A significant increase in forest regeneration failure following logging is driven by climatic and management factors.

Temporal patterns of vegetation recovery after wildfire in two obligate seeder ash forests

Variable retention harvesting is a silvicultural system that focuses on retaining key elements of stand structure at the time of logging and is increasingly being used worldwide. We describe the design and establishment of a variable retention harvesting experiment established in the Mountain Ash (Eucalyptus regnans) forests of the Central Highlands of Victoria, south-eastern Australia. The experiment was instigated in 2003, and the work to date has shown that it has environmental benefits for certain groups of small mammals, birds, and vascular plants. The experiment has been integrated with an ongoing long-term monitoring program as well as other experiments such as those in post-fire salvage-logged areas. Collectively, the results of various studies suggest that the potential value of variable retention harvesting extends beyond green-tree logging to post-fire salvage logging environments.We outline some of the challenges in, and new perspectives derived from, implementing and maintaining our experiment. This included difficulties protecting islands from high-intensity post-harvest regeneration burns and threat of declining funding undermining ongoing project viability. A critically important perspective concerns the ecological and economic context in which variable retention harvesting is implemented. In the particular case of Mountain Ash forests, assessments using formal IUCN criteria classify the ecosystem as being Critically Endangered under the Red Listed Ecosystem approach. As a result, Mountain Ash forests are at a high risk of ecosystem collapse. Further logging will increase that risk, making the basis for continued harvesting questionable. In addition, economic analyses suggest that the value of natural assets, like water production, far outweigh the value of the wood products harvested from the Mountain Ash ecosystem, again leading to questions about the viability of ongoing harvesting. We therefore conclude that whilst variable retention harvesting has the potential to contribute to biodiversity conservation in Mountain Ash forests, broader ecological and economic contextual issues (such as the values of competing resources like water yields and the heavily degraded state of the forest) may erode the case for its broader application.

Old forest, new perspectives—Insights from the Mountain Ash forests of the Central Highlands of Victoria, south-eastern Australia

AimThis study identified the factors influencing the presence, stocking rate and basal area of the southern Australian cool temperate rain forest tree myrtle beech (Nothofagus cunninghamii).LocationThis investigation was conducted in the montane ash forests of the Central Highlands of Victoria, south‐eastern Australia.MethodsThe occurrence ofN. cunninghamiiwas measured as part of field surveys of 474 sites (2844 plots) distributed widely throughout mountain ash (Eucalyptus regnans), alpine ash (E. delegatensis) and shining gum (E. nitens) forests. Statistical analysis was employed to examine relationships between the occurrence ofN. cunninghamiiand measured site attributes and interpolated and derived environmental variables.ResultsThe presence ofN. cunninghamiiwas significantly related to five variables: the age of the overstorey eucalypt stand, dominant species of overstorey eucalypt tree, topographic position in the landscape, slope and the estimated quantity of precipitation in the warmest quarter of the year.N. cunninghamiiwas more likely to occur in old growth eucalypt stands, in gullies and locations with high values for rainfall in the warmest quarter. In addition, there was an interaction between slope and dominant eucalypt tree species—in forests dominated byE. regnans,N. cunninghamiiwas more likely to occur on steep slopes whereas inE. nitensandE. delegatensis‐dominated stands,N. cunninghamiiwas more likely to occur on flatter terrain. Statistical modelling also showed a significant positive effect of slope on both the stocking rate and basal area of the species. The stocking rate ofN. cunninghamiialso was significantly higher in old growth eucalypt stands.Main conclusionsThe study demonstrated that the patterns of distribution ofN. cunninghamiimay be more complex than recognized previously and it illustrated the importance of multiple factors including climatic conditions, disturbance processes (as reflected through the age of the dominant overstorey eucalypt stands) and topography. Our work also shows there can be complex interactions between age cohorts of different tree species in the same stand—in this case, the dominant overstorey eucalypts and the understorey which includesN. cunninghamiistems.

