Quantification of wind damage to New Zealand's planted forests

Abstract

Wind has a significant impact on planted forests in New Zealand, but the extent of loss and its spatial and temporal variability have not been quantified. Therefore, a formal analysis was undertaken using a database containing 78 records of wind damage in planted forests that spanned the period between 1945 and 2010. Approximately 63 000 ha of damage was recorded during this period. The damage that occurred in a single event ranged from 2.8 to 26 000 ha, with a median of 90 ha, and in most cases was confined to a single wood supply region. The relative risk of wind damage for each wood supply region was estimated by calculating the percentage of total net stocked area (NSA) that has been damaged by wind. Across all wood supply regions, an average of 0.21 per cent of NSA per annum has been damaged by wind; however, in the most affected region, the average level of damage was 0.94 per cent of NSA per annum, whereas in the least affected region it was 0.03 per cent. The probability of different levels of annual damage was modelled along with the level of damage associated with different return periods using a generalized Pareto distribution. Using this model, the mean return period for 500 ha of damage was estimated to be 5.0 years with 95% confidence interval (4.5, 6.2) years, whereas for 1000 ha of damage it was estimated to be 6.4 (4.8, 9.9) years.