Samuel W. Wood

Alternative stable states and the role of fire–vegetation–soil feedbacks in the temperate wilderness of southwest Tasmania

Two ecological models have been put forward to explain the dynamics of fire-promoting and fire-sensitive vegetation in southwest Tasmania: the alternative stable states model of Jackson (in Proc Ecol Soc Aust 3:9–16, 1968) and the sharpening switch model of Mount (in Search 10:180–186, 1979). Assessing the efficacy of these models requires high resolution spatio-temporal data on whether vegetation patterns are stable or dynamic across landscapes. We analysed ortho-rectified sequences of aerial photography and satellite imagery from 1948, 1988 and 2010 to detect decadal scale changes in forest and non-forest vegetation cover in southwest Tasmania. There was negligible change from forest to non-forest (<0.05%) and only a modest change from non-forest to forest over the study period. Forest cover increased by 4.1% between 1948 and 1988, apparently due to the recovery of forest vegetation following stand-replacing fire prior to 1948. Forest cover increased by 0.8% between 1988 and 2010, reflecting the limited ability of forest to invade treeless areas. The two models include interactions between vegetation, fire and soil, which we investigated by analysing the chemical (phosphorus, nitrogen) and physical properties (clay, silt) of 128 soil samples collected across 34 forest–non-forest boundaries. Phosphorus in the upper horizon was typically lower in non-forest vegetation compared to forest vegetation, which is consistent with proposed fire–vegetation–soil feedbacks. Mineral horizons were dominated by sand, with low levels of clay under all vegetation types. Available field evidence lends support to the Jackson (1968) alternative stable states model as the most suitable model of vegetation dynamics on nutrient poor substrates in southwest Tasmania although modifications of the timeframes for transitions toward rainforest are required.

A fire-driven shift from forest to non-forest: evidence for alternative stable states?

We test the validity of applying the alternative stable state paradigm to account for the landscape-scale forest/non-forest mosaic that prevails in temperate Tasmania, Australia. This test is based on fine-scale pollen, spore, and charcoal analyses of sediments located within a small patch of non-forest vegetation surrounded by temperate forest. Following nearly 500 years of forest dominance at the site, a catastrophic fire drove an irreversible shift from a forested Cyperaceae–Sphagnum wetland to a non-forested Restionaceae wetland at ca. 7000 calibrated (cal) yr BP. Persistence of the non-forest/Restionaceae vegetation state over 7000 years, despite long fire-free intervals, implies that fire was not essential for the maintenance of the non-forest state. We propose that reduced interception and transpiration of the non-forest state resulted in local waterlogging, presenting an eco-hydrological barrier to forest reestablishment over the succeeding 7000 years. We further contend that the rhizomatous natur...

Effects of High-Severity Fire Drove the Population Collapse of the Subalpine Tasmanian Endemic Conifer Athrotaxis Cupressoides

Athrotaxis cupressoides is a slow-growing and long-lived conifer that occurs in the subalpine temperate forests of Tasmania, a continental island to the south of Australia. In 1960-1961, human-ignited wildfires occurred during an extremely dry summer that killed many A. cupressoides stands on the high plateau in the center of Tasmania. That fire year, coupled with subsequent regeneration failure, caused a loss of ca. 10% of the geographic extent of this endemic Tasmanian forest type. To provide historical context for these large-scale fire events, we (i) collected dendroecological, floristic, and structural data, (ii) documented the postfire survival and regeneration of A. cupressoides and co-occurring understory species, and (iii) assessed postfire understory plant community composition and flammability. We found that fire frequency did not vary following the arrival of European settlers, and that A. cupressoides populations were able to persist under a regime of low-to-mid severity fires prior to the 1960 fires. Our data indicate that the 1960 fires were (i) of greater severity than previous fires, (ii) herbivory by native marsupials may limit seedling survival in both burned and unburned A. cupressoides stands, and (iii) the loss of A. cupressoides populations is largely irreversible given the relatively high fuel loads of postfire vegetation communities that are dominated by resprouting shrubs. We suggest that the feedback between regeneration failure and increased flammability will be further exacerbated by a warmer and drier climate causing A. cupressoides to contract to the most fire-proof landscape settings.

