303 Assessable crown
The estimation of crown condition strongly depends on the definition of the assessable crown.
The crown present at the moment of the assessment is to be considered, regardless of the potential or theoretical crown which may have existed in previous years.
The influence of any present or absent trees on the crown of the sample tree must be taken into account when determining its condition. In cases where the sample tree crown is influenced by competition, the assessable crown includes only those parts that are not influenced by other crowns (i.e. shading). Parts of the crown directly influenced by interactions between crowns or competition are excluded (see figure below).
The following parts of a crown must be excluded from the assessment:
- Epicormic shoots below the crown
- Gaps in the crown where it is assumed, that no branches ever existed
The assessable crown includes recently died branches, but excludes snags that have been dead for many years (i.e. which have already lost their side-shoots). Snags represent the historic mortality of parts of the crown and have no influence on the current condition of the tree. They are therefore excluded from the assessment. Dieback of shoots and branches represents an active process in the crown and is therefore included.
The definition of the assessable crown varies between countries. It is therefore essential that for each country, region and tree species the definition of assessable crown is documented.
In coppice (and macchia) stands it may be necessary to consider the assessable crown as a single unit consisting of crown parts from different stems.
A suggestion of the assessable crown for freely grown trees and trees in the stand:
A - freely grown trees (assessable crown upwards from the black line)
B - stand (lighter colour indicates assessable crown)