Despite rampant deforestation in the last 10,000 years, there are still 400 trees for every one of the eight billion people on Earth. They occupy every continent on Earth, save for Antarctica. The tallest is almost the same height as the great pyramid of Giza. The smallest is about the size of a golf tee. But such a massive diversity in distribution and size begs the questions, ‘what do they all have in common? What makes a tree a tree?’ Well, what better place to find out than at the Tree Trail at Cambridge University Botanic Garden. I’m off to meet the oaks-pert who can help me separate the wood from the trees...

Raffy - Hi, I'm Raffy. I work in the learning team at Cambridge University Botanic Garden.

Will - And we are on the Tree Trail here. This is a beautiful tree trail and you've got redwoods and pines and even a few magnolias. Such a wide variety of trees. But how are you able to look at something and go, 'that is definitely a tree,' as opposed to looking at something which may be similar, but go, ' is definitely not a tree.'

Raffy - Well, a tree is a plant that's primarily characterised by its woody structure and its ability to put on growth in height and girth by secondary thickening. So that's growth at the stem or the trunk that allows it to get wider and thicker. Essentially, a tree is a plant with an elongated stem, branches, and leaves that are held up to the sun.

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

Raffy - There are around 60,000 species of trees in the world that are known to science. So trees can adapt to nearly every niche in the world. They're found from tropical rainforests to boreal forests and everywhere in between.

Will - In terms of what we think about when we think of trees in terms of climate, probably the first thing that springs to mind is the fact that they can store carbon. They are a carbon sink, which is a phrase that's become quite a flashpoint in recent years. But what does that actually mean?

Raffy - Well, trees can store carbon by photosynthesis. So they fix carbon from the air, they take carbon dioxide and they turn it into energy and they store this energy as biomass in their trunks, their leaves, their roots, and their associated microbiota, so their mycorrhizal fungi in the soil.

Will - When do we first think trees came about?

Raffy - Tree-like plants first evolved 360 to 390 million years ago in the Devonian period. And these trees weren't like modern trees today. They resembled our ferns and our horsetails, very ancient tree-like plants. And they actually changed the earth radically. They helped contribute to the formation of the first soils, and they also changed the atmosphere. They pulled in carbon dioxide and they released oxygen paving the way for more complex ecosystems to develop.

Will - And in a fit of irony, that compaction that happened so many hundreds of millions of years ago is responsible for many of the fossil fuels that are putting them in so much trouble today.

Raffy - Exactly, yes. Those first trees created the coal forests, which we are using today.

Will - We have a gap in the clouds unbelievably on this beautiful June afternoon. Shall we go have a look at some trees?

Raffy - Yes. Let's walk the tree trail.

Will - We've got a gorgeous looking tree here. We've got pink and white petals.

Raffy - This is a horse chestnut. So this is on our tree trail. And it has these wonderful stumpy 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 haven't been pollinated to attract insects. And they turn red to show that they have been pollinated and there's no point of an insect visiting the flower. And that's actually quite common in this family in Sapindaceae. So you'll find that 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 sort of rubbed pollen everywhere.

Raffy - No, it's a really good indication whether it's worth visiting or not. It's the same with forget-me-nots as well. They change colour when they've been pollinated. They're bright yellow and then they turn white when they've been pollinated.

Will - This is a remarkably tangled up knotted tree with sort of speckled mottled, almost camouflage-like bark. What are we looking at? We

Raffy - Are looking at the Persian ironwood. 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. Still grows really slowly and its wood is very dense, actually sinks in water. It's so dense. The remarkable thing about this tree is the way that the branches have fused with one another. It's a process called inoculation, actually comes from the Latin to kiss, and they have, they fused at multiple points and formed this webbing. So it's a really gorgeous tree to stand under. And you're right, the pattern of the bark is just like camouflage.

Will - We're still fairly close to the road here, but having stepped under it, all sound is just dropped completely.

Raffy - I love that. It's like coming in, coming into a dome. Yeah, it's a really nice, quite private space.

Will - Well, thank you so much for taking me down this exquisite tree trail. I would be doing a disservice then if I didn't ask. How can people get involved?

Raffy - The great thing about the tree trail and many of our other trails is that they're available all year round. So you can pick up a leaflet from the ticket office or you can see it online. Both offer free for you to enjoy a curated route through the garden. So we have a selection of trails, um, along with the tree trail. We've got medicines from plants, dyes from plants, plants that have inspired, um, design and technology, real breadth of subjects. So you can take your pick. And summer is a great time to come and do them. I said that they're there all year. Summer's the best time.