Fragility and damage functions for production forests exposed to volcanic ashfall and flood hazards in Aotearoa-New Zealand

This study focuses on the economic impacts of disaster-related events, specifically the impacts of floods and volcanic eruptions on production forestry in Aotearoa-New Zealand (A-NZ). A-NZ has extensive areas of production forests exposed to flood risks and active volcanoes. This exposure, coupled with the potential magnitude of impacts, and the prolonged period in which those impacts materialise, make A-NZ’s production forests an important case study for understanding the economic impacts of flood and volcanic forest damage. This research develops functions to predict economic outcomes from hazard impacts through a literature review, expert elicitation, and the modelling of revenue impacts. Predictions for flooding were based on water depth and flood duration, while ashfall thickness was considered for volcanic events. For flood events, functions predict forest-crop survival rates and the flow-on impact to forest management costs and log revenues were modelled. For volcanic events, functions predict the probability of impact states, defined by forest management cost and revenue scenarios. Impacted cashflows for three distinct forest age classes were assessed in an industry-established forest valuation framework, providing a structured impact assessment over a 29-year cycle of tree growth and harvest. Net impacts of flooding and volcanic events varied by forest age class and hazard metrics and were up to NZ$50,000/ha for older-age class forests under 3 m of water depth for three months, or 3 m of ashfall. This forest-age-class based impact assessment covered the range of hazard metrics up to and including unusually severe flood and volcanic events, presenting the full range of potential economic impacts and providing a vulnerability variable for simulating future risk.