Competition-induced mortality for Mediterranean Pinus pinaster Ait.;P. sylvestris L.

Abstract

A logistic model for determining the probability of survival in the next 5-year period of two of the main species in the Mediterranean area in Spain, Mediterranean maritime pine (Pinus pinaster Ait.) and Scots pine (Pinus sylvestris L.), is presented. Data were supplied by the Forest Research Centre of the National Institute of Agricultural and Food Research (INIA) of Spain, consisting of a Permanent Sample Plot network (PSP) covering different stand conditions. In this study, 84 plots for Mediterranean maritime pine and 74 plots for Scots pine were used. Several models were tested. Variables concerning tree size, competition and social position of the tree were introduced in stepwise selection. The ratio diameter at breast height versus quadratic mean diameter and the interaction basal area-site index resulted as the most significant variables in the case of Mediterranean maritime pine. For Scots pine, the most significant variables were site index, DBH-1 and the interaction between basal area of larger trees and the coefficient of variation of the DBH distribution as a surrogate of structure. Results indicate that competition, tree size and social position within the stand are present in the survival process of both species. One-sided competition in highly structured stands for Scots pine and two-sided competition for Mediterranean maritime pine, are shown to be also important. Model evaluation has been done with data from the PSP not used in the fitting process. The area under the Receiver Operating Characteristic curve is 0.86 for Mediterranean maritime pine and 0.85 for Scots pine. The overall predicted rates of survival and mortality are shown for different cut-off points, indicating that the use of random uniform numbers yields better results. Afterwards, the performance of the model in thinned stands is tested for both species. Results for Mediterranean maritime pine show that it is appropriate to use the same model both for unthinned and thinned stands. However, a new model should be fitted for thinned Scots pine stands. © 2005 Elsevier B.V. All rights reserved.