What factors influence the collapse of trees retained on logged sites?: A case-control study

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Living trees (green trees) are intentionally retained on logged sites for many purposes such as wildlife conservation, future wood potential and aesthetics. Minimizing the risk that these trees collapse in the short-term will improve the chances that green-tree retention meets its objectives. In a retrospective case-control study, we identified factors significantly associated with the collapse of living trees retained on logged and slash-burnt sites in south-eastern Australia within 8 years of harvesting. Trees with incomplete crowns or trees with at least one visible cavity were, on average, at three times greater risk of collapse relative to trees with complete crowns or no visible cavities, respectively. Trees with fire-scarring, trees retained greater than 50 m from intact forest or trees retained in isolation were, on average, around twice the risk of collapse relative to trees without fire-scarring, trees retained within 50 m of intact forest or trees retained among other trees, respectively. When considered jointly, the significant factors in a model predicting the collapse of retained trees – tree height, tree diameter, crown form – were all associated with the health of trees. Choosing the healthiest trees possible for fulfilling their function, protecting trees from damage (e.g. fire) and retaining trees near, or among, other trees are likely to reduce the risk of collapse among retained trees and thus contribute to the effectiveness of green-tree retention in logged eucalypt forests.