By Catherine Belzung

Sometimes, when walking in a forest, I am wondering: how is it possible that two trees that are very close one to the other survive? If they were competing for resources, only one should survive. Also, how is it possible that small trees, that do not receive natural light because of the density of the surrounding much bigger trees, do not die immediately as they need natural lighting to grow? 

Scientists like Susanne Simard have found an answer to this: a forest is not a collection of isolated organisms, but a whole in which the different trees collaborate. For this, they need to receive information about the needs of surrounding plants, which is possible because it has been shown that the roots of 90% of all plants are linked through the so-called mycorrhizal network, made up of fungi, which transfer water, nutriments (carbon nitrogen phosphorus), or protective compounds in case of infection, from one to the other plant. This transfer follows the dynamic of the source-sink model, from plants in areas with high availability (the source) to plants being in habitats with few resources (the sink). For example, plants in a place with much light transfer carbon to plants located in shaded places, were light availability limits photosynthesis. This process exists not only among trees of the same species, but also between trees from different species. 

This process results in some individuals helping others facilitate the survival and expansion of the whole forest. So, it benefits all the trees of a forest.