The effects of forest structure and site characteristics on probability of pine twisting rust damage in young Scots pine stands Ulla Mattila a,* , Risto Jalkanen b , Ari Nikula b a Faculty of Forestry, University of Joensuu, P.O. Box 111, Fin-80101, Joensuu, Finland b Finnish Forest Research Institute, Rovaniemi Research Station, P.O. Box 16, Fin-96301, Rovaniemi, Finland Received 2 June 1999; received in revised form 11 November 1999; accepted 23 January 2000 Abstract The effects of forest structure and site characteristics on the susceptibility of Scots pine (Pinus sylvestris L.) stands to pine twisting rust (Melampsora pinitorqua (Braun) Rostr.) and on the relative amount of the disease in a pine stand were analysed in 109 young Scots pine stands on mineral soils in northern Finland. The data included the occurrence of pine twisting rust in pine leader shoots, the numbers of trees by species, the mean height and mean age of the pines and site characteristics. The susceptibility of a pine stand, de®ned as the probability of rust occurrence, was modelled with logistic regression and the relative amount of pine twisting rust in a susceptible stand with multiple linear regression. The models were linked together to assess the relative amount of pine twisting rust in pine stands. The results show that the susceptibility of the stands to the disease is best explained by the presence or number of aspen, site fertility, soil stoniness and the number of pines per hectare. A combination of forest soil stoniness and site fertility has a strong effect on the probability of pine twisting rust occurring in a pine stand. The relative amount of rust in infected pine stands is best explained by the number of aspens and the number of willows (soil characteristics) in stands where aspens are present. When the relative amount of disease in the stands was calculated with the combined model, failure to correctly classify infected stands as such led to more serious mistakes than failure to classify healthy stands correctly. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Melampsora pinitorqua; Pinus sylvestris; Populus tremula; Risk management; Logistic regression 1. Introduction Pine twisting rust, Melampsora pinitorqua (Braun) Rostr., is a heteroecious macrocyclic rust fungus which causes damage in young pine stands. In Finland, its most important host species is Scots pine (Pinus sylvestris L.). The alternate host of the fungus is aspen (Populus tremula L.). Basidiospores of the fungus infect growing pine shoots. This often leads to bending and death of the infected shoots and thus to decreased height growth and quality in pine stands (Jalkanen and Kurkela, 1984; Martinsson, 1985). Since pines are most susceptible under the age of 15 years (Kardell, 1962, 1966), the damage occurs mainly in the lowest and most valuable part of the stem, i.e. in the butt log. The damage caused by pine twisting rust could probably be reduced if the damage risk were taken into account in forest management planning. The risk of pine twisting rust is associated with several char- acteristics of the forest site and the pine stand. Only Forest Ecology and Management 142 (2001) 89±97 * Corresponding author. Tel.: 358-13-251-4090; fax: 358-13-251-4444. E-mail address: ulla.mattila@forest.joensuu.fi (U. Mattila). 0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-1127(00)00342-X