3.2 Mutual benefits and competition

Well, the dipterocarps are one of the most important family of timber trees in Malaysia and Southeast Asia. This family consists of a very large number of species, many of which have very important value as commercial timber trees.

Behind every great dipterocarp lies a team of tiny fungi.

Underneath this big dipterocarp. Oh, yeah. Look at those. Oh yeah. The mycorrhizas are very active in competing for water and nutrients and providing benefit to the tree. In fact, the trees on these poor tropical soils just wouldn't survive without them

So the tree taps the fungus for nutrients and water. By climbing to the top of the forest, Su See can get to the other side of the bargain.

Up in the canopy, the trees are photosynthesising. They're capturing the energy from the sunshine and with the carbon dioxide up here, they're going to produce sugars, which are then transported down the tree.

Down here on the forest floor is where the trees have to capture their water and their nutrients. So a proportion of the sugars, which are made up there in the canopy, come down here below ground to fuel the root system to capture nutrients. And some of that sugar finds its way out into the mycorrhizal fungus.

With a Geiger counter and a mycorrhizal seedling, Jonathan can track the process.

This shoot has received radioactive carbon dioxide in this box for just two hours. The shoot has photosynthesised and fixed some of that carbon. We'll check now and see where it is and where it's gone. You can hear the large amount of carbon that's being fixed by the shoots. And then, that carbon also is being transferred on to the root system. And then from the roots through the mycorrhizal root tips onto the external mycorrhizal mycelium that you can see here.

So carbon passes not only from the shoot to the roots, but out of the plant to a totally different organism. A more sophisticated technique reveals the fungal side of the bargain. The patch contains nutrient rich leaf litter. Within weeks, the fungus will grow towards it and start to take up phosphorus and other nutrients.

Phosphorus is one of the key nutrients in forest ecosystems that controls plant growth. And it's the one major nutrient that these mycorrhizal systems are very important in terms of acquiring from the soil.

Adding radioactively labelled phosphorus reveals what happens next.

JONATHAN: You can see straight away that we've got the radioactive phosphorus that we've added, some of it has already moved up. And this is where the plant is just outside of the imaging area, so it's moving up towards the plant. And if we look in a bit more detail, we can see firstly that it's moved through the fungus. This is the fungus connecting up to the plant. And then secondly, you can see that there are root tips here, mycorrhizal root tips, which have already acquired quite high concentrations of the phosphorus. This is the same system five days later. We can see the main pathways becoming even more evident now. And you could see the accumulation of phosphate, particularly in the root tips. Large amounts accumulating there. And some of it being transferred then on throughout the root system of the plant. The other part that's very exciting here is you see the distribution of phosphate that's being transferred around by the fungus. And particularly, to the growing tips where the demand is greatest as the fungus is growing.

This all fits in to an intricate larger system. Trees and mycorrhizal fungi create a natural network with far-reaching connections.

Well, as you can see, the canopy consists of many layers of trees and the light filters down and the light intensity gets less and less. It gets darker as you get down to the forest floor. And when you have little seedlings down in the forest floor, most of them get very little light.

Dipterocarp seedlings are often overshadowed by their parents. They're in very deep shade, where many kinds of tree wouldn't grow at all. In effect, they're waiting for dead men's shoes. If an older tree dies or falls, a gap in the canopy suddenly appears. Light floods in to fuel growth. Eventually, one of the seedlings will take the place of the dead tree in the canopy. But while they're waiting, the seedlings might be dependent on others.

Or when they germinate, they're going to encounter a web, a woodwide web if you like, of mycorrhizal fungi.

As the seedlings root grows down into the soil, it releases a cocktail of chemicals. Sugars, amino acids and nutrients, an attractive meal for soil bacteria and fungi. The mycorrhizal fungus is just one of a crowd. But the plant root also releases chemicals called flavonoids. They act as a signal, and the mycorrhizal fungus is more sensitive to its message than other organisms. Under the influence of flavonoids, the fungus grows towards the root. A subtle, molecular conversation starts to take place. Closer in, a new chemical vocabulary comes into play. Cytokinins tell the fungus to branch. Now it's the turn of the fungus to release a chemical. It in turn communicates with the plant. In response, the plant switches off its natural defences and the root hairs, now redundant, disappear. The fungus has now been recognised. The stage is set for the formation of a fully fledged mycorrhizal partnership. In microcosm, this is what happens in a forest. A baby plant joins a larger one, which already has a mycorrhizal network.

This mycorrhizal network is being supported by the larger and established plant, and the seedling growing here will become part of that network when it becomes infected and will gain the benefits of being part of that network, in terms of the uptake of nutrients.

That means that they have a ready-made system for capturing water and nutrients. And they may be getting that cheap because their parents upstairs in the sunlight up there, are producing the carbon to support this fungal web. And all these guys have to do is tap into it.

It could be a case of parental care for a nursery full of seedlings.

I think the big question outstanding is to what extent does this actually mean that plants no longer compete with each other in the conventional sense? To what extent are individual plants supporting perhaps individuals of other species, or even juveniles or seedlings of their own type through this network.

If this idea is true, the seedlings are subsidised by the mycorrhizal network. The network is supported by the canopy trees. Even more controversial, something may pass from parents to offspring via the fungal web.

It's possible that some of the carbon, which comes from the over story trees and out into the web of mycorrhizal fungi, some of that may in fact find its way into these seedlings. And that means that that gives them an extra chance of surviving down here in these low light conditions until such time as a gap in the canopy opens and then, off they go.