Classic definition
The classic, restrictive definition, proposes a clear criteria for distinguishing what is a tree from what is not. However, it was composed by western scientists, who used western representations of what they called a "tree", to the detriment of arborescent tropical plants.
To be a tree, it must:
- Be tall: theoretically, able to reach (under favorable growing conditions) a minimum height above the ground. This height varies according to the definition used. For reference purposes, according to the FAO (Food and Agriculture Organization of the United Nations): a minimum of 5 metres;
- Possess secondary branches, but only at a certain distance from the ground;
- Have a single trunk;
- Have a self-supporting trunk: able to stand straight on its own;
- Have a trunk of true wood: composed of secondary xylem (a solid tissue that stores lignin) created from cambial tissue, which allows lateral (radial) growth in thickness;
- Possess a long life expectancy.
Hallé's definition
Francis Hallé, a renowned French botanist and a specialist on tropical trees and rainforests, proposed a new definition. It is based on a functional view of the tree: it is a tall plant that has evolved towards a form that raises itself above the ground in order to surpass its rivals in the search for light.
He considered a tree to be any plant that:
- Is usually perennial (lives at least two years);
- Possesses one or several trunks that grow vertically;
- Possesses the structure and anatomy of a trunk or trunks that provide sufficient rigidity to allow it to rise above plants competing for light;
- Possesses some type of photosynthetic apparatus: generally, leafy branches, or large leaves if the tree has no branches; branches that ensure photosynthesis if the leaves are too small to ensure this function or are absent;
- Theoretically reaches (under favorable growing conditions) a tall height, as compared to human proportions.
