Adapting a model for even-aged Pinus pinea L. stands to complex multi-aged structures

Stone pine (Pinus pinea L.) stands have been usually managed as even-aged stands. Main objectives in management combine two main commercial productions, timber and pine nuts with other social aspects: soil protection, recreational use and biodiversity conservation. Multifunctional management, together with the occurrence of successive events affecting regeneration have oriented managers to propose a management schedule based on the establishment and preservation of a low-stocking multi-aged complex structure on favourable locations. Despite the recent effort on modelling growth and yield on even-aged stands of stone pine, no studies focusing on modelling dynamics for uneven-aged stands have yet been developed up to present.In this study, a proposal is presented for adapting and calibrating an existing tree-level model, originally developed for even-aged stands of stone pine (model PINEA2), to multi-aged complex stands. Data from four multi-aged trials and 61 plots from the National Forest Inventory were used to adapt the whole set of functions included in the original model. In our study, four different methods have been proposed to adapt the original equations: (1) direct validation and re-parameterization; (2) size class modelling; (3) refit of functions after removing typical even-aged covariates; and (4) multilevel calibration. Adaptation is based on assuming that a multi-aged stand of stone pine can be seen as the sum of independent, smaller, even-aged groups. The low densities of the stands, the early liberation of the most vigorous trees in all size classes and the major importance of root-level competition for water in Mediterranean forests are the main factors explaining these particular dynamics. Results show the suitability of the proposed method, attaining unbiased estimates with a degree of accuracy similar to that achieved in applying the original model to even-aged stands. The adapted model (PINEA_IRR) constitutes a flexible tool for the management and maintenance of stone pine stands, covering a wide range of within stand structural complexity, including forests in transition.