Compatible models to predict tree crown ratio, crown length and height to the base of the crown for maritime pine (Pinus pinaster Aiton.)

Key message This paper provides individual tree models to estimate crown variables in maritime pine ( Pinus pinaster Aiton.). The estimation of crown variables, especially height to the base of the crown and crown length, is essential for forest fire simulators that require the calculation of canopy bulk density, a measure that relates leaf weight and canopy volume. Context The numerous forest fires that have occurred in the Portuguese forest ecosystems over the past decades justify, by themselves, the importance of estimating crown variables such as crown ratio, crown length and height to the base of the crown, which are also frequently used as predictors in individual tree models’ components. Aims A compatible system of equations to predict crown variables was developed for maritime pine ( Pinus pinaster Aiton.) in Portugal. Methods A database with a long time-series of measurements covering a wide variety of situations was used to fit models to estimate crown ratio, crown length and height to the base of the crown. To guarantee accuracy and compatibility among crown variables the models were developed as a compatible system of equations fit using seemingly unrelated regression. The parameters of the functions were expressed as a function of tree, site and stand variables and inter-tree competition. Results The system of equations for height to the base of crown and crown length, both accurate and consistent, is based on a set of parameters describing tree age, height and slenderness, basal area, dominant height and a distance-independent competition index, the ratio tree size/average tree size. Conclusion The equations developed in this study describe how the tree and stand variables control height to the base of the crown, crown ratio and crown length. These variables depend on tree age and dominant height, with crown ratio decreasing as trees and stands get older. Tree slenderness has a positive effect, with shorter crowns in trees that are closer to a cylinder (lower d/h ratio). Stand density implies shorter crowns and, within a stand, trees under lower competition (higher values of d/dg ) have longer crowns.