The catastrophic 2009 wildfires in the Mountain Ash (Eucalyptus regnans) forests of the Central Highlands of Victoria provided an opportunity to gain new insights into the responses to fire by various elements of the biota. Ongoing long-term monitoring at a large number of permanent field sites for up to 25 years prior to the fire, together with 10 years of post-fire monitoring, has provided an unparalleled series of datasets on mammal, bird, and plant responses on burned and unburned sites. The empirical studies briefly summarized in this paper show patterns of steep declines in large old trees and declines in site occupancy by arboreal marsupials and birds. These changes contrast markedly with the responses of the two most common species of small mammals (the Agile Antechinus [Antechinus agilis] and Bush Rat [Rattus fuscipes]), which recovered within two generations after the fire. Declines in arboreal marsupials, birds and large old trees have also occurred on unburned sites, indicating an ecosystem-wide trend. In general, logging had a greater impact than fire on the majority of groups of birds and plants, particularly post-fire salvage logging that occurred in some areas following the 2009 wildfires. Beyond interactions between fire and post-fire (salvage) logging and their effects on forest biota, we have uncovered evidence of other kinds of interactions in Mountain Ash forests. These include interactions between: (1) the severity of fires and logging history, (2) post-fire bird population recovery and long-term climate and short-term weather conditions, and (3) impacts on forest soils. The structure and landscape composition of the Mountain Ash ecosystem has been radically altered over the last century. This has resulted from the combined impact of several large fires, including the 2009 fires as well as widespread clearfell logging that has been conducted within state forests over the last 50 years. The ecosystem now supports old growth cover that is 1/30th to 1/60th of what it was estimated to have been prior to European settlement. The ongoing decline of key components of the Mountain Ash ecosystem has led to it being classified as Critically Endangered and at high risk of ecosystem collapse. We argue that current forest policy and practices need to better mitigate the effects of fire on this already highly disturbed forest and enhance the possible persistence of species in this ecosystem. Several key strategies are required to do this. First, there is a need to significantly expand the extent of old growth within the Mountain Ash forest estate. This is because fire severity is diminished in such areas. Spatial contagion across old-growth dominated landscapes also may be suppressed relative to landscapes composed primarily of young forest. Allied management strategies include the protection of more mesic parts of Mountain Ash landscapes as these are less likely to burn or at least burn at high severity. Such enhanced protection should include an expanded network of buffers around drainage lines and waterways as these are where fire severity is likely to be lowest and also where old growth elements like large old hollow-bearing trees are more abundant. In addition, all existing living and dead hollow-bearing trees need to be protected by buffers of unlogged forest within wood production forests to promote their standing life and better conserve cavity-dependent fauna such as the Critically Endangered Leadbeater’s Possum (Gymnobelideus leadbeateri) and other declining taxa like the Greater Glider (Petauroides volans).

Landscape change and habitat fragmentation is increasingly affecting forests worldwide. Assessments of patterns of spatial cover in forests over time can be critical as they reveal important information about landscape condition. In this study, we assessed landscape patterns across the Mountain Ash (Eucalyptus regnans) and Alpine Ash (Eucalyptus delegatensis) forests in the Central Highlands of Victoria between 1999 and 2019. These forests have experienced major disturbance over the past 20 years through a major fire (in 2009) and extensive industrial logging. We found that around 70% and 65% of the Mountain Ash and Alpine Ash forest areas, respectively, were either disturbed or within 200 m of a disturbed area. Inclusion of planned logging increased these disturbance categories to 72% and 70%, respectively. We also found that the isolation of Mountain Ash core areas (patches of undisturbed forest >1000 ha) increased significantly (P < 0.05) over our study period, with the proximity between disturbed areas conversely increasing significantly (P < 0.05). This means that continued and planned disturbance through industrial logging will have an amplified adverse effect on remaining undisturbed ash forest patches, which will become smaller and more dispersed across the landscape.

