Large, infrequent natural disturbances are important drivers of change in forested ecosystems. While the variability inherent in the impacts of these disturbances is a key element of conceptual models of disturbance severity and forest dynamics, few empirical studies have assessed the structural heterogeneity generated by a major fire event across spatial scales ranging from square meters to hundreds of square kilometers. In this study we used high-resolution LiDAR data collected before and after the 2009 Black Saturday bushfires in Victoria, Australia—one of the most extreme fire events in the past century—to assess the impacts of fires across the broader landscape. We found that only 16% of 4-ha areas within the fire footprint experienced catastrophic stand-replacing impacts (i.e., >90% of canopy cover was lost). Approximately half of the area within the fire footprint experienced partial canopy loss and one-third of the landscape experienced either a ground fire or no fire. Analysis of a range of potential drivers of canopy loss revealed that the distribution of surviving vegetation was associated with topographic position, forest composition, and management history. Over time, the impacted forest stands will move along a broad range of developmental trajectories, forming diverse structures distributed across the landscape. This variability in post-fire stand structures and stand development patterns will produce a wide range of ecosystem values within and across the Central Highlands landscape. These findings provide new insights into the dynamics of temperate eucalypt forests and can inform the development of stand-scale silviculture and landscape-scale forest management strategies.

This work provides a novel, multi-proxy approach employing dendroecological techniques, to reveal patterns of species recruitment and population dynamics for long-lived species. In doing so it informs our understanding of the impact of disturbance regimes on plant community dynamics. Using the threatened, fire sensitive understorey tree Persoonia arborea as a case study, this study combined bomb-pulse radiocarbon dating, tree-ring analysis, and field surveys to reconstruct recruitment and persistence in response to disturbance from wildfire and commercial forest harvesting. We demonstrate rings are broadly annual but irregular with occasional missing or false rings reflecting the wandering lobate growth in the species. Growth declines with age, providing further support for the use of non-linear growth models for long-lived species. Reconstructing age using tree rings is a cost-effective option within the confines of an 18% error rate. Radiocarbon dating was the most accurate (CV = 5%) but also most expensive. The age-size (diameter at breast height) allometry was less precise (CV =22%) but is useful for rapid assessment of tree age. Using the best age-size models to reconstruct age distribution for over 5500 field observations, we demonstrate that fruiting probability increases with age, reaching 50% at 15 years and 90% at 46 years. Flowering peaks from January to March and fruit production peaks from August to October. Persoonia arborea recruits in pulses after fire, is killed in high severity fire but can persist at lower density within the footprint of light to moderate fire severity resulting in multi-age cohorts. The species can regenerate in the absence of fire and can regenerate continuously after clearfelling with a wide range of tree ages observed within the footprint of older (1970–2000) logging coupes. In this way, we demonstrate the utility of fire sensitive species in reconstructing disturbance history extends beyond the most recent fire.

Rising temperatures and increased drought intensity are driving accelerated tree mortality rates worldwide. We investigated how these changes affect the carrying capacity of mountain ash forests (Eucalyptus regnans), the world's tallest flowering plant and one of the most carbon-dense forests on Earth (450-819 tonnes carbon per hectare).Using data from a large network of silvicultural experiments collected between 1947 and 2000 in southeastern Australia, we quantified temporal trends in mortality rates and carrying capacity, and their relationships to spatiotemporal climate variations. We analyzed how maximum stand density changes with tree size (self-thinning line) across different climatic conditions and over time, disentangling spatial variation among sites from temporal variation within sites.Our results show forests growing in the warmest and driest conditions (highest vapour pressure deficit) had the lowest carrying capacity. This capacity further decreased with rising temperatures. Each one-degree Celsius increase in mean annual temperature was associated with a 9% reduction in carrying capacity. Based on these relationships, a projected three-degree Celsius increase by 2080 (CSIRO RCP8.5 scenario) could reduce tree density and carbon stocks by 24%, equivalent to losing 240,000 hectares of mature mountain ash forests or releasing 108 million tonnes of carbon.Trees that died were 0.62 times the size of living trees (i.e., they were suppressed), with no detectable effect of climate on this ratio. These findings demonstrate that reduced carrying capacity could undermine carbon sequestration and global forest restoration efforts, particularly in seasonally dry regions where warming accelerates water limitations. We discuss implications for incorporating changing carrying capacity into forest management and carbon accounting.

