The concept of formation usually refers to plant communities typical of large territories. These are mainly described according to their physiognomy, resulting from the spatial organisation conferred by the life forms of the predominant plants, corresponding to the mature state of the vegetation, but also includes climatic and edaphic criteria and aspects related to the environmental adaptation of member plants.
According to the land cover classification system of the FAO (Di Gregorio & Jansen, 2005), the life form of a plant is defined by its physiognomic appearance. This allows woody plants to be classified as either trees and shrubs, and herbaceous plants as either graminoids and forbs.
Trees are usually differentiated from shrubs by their height. Woody plants measuring more than 5 m in height at maturity are classified as trees, while woody plants measuring less than 5 m in height are classified as shrubs. This general rule is subject to the following exception: a woody plant that clearly has the physiognomy of a tree can be classified as a tree, even if is less than 5 m high but greater than 3 m. In this case, another physiognomic condition is added: in order to be a tree, it must have a single well-defined stem.
The Spanish classifications (San Miguel et al., 2006) differentiates between the concept of shrub and bush. Shrubs are woody plants less than 5 m in height without a well-defined trunk, since their branches start at the base, while bushes or subshrubs reach around 1 m in height. Their respective formations are called shrubland, or fruticetum, and subshrubland, or suffruticetum.
The global importance of shrub formations is not well known, since in many cases they are plant resources of secondary importance compared to tree resources. In addition, shrubs are often a successional species that appear after the disappearance of tree cover. In most cases, shrub formations are located in one or several levels below tree formations, and in other cases, when they are one of the seral stages of woodland, they can contain varying proportions of tree species, which adds to the difficulty of studying these growth forms and their importance.