Shrub and Tree Growth Form
Woody species are defined as shrubs or trees by their growth form.

Shrub growth form
A combination of features are used to classify a woody species as having a shrub growth form -

  •  bushy or multi-stemmed (3 or more near the base)
  •  less than about 5 metres in height (the height of any associated earth bank is not taken into  account)
  •  vegetative growth in the bottom 2 metres

Some woody species tend to grow naturally as shrubs eg. hawthorn (Crataegus monogyna), blackthorn (Prunus spinosa) and hazel (Corylus avellana). Hawthorn is the classic hedge forming species and is almost always recorded as a shrub except when it has become very old, large and woody with apical dominance and the bottom 2m has become bare and devoid of leafy vegetative growth.

A bushy or multi-stemmed structure can be due to natural growth or suckering but it can be induced by management practices such as coppicing, flat-topping or flailing. Thus species such as ash (Fraxinus excelsior) and sycamore (Acer pseudoplatanus) which grow naturally as trees with apical dominance can be managed to maintain a shrub growth form and therefore be recorded as an integral part of the hedge shrub canopy. In some cases trees with woody outgrowths at the base which might be eg. flailed to produce a dense leaf growth would be recorded as having a shrub layer.
Tree growth form
Trees usually have a single stem with apical dominance. Also classified as trees are former shrubs which have out-grown their shrub growth form by becoming larger with thickened woody stems and apical growth dominating. For example a line of trees such as Ash along a Fence could be coppiced to induce a shrub growth form and be recorded as a Hedge. If left unmanaged, at some point it will grow out to be a line of trees again.
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