Forest fires of unprecedented scale and intensity have become a more frequent occurrence in many parts of the world. In southeastern Australia, the Black Summer fires of 2019–2020 impacted nearly 20 million hectares of forests. After more than a century of human impacts and other fires, there is a risk of shifts in species composition and forest structure. We studied lowland mixed-eucalypt forest in eastern Victoria to determine how variability in fire intensity influenced the structure and composition of the forest canopy. We found that resistance to low-intensity fire, as measured by avoidance of crown loss, increased with increasing tree size and varied among species. Resistance to moderate- to high-intensity fire was low, but all species showed significant resilience, with 95 As fire intensity increased, the relative proportions of the various species in the upper canopy shifted. Some species increased, while others decreased. Low-intensity fire however resulted in very little change to the structure and composition of the forest canopy. While areas of high-intensity fire may lead to shifts in relative abundance and dominance of eucalypt species, the general resilience of the eucalypt species to fire suggests a substantial inertia in the species composition in these forested landscapes. However, changes in canopy structure due to crown mortality in E. sieberi promoted increased openness which could promote this species regeneration and create a positive feedback loop which facilitates a shift in species composition in gaps created by crown and tree mortality.

Rising temperatures and increased frequency and intensity of droughts and heat waves have affected tree mortality rates worldwide. Here, we investigate how these changes have affected the carrying capacity of mountain ash forests (Eucalyptus regnans), the world's tallest flowering plant and one of the most carbon-dense forests on earth. We analyze data from a large network of silvicultural experiments collected between 1947 and 2000 in southeastern Australia to identify trends in mortality rates and carrying capacity for the species, and to quantify how these changes relate to spatiotemporal variations in climate. We show that forests growing in the warmest and highest vapor pressure deficit conditions had the lowest carrying capacity, and this capacity further decreased with rising temperatures. Key findings indicate that a projected three °C increase in temperature by 2080 could reduce tree density and carbon stock in these forests by 24%, equivalent to losing 240,000 hectares of mature mountain ash forests. Trees that died were 0.62 times the size of living trees (i.e., they were suppressed), with no detectable effect of climate on this ratio. We discuss the implications for forest conservation and management, and how reduced carrying capacity could undermine global forest restoration and carbon sequestration efforts.

Wildfire has shaped many ecosystems across Earth, and humans have in turn shaped fire and its interactions within a range of socio-ecological systems. Climate change is changing fire regimes, and recent major and disruptive fire seasons around the globe have indicated a need to reimagine and redefine how fire research is conducted. One potential path forward is increased promotion and development of interdisciplinary approaches to fire research, yet these are hindered by a lack of a common language and 'framing' of the 'problem'. In this paper, we seek to advance the field of interdisciplinary fire research by bringing together experts from a wide range of disciplines to identify the key challenges for understanding and living with wildfires of the future ('Wildfire Futures'). Through an iterative process, we identify seven major interdisciplinary challenges relating to Wildfire Futures in south-eastern Australia: data and understanding of fire; the need to reorientate cultural relationships with fire; recognising diverse tangible and intangible values of fire; exploring different ways to understand fire risk; adaptation pathways to envision alternate ways of living with fire; exploring the uncertainties and trade-offs inherent in decision making around fire; and how inertia in multiple systems hinders transformative change and interdisciplinary progress. Our paper illustrates how researchers from diverse disciplines can develop a common language for interdisciplinary fire research and identifies fire challenges relevant to many other regions around the world.

