In a New Zealand forest, the trunk of a tree, with the naked eye, remains alive by clinging to the roots of neighboring trees, exchanging water and resources through the grafted root system. That is the conclusion of a study published in iScience, which details how the surrounding trees keep the trunk alive, possibly in exchange for access to larger root systems. The findings suggest a shift in the perception of trees as individuals towards the understanding of forest ecosystems as “superorganisms.”
A team led by Martin Bader and Sebastian Leuzinger decided to investigate how nearby trees kept the tree stump alive by measuring the flow of water both in the stump and in the surrounding trees that belong to the same species. What they found is that the movement of water on the tree stump was strongly correlated with that of the other trees.
These measures suggest that the roots of the stump and those of the surrounding trees were grafted together, according to the authors. Root grafts can be formed between trees once a tree recognizes that a nearby root tissue, although genetically different, is similar enough to allow for the exchange of resources.
“This is different from how normal trees work, where the flow of water is driven by the water potential of the atmosphere,” explains Leuzinger. In this case, the stump has to follow what the rest of the trees do, because since it lacks breathable leaves, it escapes the atmospheric force. For the trunk, the advantages are obvious: it would be dead without the grafts, because it has no green tissue of its own. But why would green trees keep his grandfather alive if he gives them nothing in return?
One explanation, the authors point out, is that root grafts were formed before one of the trees lost its leaves and became a stump. Grafted roots expand tree root systems, allowing them to access more resources such as water and nutrients, and increase the stability of trees on forest slopes. When one of the trees stops providing food, this can go unnoticed and, therefore, allow the stump to continue its life.
“This has consequences of great importance in relation to our perception of trees. Possibly we are not dealing with trees as individuals, but with the forest as a superorganism,” concludes Leuzinger.
During a drought, for example, trees with less access to water may be connected to those with more access to water, allowing them to share this resource and increase their chances of survival. However, this interconnectivity could also allow the rapid spread of diseases.