Young forests are most at risk from ashfall; stands of trees less than 2 years old are likely to be destroyed by ash deposits thicker than 100 mm (4 in) (Neild et al, 1998). Ashfall alone is not likely to kill mature trees, but the accumulated weight of ash can break large branches in cases of heavy ashfall (>500 mm (20 in)). Defoliation of trees may also occur, especially if there is a coarse component of ash-sized particles or larger tephra and during heavy ashfall.
Impacts on forests are not expected to be significant until ashfall exceeds 100 mm (4 in). Branch damage may begin to occur in younger trees at around this level, with an increase in damage occurring as levels of ash increase. Access to forests will also be disrupted as roads may be blocked. Little long-term damage is expected to ensue. Depths of around 500 mm (20 in) of ash or more will cause major damage to forests. Extensive branch breakages will occur, and access to forests will be severely impeded. Access will not be possible at all for logging trucks. The area will be reusable, but the existing forest environment will be substantially altered, the burial of young trees a major part of this. Planting directly into basaltic ash is possible for many species, including Pinus Radiata. Planting directly into more silicic ash such as rhyolite is however more problematic, due to nutrient issues such as nitrogen and calcium deficiency (Neild et al., 1998).