National-scale assessment of forest site productivity in Spain

Sustainable production of wood is one of the main services provided by forest systems. Site productivity in the case of forests is often evaluated through the site quality. However, most of the works addressing the site quality have been done at local or regional scale. In this work, we aim to develop site quality models for five dominant species in Spanish forests (Fagus sylvatica, Pinus pinaster atlantica, Quercus pyrenaica, Pinus nigra, Pinus sylvestris) and create site quality maps at a national-scale from these models. First, we develop site quality models using site form (height-diameter relationship) as the reference index and the Spanish National Forest Inventory as dataset. Then, we fit spatial additive models entering physiographic and climatic variables in order to predict the site quality over the whole country. Additionally, we plot site form maps for the five species in order to describe spatial pattern in site quality at a national scale. Altitude and aspect appeared to be fundamental variables in the assessment of site quality. The accuracy of the spatial additive models ranged from 38.2% to 47.9%. The correspondence between the predicted and observed maps of site qualities is clear. Our results provide a tool which could be used by forest managers in land use planning as well as in forest policy decision-making at a national scale. We suggest that this method could be used in other countries and that the maps could be expanded to the European scale to assessing the way in which site quality varies across Europe always considering that the relationships between forest productivity and environmental variables could vary among biogeoclimatic zones.