Large old trees are critical organisms and ecological structures in forests, woodlands, savannas, and agricultural and urban environments. They play many essential ecological roles ranging from the storage of large amounts of carbon to the provision of key habitats for wildlife. Some of these roles cannot be replaced by other structures. Large old trees are disproportionately vulnerable to loss in many ecosystems worldwide as a result of accelerated rates of mortality, impaired recruitment, or both. Drivers of loss, such as the combined impacts of fire and browsing by domestic or native herbivores, chemical spray drift in agricultural environments, and postdisturbance salvage logging, are often unique to large old trees but also represent ecosystem-specific threats. Here, we argue that new policies and practices are urgently needed to conserve existing large old trees and restore ecologically effective and viable populations of such trees by managing trees and forests on much longer time scales than is currently practiced, and by protecting places where they are most likely to develop. Without these steps, large old trees will vanish from many ecosystems, and associated biota and ecosystem functions will be severely diminished or lost.

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We applied an ecosystem risk assessment to the mountain ash forest ecosystem of the Central Highlands of Victoria (hereafter 'mountain ash forest'), south-eastern Australia, using the IUCN Red List of Ecosystems criteria. Using this methodology, we quantified: (i) key aspects of the ecosystem's historical, current and future decline in spatial distribution; (ii) the extent of occurrence and area of occupancy for the mountain ash ecosystem; and (iii) the decline in key abiotic and biotic processes and features for historical, current and future time periods. We developed a probabilistic model of tree growth stages to estimate the risk of ecosystem collapse within 50 to 100 years in the mountain ash forest. There was uncertainty in our estimates of risk under the various IUCN criteria, with two sub-criteria being categorized as 'Data Deficient'. Our overall ranking of risk of collapse for the ecosystem was Critically Endangered. We are confident that this risk category is appropriate because all 39 scenarios modelled indicated a ≥92% chance of ecosystem collapse by 2067. Our findings highlight the important need for timely policy reform to facilitate improved management of the mountain ash ecosystem in Victoria. In particular, there needs to be greater protection of remaining areas of unburned forest, and restoration activities in parts of the forest estate. Implementation of these strategies will require a significant reduction in logging pressure on the mountain ash ecosystem.

Large trees with cavities provide critical ecological functions in forests worldwide, including vital nesting and denning resources for many species. However, many ecosystems are experiencing increasingly rapid loss of large trees or a failure to recruit new large trees or both. We quantify this problem in a globally iconic ecosystem in southeastern Australia – forests dominated by the world's tallest angiosperms, Mountain Ash (Eucalyptus regnans). Tree, stand and landscape-level factors influencing the death and collapse of large living cavity trees and the decay and collapse of dead trees with cavities are documented using a suite of long-term datasets gathered between 1983 and 2011. The historical rate of tree mortality on unburned sites between 1997 and 2011 was >14% with a mortality spike in the driest period (2006–2009). Following a major wildfire in 2009, 79% of large living trees with cavities died and 57–100% of large dead trees were destroyed on burned sites. Repeated measurements between 1997 and 2011 revealed no recruitment of any new large trees with cavities on any of our unburned or burned sites. Transition probability matrices of large trees with cavities through increasingly decayed condition states projects a severe shortage of large trees with cavities by 2039 that will continue until at least 2067. This large cavity tree crisis in Mountain Ash forests is a product of: (1) the prolonged time required (>120 years) for initiation of cavities; and (2) repeated past wildfires and widespread logging operations. These latter factors have resulted in all landscapes being dominated by stands ≤72 years and just 1.16% of forest being unburned and unlogged. We discuss how the features that make Mountain Ash forests vulnerable to a decline in large tree abundance are shared with many forest types worldwide.