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The trials and tribulations of trees

Despite massive deforestation over the past 10,000 years, there are still 400 trees for every one of the eight billion people on Earth. They populate every continent on Earth except Antarctica. The tallest tree is almost as tall as the Great Pyramid of Giza. The smallest is about the size of a golf tee. Yet such a wide variety in distribution and size begs the question: “What do they all have in common? What makes a tree a tree?” Well, where better to find out than on the Tree Trail in Cambridge University Botanical Garden. I’m on my way to the oak expert who can help me separate the wood from the trees…

Raffy – Hi, I’m Raffy. I work in the learning team at the University of Cambridge Botanic Garden.

Will – And we’re here on the Tree Trail. This is a beautiful tree trail and there are redwoods and pines and even a few magnolias. Such a wide variety of trees. But how can you look at something and say, “That’s definitely a tree,” as opposed to something that might be similar but say, “That’s definitely not a tree.”

Raffy – Well, a tree is a plant that is primarily characterized by its woody structure and its ability to grow in height and girth by secondary thickening. So that is the growth on the trunk or stem that allows it to become wider and thicker. Essentially, a tree is a plant with an elongated trunk, branches and leaves that face the sun.

Will – How many trees do we estimate there are in the world?

Raffy – There are about 60,000 tree species known to science worldwide, so trees can adapt to almost any niche in the world. You can find them from tropical rainforests to boreal forests and everywhere in between.

Will – When we think of trees in the context of climate, the first thing that probably comes to mind is their ability to store carbon. They are a carbon sink, a term that has been quite hotly debated in recent years. But what does that actually mean?

Raffy – Trees can store carbon through photosynthesis. They capture carbon from the air, absorb carbon dioxide, convert it into energy and store this energy as biomass in their trunks, leaves, roots and their associated microbiota, i.e. their mycorrhizal fungi in the soil.

Will – When do we think trees first appeared?

Raffy – Tree-like plants first evolved in the Devonian period, 360 to 390 million years ago. And these trees were not like today’s trees. They resembled our ferns and horsetails, very ancient tree-like plants. And they actually radically changed the Earth. They helped form the first soils and also changed the atmosphere. They took in carbon dioxide and gave off oxygen, which paved the way for the development of more complex ecosystems.

Will – And ironically, that compression that occurred many hundreds of millions of years ago is responsible for many of the fossil fuels that are causing them so much trouble today.

Raffy – Exactly, yes. Those first trees created the charcoal forests that we use today.

Will – Incredibly, there’s a break in the clouds on this beautiful June afternoon. Shall we go and look at some trees?

Raffy – Yes. Let’s go along the tree path.

Will – We have a beautiful tree here. We have pink and white petals.

Raffy – This is a horse chestnut. It’s on our tree path. It has these wonderful, stocky, candle-like flowers, pink and yellow, and they actually change colour depending on whether they’ve been pollinated or not. So they’re yellow when they’ve not been pollinated to attract insects. And they turn red to indicate that they’ve been pollinated and there’s no point in an insect visiting the flower. And that’s actually quite common in this soapberry family. So you’ll find it in species related to the horse chestnut as well.

Will – I just assumed it was a clumsy bee that stumbled out of the plant and just spread pollen everywhere.

Raffy – No, it’s a really good indication of whether it’s worth visiting or not. The same goes for forget-me-nots. They change colour when they’ve been pollinated. They’re bright yellow and then turn white when they’ve been pollinated.

Will – This is a remarkably knotted tree with a sort of mottled, almost camouflage-like bark. What do we see? We

Raffy – We’re looking at the Persian Ironwood tree. It’s called Parrotia. It’s a beautiful tree. It’s quite old. It was planted in the late 1800s but it’s quite small. It still grows very slowly and its wood is very dense, it actually sinks in water. It’s so dense. The remarkable thing about this tree is the way the branches have fused together. This process is called inoculation, which actually comes from the Latin word “kiss,” and they’ve joined together in several places and formed this web. So it’s a really beautiful tree to stand under. And you’re right, the pattern of the bark is like camouflage.

Will – We’re still pretty close to the road here, but since we’ve stepped under it, all noise has completely disappeared.

Raffy – I like that. It’s like coming into a dome. Yeah, it’s a really nice, quite private space.

Will – Thank you for taking me on this wonderful tree walk. I would be doing you a disservice if I didn’t ask. How can people get involved?

Raffy – The great thing about the tree trail and a lot of our other trails is that they’re open all year round, so you can pick up a leaflet at the ticket office or look at it online, both of which give you the opportunity to enjoy a curated route through the garden for free. So we’ve got a range of trails, um, in addition to the tree trail. We’ve got medicines from plants, dyes from plants, plants that have inspired design and technology, a really wide range of themes. So you can take your pick. And summer is a great time to come and do them. I said they’re open all year round. Summer is the best time.

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