Models for individual tree mortality in Norway

Logistic models predicting probability of survival for individual trees were developed, respectively, for Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), birch (Betula spp.), and for other broadleaved trees. The models were based on data from one remeasurement of a nation-wide grid of permanent sample plots recorded by the Norwegian National Forest Inventory. The data comprised 4506 sample plots with 46107 individual trees. Approximately 80% of the observations were used for model development and 20% for validation. The models were designed to be used in large scale forestry scenario models for even-aged and uneven-aged forests, as well as for forests with mixed and pure species composition. The explicatory variables in the models were the inverse of diameter at breast height, a competition index for individual trees, site index, and the proportion of basal area for the respective tree species. All parameter estimates were found highly significant (P < 0.001) in predicting mortality except site index in the model for Norway spruce (P < 0.05) and the inverse of diameter in the model for birch (P < 0.01). Although the phenomenon of mortality is a stochastic, rare and irregular event, the model fit and validation tests were fairly good. Given the general uncertainty related to large scale forestry scenario analyses, and the uncertainty related to mortality as a phenomenon, the mortality models presented in this paper were considered to have an appropriate level of reliability. (C) 2001 Elsevier Science B.V. All rights reserved.