Robust hydroclimate risk assessment requires a thorough understanding of past climate variability, which can be achieved by supplementing short instrumental hydroclimate records with palaeoclimate data. However, longterm continuous simulation of catchments and storage modelling, essential for hydrological risk assessment, necessitates monthly or daily time series input data, while palaeoclimate records are typically available at annual or seasonal scales. Additionally, modelling operational water storages used for hydropower is complex, requiring inputs such as water extraction information, which are difficult to replicate due to their inherent variability. Based on Lake Burbury, part of Hydro Tasmania's hydroelectric scheme in southern Australia, we demonstrate a novel method through which seasonal flow reconstructions can be used for daily palaeo water balance modelling, coupled with an Artificial Neural Network (ANN) model to simulate storage extractions. We first developed two seasonal tree-ring based inflow reconstructions, an approximately 1000-year Austral summer and a 400-year Austral winter reconstruction. We then used these as a guide to bootstrap historical daily inflows and the ANN known as Long Short-Term Memory (LSTM) was trained to simulate extractions for hydroelectricity. A Source model of the Lake Burbury hydro-electric water supply system was prepared to simulate the daily surface water balance of Lake Burbury, including inflows, outflows and resultant storage levels over some 1000 years. The simulations were used to 'stress test' the current storage system under a broader range of climatic conditions than the instrumental period. Based on our simulation, a low flow period like that in the 18th century represents the highest risk to hydroelectricity production, while a repeat of 12-13th century conditions would be associated with the highest spill volumes and most reliable electricity production. Importantly, by extending the instrumental record, we can place contemporary trends in water availability in a longer historical context, better assess the likelihood of extreme events, and hence adjust plans to decrease the vulnerability of the hydroelectric sector (among other water users) to drought/shifts in climate. This approach requires collaboration between palaeoclimatologists, the modelling community, hydrologists and managers of natural resources and the built environment.

Temperate rainforests have historically been considered highly vulnerable to disturbance. Climate change, which is expected to increase the intensity, frequency, and impacts of disturbance events, is consequently a significant threat to their long-term persistence. However, data describing the long-term response of temperate rainforests to disturbance is rare. In the cool temperate rainforests of northern New South Wales, Australia, Nothofagus moorei is considered especially vulnerable to climate change due to a decreasing number of mature individuals, limited remaining suitable habitat, and low rates of sexual regeneration. In this study, we used over 50 years of empirical data from silvicultural experiments with multiple thinning intensities to characterise the demographic responses (i.e., growth, mortality, and recruitment) of cool temperate rainforest species, including N. moorei, to disturbance over time. Cool temperate rainforest species showed resilience to disturbance, predominantly through their widespread ability to basally coppice. Nothofagus moorei, in particular, demonstrated higher rates of successful sexual and vegetative recruitment and grew faster in response to higher intensities of disturbance, in comparison to very low rates of recruitment pre-disturbance. These results challenge successional models that position rainforests as disturbance-sensitive ecosystems and identify N. moorei as a species that requires large-scale disturbance to successfully regenerate. Management regimes that actively exclude disturbance from these forests risk the local loss of disturbance-dependent rainforest species such as N. moorei.

Recent extremes of flood and drought across Australia have raised questions about the recurrence of such rare events and highlighted the importance of understanding multi-decadal climate variability. However, instrumental records over the past century are too short to adequately characterise climate variability on multi-decadal and longer timescales or robustly determine extreme event frequencies and their duration. Palaeoclimate reconstructions can provide much-needed information to help address this problem. Here, we use the 600-year hydroclimate record captured in the eastern Australian and New Zealand Drought Atlas (ANZDA) to analyse drought and pluvial frequency trends for East Australian Natural Resource Management (NRM) clusters. This partitioning of the drought atlas grid points into recognised biophysical areas (i.e. NRM clusters) revealed their differences and similarities in drought intensity and pluvial events over time. We find sustained multi-decadal periods of a wet–dry geographic 'seesaw' between eastern to central and southern NRMs (e.g. 1550–1600 CE and 1700–1750 CE). In contrast, other periods reveal spatially consistent wetting (e.g. 1500–1550 CE) or drying (e.g. 1750–1800 CE). Emerging hot spot analysis further shows that some areas that appear naturally buffered from severe drought during the instrumental period have a greater exposure risk when the longer 600-year record is considered. These findings are particularly relevant to management plans when dealing with the impacts of climate extremes developed at regional scales. Our results demonstrate that integrating and extending instrumental records with palaeoclimate datasets will become increasingly important for developing robust and locally specific extreme event frequency information for managing water resources.

