In recent years, there has been an expansion of mixed-species forest areas of cork oak (Quercus suber L.) and stone pine (Pinus pinea L.) in Portugal. This tendency requires the development of forest growth models to support the management of these stands and optimize productivity. In forestry, the total height-diameter (h-d) relationship and the height to the crown base (hcb) are fundamental to forest inventory, especially when direct total height or hcb measurements are unavailable.

This study aimed to develop generalized h–d models for cork oak and stone pine, as well as an hcb–d model for cork oak growing in pure stands or mixed stands with stone pine across Portugal. The analysis was based on data from the 5th and 6th National Forest Inventories and from permanent research plots. The main hypothesis was that the inclusion of species composition variables would better characterize stand structure and improve model performance.

The models were fitted using nonlinear least-squares and nonlinear mixed-effects models for both h–d and hcb–d relationships, and generalized additive models for the hcb–d model. Model parameters were expressed as linear functions of stand characteristics, competition indices, and variables describing species mixture. The results showed that the sequential inclusion of stand variables, competition indices, and species composition variables significantly improved the h–d models for both cork oak and stone pine. For the cork oak hcb–d model, including stand variables and a species composition index also improved model performance. Overall, this research provides practical tools for estimating tree height and height to crown base in mixed-species cork oak and stone pine stands. These models can support forest inventory, improve stand-level assessments, and are expected to support decision-making in Mediterranean forest management.