The role of species composition in the emergence of alternate vegetation states in a temperate rainforest system

Context: Forest systems are dynamic and can alternate between alternative stable states in response to climate, disturbance and internal abiotic and biotic conditions. Switching between states depends on the crossing of critical thresholds and the establishment of feedbacks that drive (and maintain) changes in ecosystem functioning. The nature of these thresholds and the workings of these feedbacks have been well-researched, however, the factors that instigate movement toward and across a threshold remain poorly understood. Objectives: In this paper, we explore the role of species composition in initiating ecosystem state change in a temperate landscape mosaic of fire-prone and fire-sensitive vegetation systems.Methods: We construct two 12-kyr palaeocecological records from two proximal (230 m apart) sites in Tasmania, Australia, and apply the Alternative Stable States model as a framework to investigate ecosystem feedbacks and resilience threshold dynamics. Results: Our results indicate that, in this system, invasion by pyrogenic Eucalyptus species is a key factor in breaking down negative (stabilising) feedbacks that maintain pyrophobic sub-alpine rainforest.Conclusions: We conclude that the emergence of an alternative stable pyrogenic state in these relic rainforest systems depends on the extent of pyrophytic species within the system. These findings are critical for understanding resilience in forest ecosystems under future climate and land management changes and are relevant to fire-adapted cool-temperate ecosystems globally

Development of a southern hemisphere subtropical wetland (Welsby Lagoon, south-east Queensland, Australia) through the last glacial cycle

Continuous records of terrestrial environmental and climatic variability that extend beyond the Last Glacial Maximum (LGM) in Australia are rare. Furthermore, where long records do exist, interpretations of climate and ecological change can be hampered by marked changes in sedimentary environment which, in turn, affect the taphonomy of palaeoecological remains. As a consequence, in order to determine how wetland systems responded to climatic and environmental changes, we first need to understand how their depositional environment changed through time. Here we document the development of freshwater Welsby Lagoon, south-east Queensland, from a 12.7 m sediment sequence with a basal age of ca. 130,000 years. We present a variety of proxies reflecting change within the wetland. Carbon and nitrogen concentrations and carbon and nitrogen isotope ratios are used to infer the source of organic matter. However, the nitrogen limited nature of the catchment soils and presence of the colonial algae Botryococcus meant that organic material with C:N ≥ 20 is likely to be derived from autochthonous sources rather than terrestrial sources. A combination of photosynthetic pigments, plant macrofossils, aquatic pollen and sedimentary lignin was used to identify the sources of organic matter and the changing nature of this wetland. Since its formation, Welsby Lagoon has undergone a progressive change from an open-water, algae and cyanobacteria dominated, freshwater lacustrine system, to an aquatic macrophyte-dominated palustrine swamp after ca. 40 ka. It did not revert to lacustrine conditions during the Holocene, despite what is widely viewed as an increase in the regional moisture balance, most likely due to continual infilling of the wetland with sediment. With so few records of terrestrial change throughout MIS3 and MIS4, adequately understanding the development of sites like Welsby Lagoon is imperative to advancing our knowledge of this important environmental and cultural period in Australia's history, which encompasses events such as the extinction of megafauna and human colonisation of the continent

Disruption of cultural burning promotes shrub encroachment and unprecedented wildfires

Recent catastrophic fires in Australia and North America have raised broad-scale questions about how the cessation of Indigenous burning practices has impacted fuel accumulation and structure. For sustainable coexistence with fire, a better understanding of the ancient nexus between humans and flammable landscapes is needed. We used novel palaeoecological modeling and charcoal compilations to reassess evidence for changes in land cover and fire activity, focusing on southeast Australia before and after British colonization. Here, we provide what we believe is the first quantitative evidence that the region’s forests and woodlands contained fewer shrubs and more grass before colonization. Changes in vegetation, fuel structures, and connectivity followed different trajectories in different vegetation types. The pattern is best explained by the disruption of Indigenous vegetation management caused by European settlement. Combined with climate-change impacts on fire weather and drought, the widespread absence of Indigenous fire management practices likely preconditioned fire-prone regions for wildfires of unprecedented extent

