A comparison of the relative risk of wind damage to planted forests in Border Forest Park, Great Britain, and the Central North Island, New Zealand

The risk of wind damage was assessed for Sitka spruce growing in the Border Forest Park, Great Britain and radiata pine growing in the Central North Island, New Zealand using a quantitative wind risk model (FORESTGALES). The model has been adapted for the two species using estimates of maximum resistive bending moment from historical tree winching studies performed in the two regions, drag coefficients obtained from wind tunnel tests, and wind data from representative meteorological stations. The risk is calculated as the probability that the threshold wind speed for damage will be exceeded at a particular site. This approach enables the effect of different growth rates, soil types, management regimes and wind climates to be investigated. The threshold wind speed for tree failure was calculated for stands of both species grown under standard regimes on typical soils and characteristic topography, The probability that the threshold wind speed is exceeded was determined using a recurrence function fitted to time-series of annual maximum hourly wind speeds from a representative long-term meteorological station for each region. The two species are grown over substantially different rotation lengths, so comparisons were made between the profile of risk over a single rotation, and also as cumulative risk over a time-span sufficient to contain multiple rotations. The results demonstrate substantial differences in the probability of wind damage between forests in the two regions as they are currently managed. Contrary to expectation, the risk is lowest for forests in the region of the most severe wind climate, because of the selection of a risk-minimising strategy. This finding emphasises the degree to which management of forests can affect the frequency and amount of damage that they experience. The model was portable and provided objective estimates of risk for two very different locations and silvicultural regimes. Further development should provide a tool of practical importance for those seeking to make comparisons between very different situations, such as multi-national companies and reinsurers. (C) 2000 Elsevier Science B.V. All rights reserved.