What strategies are effective for perpetuating structures provided by old trees in harvested forests? A case study on trees with hollows in south-eastern Australia

Some habitat resources provided by trees take longer to form than the period between harvesting events in forests managed for wood production. In response, governments and forest certification schemes often prescribe that old trees must be retained in harvested stands. Despite these prescriptions there is evidence from around the world that structures provided by old trees have declined over successive harvesting events in wood production forests. This undermines the ecological sustainability of harvesting from natural forests and hinders the certification of wood products derived from them. What strategies are effective for perpetuating structures provided by old trees in harvested stands? We addressed this question using the retention of trees with hollows (or cavities) in the forests of south-eastern Australia—which take >120-220 years to develop—as a case study. We developed a simulation model populated with comprehensive data from these forests to simulate existing harvesting and a range of alternative management scenarios. We predicted that, under existing practice, only 35-79% of the intended numbers of hollow-bearing trees will be perpetuated. In a sensitivity analysis we found that 75% of the variation in predicted numbers of trees with hollows over multiple harvesting rotations could be explained by the number of recruitment trees retained for each hollow-bearing tree, the rate of mortality among retained trees, the length of the harvesting rotation and the rate at which trees developed hollows. Our results indicated that trees with hollows can only be perpetuated in harvested stands over multiple harvesting rotations if ≥2 recruitment trees are retained for each hollow-bearing tree and measures are employed to minimise mortality among all retained trees. Accelerating the development of old features in trees is beneficial. Managing stands on a long rotation (200 years) is only beneficial where mortality is unavoidably high. Predicting how many old trees will be perpetuated in harvested stands over multiple rotations is not a trivial exercise because of the number of variables that influence the outcome, uncertainty around some of these and the time-lag between putting strategies in place and observing their effects. Prescriptions for retaining structures provided by old trees in harvested stands can only be developed or audited if each of these issues is considered. We present a methodology that explicitly does this.