Population collapse and retreat to fire refugia of the Tasmanian endemic conifer Athrotaxis selaginoides following the transition from Aboriginal to European fire management.

Untangling the nuanced relationships between landscape, fire disturbance, human agency, and climate is key to understanding rapid population declines of fire-sensitive plant species. Using multiple lines of evidence across temporal and spatial scales (vegetation survey, stand structure analysis, dendrochronology, and fire history reconstruction), we document landscape-scale population collapse of the long-lived, endemic Tasmanian conifer Athrotaxis selaginoides in remote montane catchments in southern Tasmania. We contextualized the findings of this field-based study with a Tasmanian-wide geospatial analysis of fire-killed and unburned populations of the species. Population declines followed European colonization commencing in 1802 ad that disrupted Aboriginal landscape burning. Prior to European colonization, fire events were infrequent but frequency sharply increased afterwards. Dendrochronological analysis revealed that reconstructed fire years were associated with abnormally warm/dry conditions, with below-average streamflow, and were strongly teleconnected to the Southern Annular Mode. The multiple fires that followed European colonization caused near total mortality of A. selaginoides and resulted in pronounced floristic, structural vegetation, and fuel load changes. Burned stands have very few regenerating A. selaginoides juveniles yet tree-establishment reconstruction of fire-killed adults exhibited persistent recruitment in the period prior to European colonization. Collectively, our findings indicate that this fire-sensitive Gondwanan conifer was able to persist with burning by Aboriginal Tasmanians, despite episodic widespread forest fires. By contrast, European burning led to the restriction of A. selaginoides to prime topographic fire refugia. Increasingly, frequent fires caused by regional dry and warming trends and increased ignitions by humans and lightning are breaching fire refugia; hence, the survival Tasmanian Gondwanan species demands sustained and targeted fire management.

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.