Late Holocene climate anomaly concurrent with fire activity and ecosystem shifts in the eastern Australian Highlands

The alpine area of the Australian mainland is highly sensitive to climate and environmental change, and potentially vulnerable to ecosystem tipping points. Over the next two decades the Australian alpine region is predicted to experience temperature increases of at least 1 °C, coupled with a substantial decrease in snow cover. Extending the short instrumental record in these regions is imperative to put future change into context, and potentially provide analogues of warming. We reconstructed past temperatures, using a lipid biomarker palaeothermometer technique and mercury flux changes for the past 3500 years from the sediments of Club Lake, a high-altitude alpine tarn in the Snowy Mountains, southeastern Australia. Using a multi-proxy framework, including pollen and charcoal analyses, high-resolution geochemistry, and ancient microbial community composition, supported by high-resolution 210Pb and AMS 14C dating, we investigated local and regional ecological and environmental changes occurring in response to changes in temperature. We find the region experienced a general warming trend over the last 3500 years, with a pronounced climate anomaly occurring between 1000 and 1600 cal yrs. BP. Shifts in vegetation took place during this warm period, characterised by a decline in alpine species and an increase in open woodland taxa which co-occurred with an increase in regional fire activity. Given the narrow altitudinal band of Australian alpine vegetation, any future warming has the potential to result in the extinction of alpine species, including several endemic to the area, as treelines are driven to higher elevations. These findings suggest ongoing conservation efforts will be needed to protect the vulnerable alpine environments from the combined threats of climate changes, fire and invasive species.Zoë A. Thomas, Scott Mooney, Haidee Cadd, Andy Baker, Chris Turney, Larissa Schneider, Alan Hogg, Simon Haberle, Ken Green, Laura S. Weyrich, Vilma Pérez, Nicole E. Moore, Atun Zawadzki i, Sarah J. Kelloway, Stuart J. Kha

Assessing changes in global fire regimes

PAGES, Past Global Changes, is funded by the Swiss Academy of Sciences and the Chinese Academy of Sciences and supported in kind by the University of Bern, Switzerland. Financial support was provided by the U.S. National Science Foundation award numbers 1916565, EAR-2011439, and EAR-2012123. Additional support was provided by the Utah Department of Natural Resources Watershed Restoration Initiative. SSS was supported by Brigham Young University Graduate Studies. MS was supported by National Science Centre, Poland (grant no. 2018/31/B/ST10/02498 and 2021/41/B/ST10/00060). JCA was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 101026211. PF contributed within the framework of the FCT-funded project no. UIDB/04033/2020. SGAF acknowledges support from Trond Mohn Stiftelse (TMS) and University of Bergen for the startup grant ‘TMS2022STG03’. JMP participation in this research was supported by the Forest Research Centre, a research unit funded by Fundação para a Ciência e a Tecnologia I.P. (FCT), Portugal (UIDB/00239/2020). A.-LD acknowledge PAGES, PICS CNRS 06484 project, CNRS-INSU, Région Nouvelle-Aquitaine, University of Bordeaux DRI and INQUA for workshop support.Background The global human footprint has fundamentally altered wildfire regimes, creating serious consequences for human health, biodiversity, and climate. However, it remains difficult to project how long-term interactions among land use, management, and climate change will affect fire behavior, representing a key knowledge gap for sustainable management. We used expert assessment to combine opinions about past and future fire regimes from 99 wildfire researchers. We asked for quantitative and qualitative assessments of the frequency, type, and implications of fire regime change from the beginning of the Holocene through the year 2300. Results Respondents indicated some direct human influence on wildfire since at least ~ 12,000 years BP, though natural climate variability remained the dominant driver of fire regime change until around 5,000 years BP, for most study regions. Responses suggested a ten-fold increase in the frequency of fire regime change during the last 250 years compared with the rest of the Holocene, corresponding first with the intensification and extensification of land use and later with anthropogenic climate change. Looking to the future, fire regimes were predicted to intensify, with increases in frequency, severity, and size in all biomes except grassland ecosystems. Fire regimes showed different climate sensitivities across biomes, but the likelihood of fire regime change increased with higher warming scenarios for all biomes. Biodiversity, carbon storage, and other ecosystem services were predicted to decrease for most biomes under higher emission scenarios. We present recommendations for adaptation and mitigation under emerging fire regimes, while recognizing that management options are constrained under higher emission scenarios. Conclusion The influence of humans on wildfire regimes has increased over the last two centuries. The perspective gained from past fires should be considered in land and fire management strategies, but novel fire behavior is likely given the unprecedented human disruption of plant communities, climate, and other factors. Future fire regimes are likely to degrade key ecosystem services, unless climate change is aggressively mitigated. Expert assessment complements empirical data and modeling, providing a broader perspective of fire science to inform decision making and future research priorities.Peer reviewe

