What makes a tree a tree?

It was once a much bigger number, but these days about 31% of the Earth’s surface is covered by forests, and they’re home to about 3 trillion trees. Trees store carbon; they give out oxygen; and they’re also nice to look at, meaning they enrich the environment in many ways. Adam Murphy spoke to Ángela Cano from the Botanic Gardens in Cambridge about what makes a tree...

Adam - I love trees. I might be a city boy, but put me in a forest, and I'm quite happy to have a good time wandering around, looking up to the canopy. There's nothing quite like being surrounded by trees to make you feel good, but I'm not. There's a pandemic on and I'm stuck inside. So to make myself feel better, I asked Ángela Cano from the Botanic gardens at Cambridge - what is it that makes a tree, a tree?

Ángela - A woody plant that usually has a main trunk, right? This is opposed to, for example, a herbaceous plant that doesn't have wood in its tissues

Adam - That seems pretty straight forward. So how do trees grow differently to make them a tree and not just some weird big vine?

Ángela - Well, all plants grow through a process that is called photosynthesis, where they actually capture the light of the sun and the CO2 from the environment. And through the roots, they also take water from the ground. And with these three elements, they transform them into sugar and oxygen. And the difference between a herbaceous plant and the tree is that the tree can stock that sugar in the trunk and sugars are basically made of carbon. And that's why we think that trees are really good at stocking carbon because their stems basically are made of carbon in a solid state.

Adam - And what is that's going on in those cells to make them so strong?

Ángela - The cells in the stem of a tree, they are, most of them, dead. We call that lignified. So the cells of all plants, they have this wall inside. That's what gives the plant the structure. But then these cells in particular, their walls are full of lignin, which is a compound that is made of carbon and gives the really hard structure of the wood. So if you compare a herbaceous plant that can't really grow tall because they don't have that structure; the tree, because it is able to transform that sugar into really hard lignin it can provide some structure and grow really tall. Because trees, eventually what they are looking for is to reach the light. It's an adaptation to compete against other plants for the light, right.

Adam - How long can trees stick around for, is it a flash in the pan or they're here forever?

安琪拉,这里有各种各样的范围。因为se you have what we call pioneer trees that grow very quickly, but then they also die quite fast. And so these are trees that, for example, if you think of a forest and a tree has fallen and then suddenly there is a lot of light in the ground of the forest, you have seeds that are waiting for light to germinate. And so the first seeds that germinate are those that are called pioneers and they can germinate and grow very quickly, but they will die also quite quickly because their wood is not really good quality, right? But then you have these other trees that can grow under less light, that can stand shaded conditions, but they grow slower and these can be really, really old. So there are trees that are thousands of years old. So there is a whole range. The one of the oldest trees that we know about, it's actually a population of trees. The species is called populus tremuloides, and it's found in Utah in the United States, this is what we call in English 'quaking Aspen'.

Adam - There Ángela's talking about Pando, a group of quaking Aspen that is by some measure, the heaviest organism in the world.

Ángela - So these trees can actually form colonies. A single individual has multiple stems that all individually look like a tree, but it's actually, if you look at the roots, they are all connected. It's only one individual that makes clones. And this specific community, if you want, is thought to be 14,000 years old.