Understanding how past disturbances have influenced the development of forests is critical for deciphering their current structure and composition and forecasting future changes. In this study, dendrochronological methods were applied to uncover the disturbance history of old-growth hemlock-dominated forests in central Bhutan. Analysis of tree-ring samples from two old-growth hemlock stands, located in two different topographic settings, identified the importance of gap-phase dynamics in facilitating recruitment and growth releases and producing complex, multi-aged structures over time. One site showed evidence of a near stand-replacing disturbance in the late 1700s, while the other showed no evidence of high-severity disturbance at any time over the last 400 years. At both sites low-to medium-severity disturbances, some of which appear to be associated with cyclones originating in the Bay of Bengal, dominated the disturbance regime. The hemlock stands exhibited a significant positive association between cyclone occurrence and growth release events and between recruitment pulses and growth release events. From 1800 to 1970 there was an increase in recruitment of angiosperm tree species at most sites and a corresponding decline in conifer recruitment. Over the past 50 years there has been little new recruitment; this may be due to light limitation in the understory from shade-tolerant angiosperms and bamboo in the lower strata of these stands. Significant variations in disturbance dynamics and recruitment were observed across the study sites, suggesting that other factors, such as topography and climate, may be influencing long-term stand development patterns. This study highlights the complex interplay between historical disturbance regimes and tree recruitment in shaping the age and size structures of old-growth hemlock forests in central Bhutan. It also provides new insights into the dynamics of these forests that can be used to support effective forest conservation and management in the future.

Study region Western Tasmania, southeastern Australia.Study focus We present two new tree-ring based inflow reconstructions from western Tasmania in southeastern Australia.The warm season reconstruction (Dec–Feb) extends from 1030–2007 CE and explains up to 42% of the variance in instrumental flow, while the cool season (JA) extends from 1550–2007 CE and explains 27% of instrumental flow variance. Key features include an extended pluvial period in the 11th Century and a protracted dry period in ∼1500CE, neither of which are represented in the DJF instrumental record. Decreasing JA flow since the 19th Century is consistent with a local sediment-based hydroclimate record.New hydrological insights for the region The reconstructions confirm that the instrumental data do not capture how protracted past low or high flow periods have been. It is therefore important to consider pre-instrumental flow data when planning for the future. The reconstructions provide new insights into regional variability through their association with the Subtropical Ridge (STR) and the Southern Annular Mode (SAM). Differing spatial signatures of the seasonal reconstructions, and their associations with season-specific impacts of STR and SAM, highlight the need for caution when considering the use of remote hydroclimate proxy records with strong seasonal signatures. The reconstructions suggest that extrapolation of seasonally defined reconstructions to represent annual flow for regions beyond the extent of their spatial footprint may be problematic.

Blue pine (Pinus wallichiana) forests are of significant ecological, economic, and cultural importance in the Himalayas. We used dendrochronological methods to investigate the role of natural and human disturbance in shaping the development of these forests. Analyses of the age structure and growth patterns of blue pine populations over the period 1760-2020 at two different sites in central Bhutan revealed that blue pine tends to establish as single-cohort stands following relatively intense disturbances and as multi-cohort stands after low-tomoderate severity disturbances. Shifting cultivation, which was common across the region, likely led to the establishment of single-cohort stands, particularly near human settlements, whereas natural disturbances are likely responsible for the development of multi-cohort stands. Tree-ring records revealed an acute change in recruitment patterns in the early 1970s associated with the 1969 Forest Act of Bhutan, which limited traditional practices, such as firewood collection and grazing, within the forests. This led to a sudden and sustained increase in the recruitment of broadleaf tree species and effectively curtailed blue pine regeneration over the past half century due to thick understory and midstory vegetation reducing the amount of light reaching the forest floor. These results highlight the role of disturbances, both human and natural, in driving forest stand dynamics in Himalayan forests and how forest policy and traditional practices can alter those dynamics.