Macroecology of Australian tall eucalypt forests: baseline data from a continental-scale permanent plot network

Tracking the response of forest ecosystems to climate change demands large (≥1 ha) monitoring plots that are repeatedly measured over long time frames and arranged across macro-ecological gradients. Continental scale networks of permanent forest plots have identified links between climate and carbon fluxes by monitoring trends in tree growth, mortality and recruitment. The relationship between tree growth and climate in Australia has been recently articulated through analysis of data from smaller forest plots, but conclusions were limited by (a) absence of data on recruitment and mortality, (b) exclusion of non-eucalypt species, and (c) lack of knowledge of stand age or disturbance histories. To remedy these gaps we established the Ausplots Forest Monitoring Network: a continental scale network of 48 1 ha permanent plots in highly productive tall eucalypt forests in the mature growth stage. These plots are distributed across cool temperate, Mediterranean, subtropical and tropical climates (mean annual precipitation 850 to 1900 mm per year; mean annual temperature 6 to 21°C). Aboveground carbon stocks (AGC) in these forests are dominated by eucalypts (90% of AGC) whilst non-eucalypts in the understorey dominated species diversity and tree abundance (84% of species; 60% of stems). Aboveground carbon stocks were negatively related to mean annual temperature, with forests at the warm end of the temperature range storing approximately half the amount of carbon as forests at the cool end of the temperature range. This may reflect thermal constraints on tree growth detected through other plot networks and physiological studies. Through common protocols and careful sampling design, the Ausplots Forest Monitoring Network will facilitate the integration of tall eucalypt forests into established global forest monitoring initiatives. In the context of projections of rapidly warming and drying climates in Australia, this plot network will enable detection of links between climate and growth, mortality and carbon dynamics of eucalypt forests.

Fire-patterned vegetation and the development of organic soils in the lowland vegetation mosaics of south-west Tasmania

Two contrasting ecological models have been proposed for the forest–moorland vegetation mosaics of south-west Tasmania that stress different interactions between fire, soils, vegetation and the physical environment to produce either stable or dynamic vegetation patterns. We investigated aspects of these models by sampling organic soil profiles across vegetation mosaics to determine variation in soil depth, organic carbon (C) content, nutrient capital, stable C isotope composition (δ13C) and 14C radiocarbon age in two contrasting landscape settings. 14C basal ages of organic soils ranged from recent (<400 calibrated (cal.) years BP) to mid Holocene (~7200 cal. years BP), with a tendency for older soils to be from poorly drained moorlands and younger soils from the forest. The long-term net rate of C accumulation ranged from 2.7 to 19.2 gC m–2 year–1, which is low compared with northern hemisphere peatland systems. We found that δ13C in organic soil profiles cannot be used to infer Holocene vegetation boundary dynamics in these systems. We found a systematic decrease of phosphorus from rainforest through eucalypt to moorland, but estimated that phosphorus capital in moorland soils was still sufficient for the development of forest vegetation. Our results suggest that the characteristics of organic soils across the landscape are the result of interactions between not only vegetation and fire frequency, but also other factors such as drainage and topography.

A systematic review of the impacts and management of introduced deer (family Cervidae) in Australia

Abstract. Deer are among the world’s most successful invasive mammals and can have substantial deleterious impacts on natural and agricultural ecosystems. Six species have established wild populations in Australia, and the distributions and abundances of some species are increasing. Approaches to managing wild deer in Australia are diverse and complex, with some populations managed as ‘game’ and others as ‘pests’. Implementation of cost-effective management strategies that account for this complexity is hindered by a lack of knowledge of the nature, extent and severity of deer impacts. To clarify the knowledge base and identify research needs, we conducted a systematic review of the impacts and management of wild deer in Australia. Most wild deer are in south-eastern Australia, but bioclimatic analysis suggested that four species are well suited to the tropical and subtropical climates of northern Australia. Deer could potentially occupy most of the continent, including parts of the arid interior. The most significant impacts are likely to occur through direct effects of herbivory, with potentially cascading indirect effects on fauna and ecosystem processes. However, evidence of impacts in Australia is largely observational, and few studies have experimentally partitioned the impacts of deer from those of sympatric native and other introduced herbivores. Furthermore, there has been little rigorous testing of the efficacy of deer management in Australia, and our understanding of the deer ecology required to guide deer management is limited. We identified the following six priority research areas: (i) identifying long-term changes in plant communities caused by deer; (ii) understanding interactions with other fauna; (iii) measuring impacts on water quality; (iv) assessing economic impacts on agriculture (including as disease vectors); (v) evaluating efficacy of management for mitigating deer impacts; and (vi) quantifying changes in distribution and abundance. Addressing these knowledge gaps will assist the development and prioritisation of cost-effective management strategies and help increase stakeholder support for managing the impacts of deer on Australian ecosystems.

