Environmental and human drivers influencing large old tree abundance in Australian wet forests

Large old trees are keystone structures in numerous ecosystems globally. They play a wide range of critical ecological roles and therefore quantifying the factors influencing their distribution and abundance therefore has significant management implications. Yet, there are few ecosystems worldwide for which quantitative statistical models of the factors affecting large old tree distribution and abundance have been produced. We constructed a suite of such models using cross-sectional data on the occurrence of large old hollow-bearing trees gathered in 2015 on 166 sites, each of 1 ha in size within the montane ash forests of the Central Highlands of Victoria, south-eastern Australia. Our analyses included two broad groups of models, those for: (1) the overall abundance of large old hollow-bearing trees at a site, and (2) the abundance of large old hollow-bearing trees in four different morphological states of decay. These were large old living trees, large old hollow-bearing trees deemed potentially suitable for marsupial gliders, large old hollow-bearing trees deemed potentially suitable for non-gliding marsupial possums, and large old collapsed hollow-bearing trees. Most of the models we built encompassed a combination of covariates encompassing environmental factors (such as elevation and topographic wetness), human disturbance (e.g. land tenure), and natural disturbance (wildfire). The overall total abundance of large old hollow-bearing trees (irrespective of morphological form) was greatest at unburned sites, within stands of old-growth forest, within reserves, and on wet sites (as reflected by a topographic wetness index). Conversely, sites in young forests and sites subject to moderate or high severity fire supported the highest abundance of collapsed large old hollow-bearing trees. Our results demonstrate that different sets of environmental factors and attributes reflecting human disturbance, and natural disturbance affect the abundance of different morphological forms of large old hollow-bearing trees. Therefore, different parts of landscapes are most suitable for different kinds of large old hollow-bearing trees. The findings of this study can help direct management toward places where actions to recover populations of large old hollow-bearing trees are needed and/or are most likely to be effective, such as for conserving cavity-dependent animals