12: Thrifty Trees- why change colour in autumn?

We often talk about the social interaction between things like monkeys, or how elephants look after their young, but trees are actually really complex in their interactions with each other too. Right now science is only beginning to understand the amazing world that goes on above and below the soil which includes wars, murder and sacrifice. Who knew trees lived such dramatic lives? Perhaps they do deserve a hug once in a while. This week we’ll be taking a look at some of the unexpected parts of tree life including why they go yellow in autumn, how they can steal and share nutrients, and how some trees may even be able to let others know when they’re afraid… Curious? Read on!

On the left is the tough, hardy needles and cones of an evergreen, the Norwegian Spruce. On the right are some deciduous birch trees dropping their leaves in autumn

Evergreen vs deciduous

Let’s start with a brief over view. There are two main types of tree, evergreen (or coniferous) and deciduous. Evergreen trees are, well ever-green keeping their leaves over winter and summer e.g spruce and pine. Deciduous trees drop their leaves in autumn and grow new ones in spring e.g maple and birch. These are two different survival strategies, which work with different success in different places. If you have leaves during a cold, wet winter, they might get frost damage and so the tree itself might be more prone to things like infection. Then it makes sense to drop leaves at the time of year when it’s cold. On the other hand, regrowing leaves takes a lot of energy, so if you have really tough leaves that can survive the cold, and if the tree is in a location that gets covered by snow so is protected from the harsh winds, it makes sense to keep the leaves year-round. You’ll notice if you look that evergreen trees do tend to have very hardy tough leaves like pine needles. Either way, leaves are important as they are the main way trees get their energy. Think back to high school biology, this is simply the whole photosynthesis sunlight=energy thing. But if the leaves collect energy, and then all of a sudden are dropped, isn’t that a massive waste? Well, nature, who doesn’t like waste, has got an excellent way of dealing with that.

A leaf in autumn. You can see that the areas near the veins of the leaf stay greenest the longest.

Autumn leaf change

Leaves are green because they are filled with a pigment called chlorophyll, which is the best colour to absorb sunlight. As winter approaches and there’s less sun and more frost risk, trees start to send less nutrient and water to the leaves. Over time the chlorophyll breaks down taking the green colour with it. This leaves (pardon the pun!) other pigments, which are red or yellow or orange depending on the tree, and as they are now longer being overshadowed by the green, they become the main colour of the leaf. This gives us the classic autumn colours. At the same time the tree keeps on sucking the nutrients and water out of the leaves until they’re a crumpled empty shell which is no longer needed. This is where they drop to the ground, where whatever nutrients are still in them will decompose into the soil and be sucked back up through the tree’s roots. A nice closed system! So next time you rake leaves, you’re actually stealing a tree’s lunch. Well, maybe not the whole lunch, but you’re taking some of their side dishes.

Here is a plant with it’s fungal network. The brown rooty things underground are roots, and all the white bits are the fungi

Sharing and stealing nutrients

So what else is going on underground?

Tree’s get nutrients from the soil through their roots. Well, actually they get it from the fungus attached to their roots. Trees, and many other plants, have a clever relationship with microscopic fungus under the ground called mycorrhizal fungi. The fungus attaches onto the root, taking sugars made from photosynthesis. In return they collect water and nutrients from the soil, like phosphorus, and pop it into the root for the plant to suck up and use. This is called a symbiotic relationship, where two things work together in nature. Another example is cleaner fish, who eat parasites off larger fish. The big fish gets clean and the cleaner fish gets lunch. Win-win!

These fungal networks that are attached to tree roots can grow throughout the soil. They can even interact with one another. It turns out that through these fungal networks trees can and do actually share nutrients. Sometimes big older trees will share with younger ones to help them grow and survive. There is debate on how useful and impactful this sharing is, but in one experiment it was found that when a ‘mother tree’ who was connected to lots of saplings around it was chopped down the saplings had a far lower survival rate as they had lost a source of nutrients.

This opportunity can also be used for evil however. Some plants like the Phantom orchid don’t have any chlorophyll. Instead they get their energy by tapping into the fungal networks of other plants and stealing their nutrients. Other plants release toxins into fungal networks to take down unwanted neighbours giving them extra room and resources to grown with. Acacias and American sycamores are known to do this.

The wood wide web, tree connected by a network underground (Picture: BBC)

Having a chat

Aside from taking and giving, it is thought that trees can actually communicate with each other and warn their neighbours of impending disaster. Some experiments have been done which have found that if a plant is sprayed with a toxic fungi or attacked by aphids they send stress signals through their fungal networks to the other trees. When the other trees get this, they can increase their defences in case they get attacked too, and so have a far better survival rate and far less damage.

This complex underground world where trees talk, fight, share and care is only just beginning to be understood so we could really be at the tip of the iceberg. Who knows what else they do! The fact that it is all so complex and interconnected has led many to call this fungal network the ‘Wood wide web’. Perhaps Tolkein’s creation of the Ents in Lord of the Rings isn’t so far-fetched after all!

For more info:

A more detailed explanation of what I’ve written: http://www.bbc.com/earth/story/20141111-plants-have-a-hidden-internet

Music: kongano.com

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