Long-term patterns in the decay, collapse, and abundance of trees with hollows in the mountain ash (Eucalyptus regnans) forests of Victoria, southeastern Australia

Large trees with hollows are an important component of stand structural complexity worldwide. Understanding their population dynamics is needed to manage cavity-dependent biota. We quantified long-term rates of collapse of 302 measured trees with hollows in 1939-aged regrowth mountain ash (Eucalyptus regnans F. Muell.) forest in southeastern Australia. We identified time-dependent dynamics in which the collapse rates of trees slowed from ~4% annually between 1983 and 1993 to ~2.2% between 1993 and 2007. Transitions of trees between different decay states (forms) also slowed over time. Nevertheless, during the 24-year period of our study, over half of our marked and measured trees had fallen, but there was no recruitment of new trees with hollows. Under current projections, few trees with hollows will occur on our field sites by ~2050, although more had been forecast in earlier investigations. Such a paucity of trees with hollows in extensive areas of regrowth mountain ash forests will lead to a shortage of nesting and sheltering sites for cavity-dependent biota. We suggest a short-medium (10- to 100-year) focus on the conservation of old growth and multi-aged stands will be needed to maintain populations of those species strongly associated with trees with hollows in mountain ash forests.