Climate, humans, fire and megafauna - key drivers of Australian subtropical vegetation change

The timing and cause of megafauna extinctions across Australian, and indeed around the world, have been strongly debated particularly since Martin (1967) first implicated human agency as a major factor in megafauna disappearances. The cause of the demise of the megafauna has been the focus of many studies, yet no work to date has developed independent environmental and climate reconstructions from a single Australian location in relation to megafauna extinctions. Sedimentary records from wetlands provide a particularly powerful archive to examine terrestrial ecosystem change in response to internal and external drivers across a variety of spatial and temporal scales. The presence of coprophilous fungi spores, such as Sporormiella, preserved in wetland sediments can indicate the local presence of large herbivorous, including extinct megafauna. Records of megafauna presence can then be coupled with palaeoecological and palaeoclimatological proxies to disentangle the influence of climate on terrestrial ecosystem change and the timing of megafauna disappearance. Sedimentary records that extend beyond the Holocene in Australia are rare. Rarer still are long, continuous, high-resolution records that extend beyond Marine Isotope Stage 3 (MIS3; 57 – 29 ka), a period of substantial significance in Australia that encompasses human arrival and megafauna extinctions. In this thesis I integrate a range of proxies from an 80,000 year old, well-dated, continuous sedimentary sequence from Welsby Lagoon, North Stradbroke Island to investigate the climate and environmental variability of subtropical eastern Australia from MIS4 to present. In this thesis I present, for the first time at a single location, records of inferred megafauna presence, local fire occurrence, vegetation change and independent local climate variability. Understanding the development of the wetland system and identification of changes in depositional environment at ca. 28 ka provide a robust basis for interpretation of proxies. During MIS4 and the Holocene fire is an important component of the surrounding landscape and drives vegetation change, with a more limited influence during MIS3 and MIS2. The largest changes in the vegetation around Welsby Lagoon occur between 55 – 40 ka, in the absence of frequent fire and coincident with the timing of widespread human migration and megafauna extinction. The shifts in vegetation during this period are predominantly driven by changes in climate, as inferred from the δ13C of bulk sediment and the δ18O of aquatic cellulose. The climate of this period displays a high level of variability as well as a shift to drier climates at ca. 54 and again at 43 ka. In addition to driving changes in vegetation dynamics, Sporormiella disappeared from the record at this time, during a shift to drier climate conditions. Within chronological uncertainty, the changes in vegetation composition and hydroclimate at ca. 43 ka at Welsby Lagoon are concurrent with abrupt changes in the Darling River region and central Australia as well as five vegetation records from across the central and eastern Australia. The data suggest that climatic change was major contributor to megafauna and vegetation change during Marine Isotope Stage 3.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 202

