The initial development of a windthrow risk model for Sitka spruce in Ireland

Damage to trees by strong winds is one of the most important abiotic hazards in forestry in Ireland. A number of classification systems have been developed to assess the risk of wind damage to forests in Ireland and Great Britain. However, these models have tended to be deterministic, ranking the relative risk on different sites and/or from silvicultural treatments but not assigning a probability to the likelihood of damage. The main objective of the study described in this paper was to devise a windthrow risk model for Ireland which would yield estimates of the probability of windthrow for a combination of site and silvicultural factors. Data were collected for a range of Sitka spruce (Picea sitchensis (Bong.) Carr.) stands growing on a variety of sites. These stands were selected in two regions of varying exposure. The modelling procedure examined the relationship between a range of site and silvicultural factors and the occurrence of windthrow. Out of the 15 factors examined, the following five were shown to contribute significantly to the risk of windthrow: toy height, the regional location of the stand, the soil type on which the stand was established, as well as the compass bearing of the plough ribbons where the site had been ploughed. Whether or not the stand had been thinned was also important.