Background Tree hollows are an important habitat resource used by arboreal fauna for nesting and denning. Hollows form when trees mature and are exposed to decay and physical damage. In the absence of excavating fauna, hollow formation can take up to 200 years in Australian temperate Eucalyptus forests, making tree hollows a critical but slow forming habitat feature. The increasing frequency and severity of wildfires due to climate change has led to increased concern about the landscape-scale loss of nesting space for arboreal fauna, including endangered species such as the folivorous southern greater glider ( Petauroides volans ). To understand patterns of nesting resource availability, we assessed drivers of hollow occurrence in southeastern Australian mixed-species Eucalyptus forests and quantified the effects of an unprecedented large-scale wildfire, the 2019/2020 Black Summer bushfires, on hollow occurrence and abundance. Results Tree size and shape, as well as site productivity and topography, were important predictors for hollow occurrence both before and after the fires. The occurrence of the southern greater glider was strongly dependent on high proportions of hollow-bearing trees. While high fire severities had a negative impact on southern greater glider occurrence, the number of hollow-dependent arboreal species was not affected. While the wildfires significantly reduced hollow abundance, we did not find significant effects on hollow occurrence. Fires altered the relationship between tree size and hollow occurrence expressed as a change in the probability of hollow occurrence, with a higher likelihood at smaller tree sizes after the fires. Conclusions Our findings suggest that post-fire nesting space may be reduced at the tree-scale, while at the stand-scale, hollow-bearing trees persist as biological legacies. These persisting trees can support the recovery of hollow-dependent arboreal fauna, such as the endangered southern greater glider by providing denning and nesting space. Hollow-bearing trees that survived the fires have the potential to form new hollows faster compared to undisturbed mature trees.

Chir pine (Pinus roxburghii Sarg.) forests are distributed in the dry valleys of Bhutan Himalaya. In the past, these forests have been heavily influenced by human activities such as grazing, burning, resin tapping, and collection of non-timber forest products. Bhutan’s Forest Act of 1969, which shifted forest management from local community control to centralized governmental control, greatly restricted these activities. To understand the implications of the Forest Act on the chir pine forests, we used tree-rings and fire scars to reconstruct the fire history of a chir pine forest in eastern Bhutan. This provided an opportunity to characterize the fire regime before and after the Forest Act of 1969 was implemented and assess the scale and magnitude of changes that have occurred. We developed a 120-year chir pine fire chronology from nine sites within a single forested landscape. Between 1900 and 1970, fires were small and patchy. When fires occurred, they were limited to one to two sites within the larger study area. After 1970, there was a distinct shift in fire activity, with fires in 1985, 1989, 1996, 2000, and 2013 burning > 90

Old-growth forests provide many ecosystem services and benefits. However, they are becoming increasingly rare and thus are an urgent priority for conservation. Accurately mapping old-growth forests is a critical step in this process. Here, we used LiDAR, an improved individual tree crown delineation algorithm for broadleaved forests, Gaussian mixture modelling, and a rule-based classification key to map the extent and location of old-growth forests across a topographically and ecologically complex landscape of 337,548 ha in southeastern Australia. We found that variation in old growth extent was largely driven by the old growth definition, which is a human construct, rather than by uncertainty in the technical aspect of the work. Current regulations define a stand as old growth if it was recruited prior to 1900 (i.e., >120 years old) and is undisturbed (i.e., <10% regrowth canopy cover and no visible disturbance traces). Only 2.7% (95% confidence intervals ranging from 1.4 to 4.9%) of the forests in the study landscape met these criteria. However, this definition is overly restrictive as it leaves many multi-aged stands with ecologically mature elements (e.g., one or more legacy trees amid regrowth) unprotected. Removing the regrowth filter, an indicator of past disturbances, increased the proportion of old-growth forests from 2.7% to 15% of the landscape. Our analyses also revealed that 60% of giant trees (>250 cm in diameter at breast height) were located within 50 m of cool temperate rainforests and cool temperate mixed forests (i.e., streamlines). We discuss the implication of our findings for the conservation and management of high-conservation-value forests in the region.