Old growth is a critical growth stage in many forest types globally. It has many key ecological roles including biodiversity conservation, carbon storage and the provision of services such as water production. The extent of old growth forest has been declining in many ecosystems around the world, with major ecological and ecosystem service consequences. Important insights about such declines, as well as the structure, function and conservation of old growth forest, can be gained from detailed cross-sectional and longitudinal studies of different age cohorts within a given forest ecosystem. In this review article, we outline key insights into the characteristics of, and threats to old growth forests, using the Mountain Ash (Eucalyptus regnans) forests of the Central Highlands of Victoria, south-eastern Australia as a detailed case study. These forests are dominated by the tallest flowering plants on earth and have been subject to several decades of intense study. These studies show that old growth Mountain Ash forests are characterized by (among other features): giant trees (approaching 100 m tall and sometimes exceeding 20 m in circumference), numerous trees with hollows, an understorey ofAcaciaand rainforest trees, a range of plant and animal species that are rare or absent in younger aged stands, and moist, nutrient-rich soils. The area of old growth Mountain Ash forest has declined to 1.16% of the ∼141,000 ha area occupied by ash-type forests in the Central Highlands region. This is up to 60 times less than it was at the time of European colonization ∼220 years ago. The loss of old growth has major implications for bird, mammal and other biodiversity, as well as for carbon storage and water production for human consumption. The main drivers of old growth decline are recurrent wildfire, widespread clearcutting, and a logging-fire interaction in which cut and then regenerated forests become more flammable and are at significantly elevated risk of burning at high (stand replacing) severity. Climate change is also a driver of old growth decline both through elevating the mortality of large old living trees and underpinning an increase in the frequency of high severity wildfire. These interacting drivers mean that restoring old growth Mountain Ash forest will be an ecological and policy challenge. We argue that a first step must be to cease all commercial logging in the Mountain Ash ecosystem to allow new cohorts of old growth forest to be recruited and thereby expand the extent of the old growth estate. In addition, the Government of Victoria should revert to a past definition of old growth that made it easier for forest to qualify for protection. Given there are high risks of recurrent high-severity wildfire in the existing Mountain Ash forest estate which is dominated by highly flammable young regrowth forest, new technologies (such as the use of drones and satellites) are needed to rapidly detect and then suppress ignitions before fires become large and difficult to control. Mountain Ash forests have provided an important natural laboratory for understanding the dynamics, management and conservation of old growth forest. They have also helped generate some valuable general perspectives likely to be relevant to other forest ecosystems globally. These include: (1) the critical value of multi-facetted cross-sectional and longitudinal studies in quantifying attributes of, and threats to, old growth forest, (2) the need for a carefully crafted definition of old growth that will typically be ecosystem-specific and based on the time required to develop key ecosystem attributes (e.g., large old trees), (3) the importance of rigorous protection measures because poor decisions that result in the loss of old growth now will take prolonged periods to rectify, and (4) setting protection levels that are relative to the existing spatial coverage of remaining old growth and the extent and impacts of stressors driving old growth decline.

Soils are a fundamental component of terrestrial ecosystems, and play key roles in biogeochemical cycles and the ecology of microbial, plant and animal communities. Global increases in the intensity and frequency of ecological disturbances are driving major changes in the structure and function of forest ecosystems, yet little is known about the long-term impacts of disturbance on soils. Here we show that natural disturbance (fire) and human disturbances (clearcut logging and post-fire salvage logging) can significantly alter the composition of forest soils for far longer than previously recognized. Using extensive sampling across a multi-century chronosequence in some of the tallest and most carbon-dense forests worldwide (southern Australian, mountain ash (Eucalyptus regnans) forests), we provide compelling evidence that disturbance impacts on soils are evident up to least eight decades after disturbance, and potentially much longer. Relative to long-undisturbed forest (167 years old), sites subject to multiple fires, clearcut logging or salvage logging were characterized by soils with significantly lower values of a range of ecologically important measures at multiple depths, including available phosphorus and nitrate. Disturbance impacts on soils were most pronounced on sites subject to compounding perturbations, such as multiple fires and clearcut logging. Long-lasting impacts of disturbance on soil can have major ecological and functional implications. Fires and logging alter soil composition and result in a significant reduction of soil nutrients that lasts for decades after the disturbance, suggests an analysis of soil samples across a multi-century sequence in mountain ash forests.

Patterns of avian species richness and assemblage composition may change markedly between and within vegetation types. We compared bird species richness and assemblage composition in cool temperate rainforest and Mountain Ash (Eucalyptus regnans) forest in the Central Highlands of Victoria, south-eastern Australia. We quantified the effects of the age of stands of Mountain Ash on the extent of forest-type differences with cool temperate rainforest. We also explored the influence of the shape of stands of cool temperate rainforest on bird species richness and the composition of the bird assemblage. We found no significant differences in bird species richness between cool temperate rainforests and Mountain Ash forest. This result was consistent with subtle differences in the composition of the bird assemblage, with few bird taxa being totally excluded from either of the two forest types. Some species (e.g. Pink Robin (Petroica rodinogaster)) were significantly more likely to be recorded in cool temperate rainforest but were not uncommon in Mountain Ash forest. We found no evidence of significant effects of stand shape in cool temperate rainforest, which was consistent with analyses of bird assemblages given that most species occurred in both forest types. Hence, we uncovered no evidence of specialist taxa confined to cool temperate rainforest.

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.

