Evaluating site quality of even-aged maritime pine stands in northern Portugal using direct and indirect methods

Site indices were compiled for maritime pine ( Pinus pinaster Ait.) based on growth curves from 65 temporary sample plots in northern Portugal. The rate of height growth in maritime pine stands was examined in relation to site factors and to floristic composition of lesser vegetation. Site factors included measures of topography, soil chemical and physical variables, and estimates of mean values of climatic variables. Principal component analysis and stepwise regression were used for estimation of site quality from site factors. Variations in growth were associated mainly with average of minimum temperatures in autumn, available potassium, total porosity and fine sand content, which explained 54.4% of the total variation. Reciprocal averaging analysis and stepwise regression were applied for estimation of site quality from lesser vegetation composition. Pteridium aquilinum was associated with good sites and Chamaespartium tridentatum with poor sites. Only 14.3% of the total variation in height growth was explained by this model. The best estimates of site quality during the first 15 years of stand growth are achieved by application of the site factors model. After that age, site index curves produce better estimates.