Abstract To better understand fire regimes and their relation to climate in the seasonal tropical forests of continental Southeast Asia, we developed the first multi‐century tree‐ring based fire history chronology for the region. The chronology included 776 fire scars collected at Bidoup NuiBa National Park (BNNP) in the Central Highlands of Vietnam and spans the period 1636–2020. Fires were recorded in 116 years, representing 47% of the years covered by the 249‐year period between the first fire scar (1772) and the last (2020). While only 9% of years within the sampled BNNP forests experienced fires before 1905, 70% recorded fires between 1906 and 1963 and 90% showed evidence of fire after 1963. Fire occurrence was highly correlated with climate indices (wet season Nino 3.4 and dry season regional Palmer Drought Severity Index) during the period 1906–1963, but showed no significant correlation after 1963. Our fire reconstruction from BNNP suggests that the fire regime has shifted from one driven primarily by climate to one in which human activities dominate the occurrence of fire within these seasonal tropical landscapes.

A fundamental requirement of sustainable forest management is that stands are adequately regenerated after harvesting. To date, most research has focused on the regeneration of the dominant timber species and to a lesser degree on plant communities. Few studies have explored the impact of the regeneration success of dominant tree species on plant community composition and diversity. In this study, we quantified the influence of variability in tree density and climatic and edaphic factors on plant species diversity in montane regrowth forests dominated by Eucalyptus regnans in the Central Highlands of Victoria in southeastern Australia. We found that Acacia density shaped plant biodiversity more than Eucalyptus density. Edaphic factors, particularly soil nutrition and moisture availability, played a significant role in shaping species turnover and occurrence. Our findings suggest that the density of Acacia is a key biotic filter that influences the occurrence of many understorey plant species and shapes plant community turnover. This should be considered when assessing the impacts of both natural and anthropogenic disturbances on plant biodiversity in the montane forests of southeastern Australia.

Despite the importance of soil seed banks in diversity maintenance, our understanding of plant response changes in resource availability is largely limited to above-ground vegetation. We investigated how forest structure and edaphic properties influenced the above-ground vegetation and soil seed banks following logging in montane regrowth forests dominated by Eucalyptus regnans in the Central Highlands of southeastern Australia. We surveyed above-ground vegetation, soil seed banks, forest structure and soil properties across 20 harvest units, aged 16 to 20 years. A total of 80 species were identified in above-ground vegetation with 72 species in the soil seed bank, with only 34% of species co-occurring in both species' pools. Climate, soil, topography, and light availability shaped plant composition but the relative importance of these properties varied among individual species and pools of diversity (combined, above-ground, soil seed bank, life form). Annual heat moisture index (AHMI) was the most important factor that influenced plant community composition across all species pools with individual species positively associated with AHMI. Acacia, but not Eucalyptus stem density further moderated plant response via effects on both light availability and soil nitrogen at the stem exclusion phase of stand development. Structurally mediated controls on above-ground community composition flow through to the soil seed bank via above-ground diversity effects on below-ground composition. For the combined species pool, individual species were most frequently related to Acacia stem density that on average explained 20% of model variation. Soil nutrients accounted for most of the model variation for individual species in above-ground (26%), soil seed bank (20%) and non-woody (32%) pools of diversity, with AHMI accounting for most of the model variation for woody species (18%). The differential response of individual species and community composition gradients in resource availability across different pools of species diversity and life forms clearly demonstrates storage effect in wet temperate forests of southeastern Australia that promotes post-disturbance recruitment and persistence. Our work suggests logging interacts with broad environmental controls to shape species diversity. The key role of Acacia stem density as a biotic filter of plant diversity provides opportunity to shape future plant diversity through management interventions such as thinning in the early stages of stand development. With native forest harvesting of these forests set to end from 2024, our findings provide important information future patterns of plant diversity as logged forests recover over time, opportunities for management interventions in shaping plant composition, and potential impacts of future wildfire.