Crown and tree allometry of open-grown ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.)

The aim of this study was to set up allometric models on the development of open-grown trees (OGT) of ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.). Sampling was carried out during a study on valuable broadleaves in Europe and covers a geographical location from 0° to 9° eastern longitude and 47° to 51° northern latitude in Western Europe. We set up models describing (I) crown allometry, taking into account possible effects of (II) site-specific differences and (III) species effects. In addition, simulations were used calculating the potential width of the knotty core inside the stem for self-pruned OGTs. The models also procured insight into space occupation of OGTs. This investigation on crown and tree allometry for a broad range of tree size complements dynamic models on growth of broadleaved trees only monitored at young ages and for a small number of years. It quantifies the relationship between tree height, stem diameter, height-stem diameter ratio, height to crown base, and descriptors of crown form. Only for the models on the crown width/length ratio (WL ratio) and on the crown ratio were there no significant allometric differences between the two species. For none of the models were site effects found to be significant. Self-pruning of ash and sycamore OGTs was slow and the simulated knottiness of the trunk was considerable. Therefore artificial pruning is necessary for a long clear bole, although both species are known for fast self-pruning. In addition, comparison of crown allometric models for forest trees and open-grown ash and sycamore trees underlined the need for specific models in agroforestry based on OGT. Specifically, when tree diameter being equal, OGTs have larger crowns compared to trees from closed forests. During tree development crown form changes uniformly with no significant species-related difference: The vertical crown expansion of young ash and sycamore overweighs the lateral expansion. As trees mature, the crown WL ratio increases leading to crown width values almost equal to crown length. Thus in their early stages of development ash and sycamore OGTs invest more in the vertical space occupation and only at later stages in horizontal expansion.