What is a tree?

Everyone knows what a tree is: a large woody thing that provides shade. Oaks, pines and similarly large majestic trees probably come immediately to mind. A stricter, botanical definition is that a tree is any plant with a self-supporting, perennial woody stem (i.e. living for more than one year). The first question that normally comes back at this point is to ask what then is a shrub? To horticulturalists, a ‘tree’ is defined as having a single stem more than 6 m (20 ft) tall, which branches at some distance above ground, whereas a shrub has multiple stems from the ground and is less than 6 m tall. This is a convenient definition for those writing tree identification books who wish to limit the number of species they must include. In this book, however, shrubs are thought of as being just small trees since they work in exactly the same way as their bigger neighbours. Thus, ‘trees’ cover the towering giants over 100 m through to the little sprawling alpine willows no more than a few centimetres tall.

Some plants can be clearly excluded from the tree definition. Lianas and other climbers are not self-supporting (although some examples are included in this book), and those plants with woody stems that die down to the ground each year, such as asparagus, do not have a perennial woody stem.