Fire-driven land cover change in Australia and W.D. Jackson’s theory of the fire ecology of southwest Tasmania

There are few conceptual frameworks of the effects of fire regimes on landscape-scale vegetation dynamics. A foundational contribution is W.D. Jackson’s “Ecological Drift” model, which, although it was originally developed to explain vegetation patterns in southwest Tasmania, anticipated many leading edge research questions in fire ecology. Jackson proposed that the interactions between plant life histories, soil fertility, and fire frequency cause mosaics of vegetation with fire frequency distributions that have characteristic central tendencies. Importantly, he argued that landscape vegetation patterns could change due to chance variation in these fire frequencies via a process analogous to genetic drift. We reflect on the applicability of this model as an organizing principal for research into fire-driven vegetation dynamics by considering the fire ecology of four distinct Australian ecosystems: (1) the forest-sedgeland mosaic of southwest Tasmania, where the theory was developed, (2) the tall Eucalyptus regnans forests of southeastern Australia, (3) the savanna landscapes, including isolated monsoon forest patches, in the tropics of northern Australia, and (4) the grassland Acacia shrubland mosaic in central Australia. We show that basic impediment in understanding these vegetation dynamics in terms of Jackson’s model is an adequate quantification of fire frequencies, particularly in systems with infrequent fire return times and long-lived trees with limited dendrochronological potential. Even in the northern Australian systems that are burnt every few years, and thus where direct measurement of fire frequencies is possible by analysis of the available satellite record, fire frequency characterization is inevitably complicated by (a) recent changes by the transition from Aboriginal to European-influenced fire management, (b) the inherent complexity of landscape geographic settings, and (c) the increasing effects of climate change. Nonetheless, the dynamics of many of our case study systems substantiated Jackson’s concept of fire-vegetation-soil feedbacks. Such fire feedbacks may cause “self-organization” of landscape vegetation patterns, which would render mosaies vulnerable to abrupt switches in land cover in response to changed fire regimes.

Fire History and Age Structure of an Oakpine Forest on Price Mountain, Virginia, USA

ABSTRACT: Fire history is an important aspect of the natural disturbance pattern of many types of forested ecosystems. Nonetheless, many forests and corresponding management plans lack quantitative information on fire interval, frequency, and seasonality. This project examined the fire history at Price Mountain, Virginia, using fire scar samples and tree-ring analyses from live tree chronologies. Additionally, this project investigated the fire scarring potential of two little-studied species, black gum (Nyssa sylvatica) and sourwood (Oxydendrum arboreum), as well as described the age-structure of the current stand. We hypothesized that fire frequency would be high prior to the fire suppression era, given the proximity to an historical railroad track at the base of the mountain and susceptibility to lightning due to elevation. Six major fire years occurred between 1861 and 1925 at an average interval of 14 years, followed by a period of no fires. Two-thirds of the fires burned early in the season. There was an initial establishment of sourwood and chestnut oak (Quercus prinus) from 1930–1940 as well as another establishment peak between 1950 and 1960 after a major logging event. Pine (Pinus pungens and Pinus rigida) species established between 1870 and 1930. Reconstructed fire history and age structure informs land managers that repeated fires occurred in this Appalachian ridge top forest and that modern forest structure is in part the legacy of historic fires and fire suppression.