Evaluation of age-independent methods of estimating site index and predicting height growth: a case study for maritime pine in Asturias (NW Spain)

Site index is the most common measure of forest site productivity used worldwide, serving as an important baseline for forest-level planning and helping to formulate silviculture strategies. Site index is usually obtained through measurement of dominant height and stand age. However, age is not available in many cases, such as in most national forest inventories or when it is not a main determinant of growth. Age-independent methods have been developed to overcome this problem, allowing modelling of the dominant height growth and estimation of site index in the absence of age data. The purpose of this study was to compare the most widely used age-dependent method and several age-independent alternatives, namely the growth intercept method, the age-independent difference equation method and an iterative method proposed in this study. We used stem analysis and permanent sample plot data from Pinus pinaster Ait. stands in Asturias, Spain, to test the alternative methods. It was found that for site index estimation, a 7-year growth intercept was the most accurate method. However, for prediction of dominant height growth, an age-independent equation that depends on climatic factors performed best, even better than the traditional age-dependent site quality curves. The iterative method proposed in this study proved to be the least accurate alternative, although it performed well in predicting height growth for long intervals starting from low heights. Finally, we provide recommendations regarding when the different methods should be used.