Science for Saving Species Research findings factsheet Project 3.3.2 Implications of the rapid loss of large old hollow-bearing trees in Victorian Mountain Ash forests What do we mean by What are large old trees and why are they so important? forest age class? Forest age is often used a proxy Large old trees are keystone structures that is stored in these forests,5 and in for measuring condition of in many ecosystems around the world. key ecological processes like nutrient Mountain Ash forests. Forest They are termed keystone structures cycling (e.g. storing large amount of age impacts on the range and because of their disproportionate carbon).2 This Fact Sheet focusses on value of ecosystems services the ecological value relative to the area the many ecological values of large forest provides, including water, they occupy.1 They are characterized by old hollow-bearing trees in Mountain carbon, timber, aesthetics and features not found in smaller, younger Ash forests in the Central Highlands of biodiversity.9 The oldest forest age trees like hollows, large lateral branches, Victoria, where populations of these classes in the Central Highlands buttresses, and extensive canopies key structures are declining rapidly.6,7 system include old growth and with large numbers of flowers.2 The Mountain Ash forests in Victoria regrowth following the 1939 Large old hollow-bearing trees provide form an ecosystem of competing and fires (commonly known as vital habitat for cavity-dependent contested land uses, most prominently 1939 regrowth). These have animals such as Leadbeater’s Possum water provisioning, tourism, biodiversity a significantly higher abundance and other species of arboreal conservation, and timber harvesting. of hollow-bearing trees than marsupials.3,4 They also play a critical Decisions about land use in this system, younger regrowth areas. role in the structure of Mountain Ash and in particular the allocation of forests of different ages, including forest coupes to logging, come under influencing the amount of carbon the auspices of the Regional Forest Agreement, which is under negotiation. The current agreement is scheduled to expire in 2018. The condition of a native forest is an indication of the ecosystems services it can provide. Both water provisioning and biodiversity values are directly dependent on the presence of large old trees, and the ecosystems they support, while large old trees store carbon, and recreational and aesthetic values also benefit from their presence.8 There are two main types of disturbance in Mountain Ash forests which impact on the condition of the forest ecosystem, including the abundance and condition of large old trees: logging and fire. Climate change also represents a growing risk to the system. Image: Large Mountain Ash, Toolangi, Victoria. Photo: David Blair Researching the condition of large old trees Intensive studies of the Mountain Ash forests of the Central Highlands of Victoria over the past 35 years highlight changes in the condition and abundance of large old trees in the system. This includes: 1. repeatedly reassessing the condition and decay status of a marked population of these trees on long-term sites (e.g. whether trees have declined or not), 2. tracking radio-collared animals to large old trees used as den trees during the day, 3. repeated surveys of the occurrence of arboreal Image: Leadbeater’s Possum is dependent on tree hollows and prefers hollows in very old trees. marsupials and birds on Photo: D Lindenmayer and M Greer long-term field sites, 4. studies quantifying relationships between the Where are large old hollow-bearing trees occupancy of large old most likely to occur? hollow-bearing trees by arboreal marsupials and the Large old hollow-bearing trees in Recent (currently unpublished) measurable characteristics Mountain Ash forests are most likely work has indicated that changes in of those trees (e.g. diameter, to be found in reserves and closed the composition of the landscape height, decay state), water catchments where there is also influence the abundance of little or no history of past timber large old hollow-bearing trees, with 5. measuring and estimating harvesting.6,10 They are also more significantly fewer trees likely to be the amount of carbon stored likely to be found in old growth found on sites where there has been in trees of different sizes stands, where there has not been extensive fire and extensive logging and ages, and recent fire and in wetter parts in the surrounding landscape. 6. exploring relationships of the landscape.6 between the abundance of hollow-bearing trees at a site and environmental The formation of large old hollow-bearing trees factors (e.g slope and aspect) A hollow-bearing tree is defined as Mountain Ash trees typically begin as well as natural disturbances any tree that contains an obvious to develop cavities at approximately (e.g. fire) and human cavity or hollow – as determined 120 years.11 Trees with cavities disturbances (i.e. logging). by visual inspection using a pair of generally suitable for occupancy Most of these studies are binoculars. Researchers routinely by arboreal marsupials are based on a long-term dataset measure the diameter, height and approximately 190 years old comprising more than 160 decay state of large old hollow- (as determined by relationships permanent field sites with a bearing trees in Mountain Ash forests between tree diameter and marked population of large to understand their condition, age tree age).3 old hollow-bearing trees. and suitability as habitat for various hollow-dwelling species.7 How do cavity-dependent arboreal marsupials use large old hollow-bearing trees? Some species of arboreal marsupials bearing trees with these trees will have different characteristics in Mountain Ash Forests such as sometimes located several hundred than those occupied by the Greater Leadbeater’s Possum spend up to metres apart.14-16 In addition, different Glider.4,12 Some species maintain very 75% of their lives within a cavity species of arboreal marsupials long-term use of a given set of large inside a large old hollow-bearing typically have different nest tree old trees, sometimes exceeding a tree.12,13 Almost all species swap requirements, e.g. trees typically decade or more.12,17 regularly between different hollow- occupied by Leadbeater’s Possum Image: Mountain Brushtail Possums use this stag hollow - which bears their tracks. Photo: David Blair Image: This Stag tree can provide hollows for the Greater Glider and Leadbeater’s Possum and other species. Photo: David Blair Image: ANU Researchers checking Nestboxes in a Mountain Ash regrowth forest. Research is testing the performance of nest boxes used to provide artificial tree hollows Photo: Lachlan McBurney Factors threatening large old hollow-bearing trees Several processes are threatening In old growth forests that experience The result of the interactions populations of large old hollow- a fire, the large old trees will become between logging and fire could lead bearing trees in Mountain Ash large dead trees or fire-scarred the Mountain Ash forest to become forests. living old trees. These trees are an caught in a ‘landscape trap’,24 i.e. a important source of nesting habitat positive feedback loop where the Logging for a wide variety of species which planting of young trees post-logging Logging is a key driver of human depend on the availability of suitable increases the risk of re-burning of the disturbance to the Victorian tree hollows such as Leadbeater’s forest at a high severity.22 If young Mountain Ash Forest ecosystem. possum.19 In young burned forest, trees are unable to reach maturity Native forest logging has long history this nesting habitat does not develop and maintain their population, it in the Central Highlands, beginning because trees with a small diameter could create irreversible changes in in the 19th century with selective are unable to remain standing long the patterns of forest disturbance logging, and intensifying through the after they are burned, and they lack dynamics, forest cover, landscape 20th century. The system continues the wood volume to develop hollows pattern, and vegetation, potentially to be a major source of native of a suitable nesting size for native leading to a regime shift.24 hardwood ash and mixed species species.20 When fires occur in rapid This regime shift would be timber. Clearfelling has been the succession they destroy young trees, categorised by the development conventional method of logging in which are unable to reach maturity of vegetation cover dominated by the region over the last 40 years, and produce seeds to maintain their wattle species (Acacia spp.), because with 15-40 ha of saleable trees population, resulting in the loss of Mountain Ash is maladapted to cleared in a single operation.18 Mountain Ash trees and a change altered landscape conditions created to the composition of species in Logging impairs the development by recurrent logging and wildfire.24 a forest.21 of large old hollow-bearing trees This shift would negatively impact because the rotation time between Fire and logging interactions ecosystem services including carbon harvesting operations means that storage, water production, and Fire and logging also interact to trees are cut down before they biodiversity conservation. The impact affect the abundance of hollow- develop hollows.7 A recent study on tree hollow dependent species bearing trees. This is because forest has shown that the abundance of of fauna