode 225 – Forests

Listen to Episode 225 on Podbean, YouTube, Spotify, or wherever you listen to your favorite podcasts!

Put together a whole lot of trees, a certain amount of heat and precipitation, and a variety of animals and plants, and you’ve got one of the most important biomes on Earth. This episode, Aly joins us to explore the diversity and evolution of Forests.

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Deer from the Gray Fossil Site

For the Trees

There are various ways to define a forest, but generally speaking it’s a biome with enough trees to block out a good portion of the sky. Because of all these trees, forests tend to have a variety of habitats not only spread out horizontally, but also vertically, with each layer from the floor to the canopy receiving different amounts of light, water, and animal activity.

Broadly, there are five major categories of forests in the world today:

• Boreal forests are limited to high latitudes of the Northern Hemisphere, and sometimes high elevations. These are home to relatively few tree species, most of which are evergreen conifers. This is the most widespread terrestrial biome on our planet.
• Temperate forests come in wet (temperate rainforest) and dry (deciduous forest) varieties. These exist at various middle latitudes and contain a mixture of tree types, including in some cases the tallest trees on Earth.
• Tropical forests also come in wet (tropical rainforest) and dry/seasonal forest varieties. Found close to the equator, these are highly variable and often very biodiverse habitats, home to some 50% of living species.

Paleobotanists study the structure of ancient forests by examining the fossil animals that lived there, the fossil plants – features like leaves can be very useful for understanding ancient temperature and precipitation – and ancient soils (paleosols).

Earth’s first forests developed during the Devonian Period, more than 360 million years ago. These were sparse forests, home to a variety of unusual tree-like plants, and they ultimately gave way to the expansive forests of the Carboniferous Period, which spread across the globe and left behind extensive coal deposits.

Although the general shape of a forest has remained the same for the last 350 million years or so, the trees themselves have changed, from the early tree-like plants of the oldest forests to the gymnosperms of more familiar-looking habitats to the angiosperm-dominated forests that we still have today.

Through time, there have even been forest types that don’t exist today. Before modern tropical rainforests first appeared around 60 million years ago, earlier tropical forests were more open-canopy and with their own distinctive ecosystems. During the warm parts of the Mesozoic Era, dinosaurs and other species lived in polar forests that persisted at higher latitudes than any forest could exist today; exactly how these forest ecosystems functioned is still a bit of a mystery. And modern boreal forests couldn’t exist until the Earth cooled down closer to modern-day temperatures around 30 million years ago.

Learn More

Tropical rainforests and the need for cross-continental comparisons (technical)

Triassic climate and the rise of the dinosaur empire in South America (technical)

Biomes of Earth: Terrestrial, Aquatic, and Human-Dominated (technical)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 73 – Trees
• Episode 115 – Biomes
• Episode 195 – Leaves

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