It is often stated that the availability of N limits the rate of growth of native forests. We discuss this hypothesis with particular reference to the mountain ash ( Eucalyptus regnans ) forests of south-eastern Australia. The abundance of 15 N in leaves and soil of mountain ash forest is in accord with data for Northern Hemisphere temperate forests and for tropical forests,and indicates that N availability is relatively high.None of the nutrient elements has limited the rate of growth of mountain ash forest regenerating after major disturbance (clear-felling and intense wild-fire). There is some evidence that P may be limiting to some ecological processes (e.g. the rate of litter decomposition). We conclude that phosphorus is more likely to be limiting than nitrogen in mountain ash forest because nitrogen cycling is conservative and continual inputs of N through biological fixation supplement this conservative N supply, and the stands never become N-deficient. The development of methodologies to determine the rate of N2-fixation in forests should be of high priority in ecological research.

The midstorey is a critical structural component of many forests globally. Using statistical models, we quantified the influence of two sets of variables on the percentage cover and basal area of two dominant Acacia spp. (Montane Wattle [Acacia frigescens] and Silver Wattle [Acacia dealbata]) in the midstorey of Mountain Ash (Eucalyptus regnans) forests in mainland south‐eastern Australia. Specifically, we focused on the influence of (1) the age of the overstorey eucalypts (corresponding to the time since the last stand‐replacing disturbance), and (2) environmental drivers (aspect, topographic wetness index, slope, elevation). We found evidence for generally non‐linear relationships between stand age and the percentage cover and the basal area of both Silver Wattle and Montane Wattle. Silver Wattle had the highest values for percentage cover, and Montane Wattle the lowest, in stands regenerating from fire in 2009. The basal area of Silver Wattle was highest in stands that regenerated after the 2009 wildfires and after disturbance that occurred between 1960 and 1990s. For Montane Wattle, basal area was lowest in stands that regenerated in 2009 but values did not differ among stands of other ages. Both Acacia species were a midstorey component in old‐growth Mountain Ash forest. No environmental covariates influenced the percentage cover of Montane Wattle or Silver Wattle. However, our model for the basal area of Montane Wattle contained evidence of a positive relationship with topographic wetness. The general paucity of environmental drivers in most of the models we constructed is likely due to the fact that both tree species occur well beyond our study region. Hence, the set of environmental conditions modelled may not be limiting the percentage cover or basal area of these midstorey tree species. Disturbance appears to be the key driver of dynamics of Montane Wattle and Silver Wattle in Mountain Ash forests.

Forest disturbance, forest wildlife conservation and the conservative basis for forest management in the mountain ash forests of Victoria—Comment

Effect of gap size on seedling establishment, growth and survival at three years in mountain ash ( Eucalyptus regnans F. Muell.) forest in Victoria, Australia

Mountain ash (Sorbus aucuparia) is widespread in Europe from sea level to the timberline and reaches its most northern range in N-Norway at 71 °N. Sorbus aucupariaoccurs on acid, dry to moist and mesotrophic to oligotrophic soils. The species often grows in secondary forests together with Alnus viridis, Sambucus racemosa, Betula pendula, Frangula alnus and some Salix spp. In most climax forests mountain ash grows only in the shrub layer. In the subalpine belt, the tree is present in green alder scrub and European larch-Swiss stone pine forests. On some N-exposed mountain slopes of the S-Alps, small mountain ash forests with green alder build the upper forest limit above 1500 m. In this study, ecological and phytosociological aspects of this particular forest community along a N-S-transect from the Swiss Central Alps (Grisons) to the South Alps (Ticino/ N-Italy) were investigated. We consider site conditions, local spread, rejuvenation, tree age structure, infestation and phytosociological aspects of S. aucuparia. The occurrence of A. viridis in the N-S-transect and the role of S. aucupariain the secondary and climax forests have been investigated. While in the green alder scrub (Alnetum viridis) of the North single mountain ash trees are present, alder-mountain ash forests have been found at the S-Alpine sites with S. aucupariacovers 25–80%. Here, the Alnetum viridis is absent, and A.viridis is element of the mountain ash forests. In the montane and lower subalpine zone, S. aucupariawill often be replaced later by beech or other climax trees. Above the upper beech limit, we consider the green alder-mountain ash forests as climax forests. At the driest forest sites, grass species are pre-dominant – mainly Calamagrostis spp. and Avenella flexuosa. Based on the earlier provisional phytosociological name of S-Alpine green alder-mountain ash forests, we suggest the name Alno viridi-Sorbetum aucupariae Hari, Leisinger et Zysset 1993 – according to the first description by HARI et al. (1993). Based on our vegetation records of green alder-mountain ash forests, we propose a new sub-association Alno viridi-Sorbetum aucupariae calamagrostietosum subass. nov.