The transition from an Indigenous to a European influenced fire regime at Lake Werri Berri, south-east Australia

The drivers of fire regimes prior to the European occupation of Australia are still contentious, with some advocating regimes dominated by anthropogenic ignitions and others advocating a climate source or mixture of these elements. Here, we examine an 850-year history of fire regimes at Lake Werri Berri in south-east Australia, prior to and following European occupation. Macroscopic charcoal and FTIR spectroscopy were used to infer broad changes of the fire regime in proximity to the lake. We found little change through much of the 850-year period and most interesting, no apparent change following the initial displacement of Indigenous peoples and the introduction of farming and woodcutting to the region by Europeans. From the mid-20th Century onwards, there was an increase in both area burnt and fire severity or intensity, likely the result of increased fuel load and connectivity following an extended period of increased precipitation and heavier recreational land usage, which likely led to an increase in anthropogenic ignitions

Investigating the Effect of Oxidants on the Quantification and Characterization of Charcoal in Two Southeast Australian Sedimentary Records

This study examined the effects of commonly used oxidants in sedimentary macroscopic charcoal analysis on two sediment cores from Thirlmere Lakes National Park, Southeast Australia. The cores, from Lake Werri Berri (WB3) and Lake Couridjah (LC2), span ~900 years and 135,000 years, respectively. The Charcoal Accumulation Rate (CHAR) for both charcoal area and count was quantified using four different chemical treatments and compared to a control using only water. We also quantified the Charring Intensity (CI) of isolated charcoal fragments, a proxy for the severity/intensity of fire, determined using the FTIR spectral characteristics of the remaining charcoal after each treatment. We found significant differences in both the area and number of particles across all treatments in both cores. Significantly, we found substantial differences in CI between treatments, with few charcoal particles formed in low-severity fire (e.g., below ~400 °C or 3.0 °C.s.106) in groups treated with an oxidant. In contrast, the control group displayed a wider range of CI values and contained lightly pyrolyzed particles. This suggests that methods using an oxidant to concentrate sedimentary charcoal are potentially biasing records towards high-intensity or -severity fires. We suggest that consideration should be used when choosing laboratory methods based on the hypotheses being tested

The potential for rapid determination of charcoal from wetland sediments using infrared spectroscopy

Wetland sediments archive information about past terrestrial ecosystem change including variations in fire occurrence and terrestrial carbon fluxes. The charcoal content of sediments is important for understanding past fire regimes, as well as the role this recalcitrant carbon plays in the global carbon cycle. Infrared (IR) spectroscopy provides a rapid, non-destructive and cost effective method for simultaneously analysing numerous organic and inorganic sediment properties. The use of IR spectroscopy is well developed for determining concentrations of total organic carbon (TOC), total nitrogen (TN), biogenic silica and carbonate in lacustrine sediments. In soil science IR spectroscopy is also routinely used to determine charcoal content, however the potential for analysing charcoal content from lacustrine sediments has yet to be investigated. Here we develop IR spectroscopy and partial least squares regressions (PLSR) to predict the charcoal and TOC content of an organic, 130,000 year old sediment sequence from North Stradbroke Island (Minjerribah), Australia. Charcoal concentrations used for model development were derived using both traditional palaeoecological area measures (cm2 g−1) and solid state 13C nuclear magnetic resonance (13C NMR) of poly-aryl structures. The IR PLSR models yielded significant correlations for the two charcoal methodologies (area measurements, R2 = 0.57, p < .05; 13C NMR, R2 = 0.70, p < .05). We additionally find a very strong, significant, correlation for TOC (R2 = 0.92, p < .05), consistent with previous studies. Hence, IR is a promising tool for determining the charcoal content of lacustrine sediments, particularly for first-order sample screening, as part of a multi-proxy framework. IR spectroscopy can therefore provide a reliable and rapid technique for the initial investigation of fir