302 Reference tree
Two different types of reference trees are distinguished:
A: local reference tree or a conceptual (imaginary) tree
Defined here as the best tree with full foliage (0% defoliation) that could grow at a particular site, taking into account factors such as altitude, latitude, tree age, site conditions and social status.
This tree should represent the typical crown morphology and age of trees in the plot.
B: absolute reference tree
The best possible trees of a genotype or species, regardless of site conditions, tree age, etc.
The reference tree can be
a healthy tree (of the same crown type) in the vicinity
a photograph locally applicable, representing a tree with full foliage
a conceptual (imaginary) tree.
A number of photo guides exist which provide guidelines on absolute reference trees in different parts of Europe.
The concept of the reference tree is one of the most controversial issues in the monitoring programme, yet it is crucial for the assessments. Use of absolute reference trees may lead to higher defoliation estimates than the application of local reference trees, but the results are perhaps more amenable to temporal and spatial analyses. Most countries have adopted local reference trees as standards. This local reference takes into account the build-up and the development stage of the tree.