What is a Tree?

Roland Ennos gives an excellent explanation of the evolution of trees and their differences in his book Trees: A complete guide to their biology and structure and most of the below comes from Chapter 1 of his book. But the simple version comes from Colin Tudge: “‘Tree’ is not a distinct category, like ‘dog’ or ‘horse’. It is just a way of being a plant.”

A botanical definition for ‘tree’ is ‘any plant with a self-supporting, perennial (living for more than one year) woody stem’. The main way that trees become self-supporting is through a process known as secondary growth, where a layer of stem cells around the outside of the stem divides to produce xylem tissue on the inside and phloem tissue on the outside. The xylem transports water but also gives structural strength to the tree, and this annual growth is responsible for trunk thickening.

From a biological taxonomy point of view, the tree form exists in several classes and families within the Tracheophyta phyllum, which is the phyllum within the Plantae kingdom containing all vascular plants (that is, plants with conducting vessels for water and phloem). You can read more about this in: The kingdom Plantae and where trees fit in.

The angiosperms (flowering plants), as the latest evolving and most successful class have some differences from other trees which is relevant to bonsai-ists. These differences include:

  • Angiosperms have specialised water transport vessels in their xylem which allows them to move more water more quickly than non-angiosperms (leaving these species more subject to embolisms and less drought-proof).
  • Their leaves are a lot more variable in terms of size and shape, and are often deciduous (there are a few deciduous species in non-angiosperm families but these are a minority – including Ginkgo, Dawn redwood and Swamp cypress). Deciduousness means that these trees do not need to create frostproof leaves, so they can take different, more productive forms (such as large leaves with high photosynthetic capability).
  • Even so, leaves of evergreeen angiosperms are still more productive than those of their counterparts in other families – possibly because their more efficient water transport allows for more transpiration and so larger leaves with more stomata (hence more photosynthesis).ref
  • Angiosperms produce ‘tension wood’ in response to gravity – if they detect a displaced stem they react by creating wood on the upper side of the stem to pull it back up again. Conifers do the opposite – they produce compression wood on the underside of a stem to change its position.ref

The next question you might want to ask is what is a bonsai?