Assessing site quality using the National Forest Inventory

Abstract

Site productivity of forests is often evaluated thorough the site quality. Different indices can be used to measure site quality but the site index (relationship between dominant height and age) is the most employed. However, site index can only be applied in even-aged forests whereas the site form (height-diameter relationship) has been proved to be a good measure of site quality in uneven-aged stands and multispecies forests. In this work, we aim to develop national-scale site quality maps for five dominant species of Spanish forests (Fagus sylvatica, Pinus pinaster atlantica, Quercus pyrenaica, Pinus nigra, Pinus sylvestris) with landscape planning purposes. First, we develop site form non-linear models using the Spanish National Forest Inventory. Then, we fit spatial additive models entering topographic and climatic variables in order to predict the site quality over the whole country. The accuracy of the models ranged from 38.2% to 47.9%. On the other hand, our results fall well with the autoecology of the species studied. Thus, we confirm the suitability of this approach to estimate the site quality in forest with different structures, such as even or uneven stands. Finally, we suggest that this method can be used in other countries to expand the current maps to the whole Europe