Araucaria bidwillii, commonly known as the bunya pine, is a widely recognised Australian conifer celebrated for its cultural, biogeographic and evolutionary significance. Belonging to the Araucariaceae, one of the oldest surviving families of trees, A. bidwillii is today found only in Far North and South East Queensland. This thesis provides the first quantified assessment of the habitat, population structure, abundance and conservation status of the Mount Lewis subpopulation of A. bidwillii.
My objective was to provide life history and ecological data in support of management planning and conservation practices that do not rely on assumption and anecdote. Prior to this study, the Mount Lewis subpopulation of A. bidwillii was thought to be restricted to approximately six discontinuous stands in two adjacent sub-catchments. With new occurrence records obtained from these catchments, a species distribution model was developed to identify potentially suitable habitat at both the local and regional scales. Rainfall, temperature and proximity to the rainforest boundary made significant contributions to the model’s performance. With records from both tall open forest and rainforest included within the model, predicted habitat was often in alignment with the margin of these two vegetation types. Locally, predicted habitat aligned with known locations and identified other nearby sites with high habitat suitability values. However, the model failed to align with the other known Wet Tropics subpopulation approximately 100 km to the south in Wooroonooran National Park.
Population structure and recruitment patterns were investigated by establishing 13 study sites across tall open forest and rainforest. Size-class data, as a proxy for age, and individual counts of seedlings were obtained for A. bidwillii across these sites. The aggregated data for the entire sample population displays a reverse J-curve structure, often interpreted as an indication of continuous recruitment. However, further analysis identified variation in pattern in response to vegetation type. The results identify recruitment but there is no clear single strategy displayed. While there is an abundance of seedlings, there are very few subsequent size-classes and an almost constant low number of trees between 15cm DBH to approximately 100cm DBH. Interpretation of these results leads to some support for a temporal recruitment strategy interwoven with the vegetation dynamics of tall open forest and rainforest. Seedlings are abundant in tall open forest and in open rocky sites within rainforest. In well-developed rainforest seedlings are sparse and the population structure resembles a slight pulse or modal pattern. Beyond seedling establishment, persistence into reproductively mature size-classes is low overall.
The Mount Lewis subpopulation of A. bidwillii was identified as a genetically distinct and evolutionary significant unit in 2004. It is also isolated from other populations, is at the northern extremity of the taxon’s range and is exposed to unique threats not faced by subpopulations elsewhere. For these reasons, a conservation assessment of the Mount Lewis subpopulation was undertaken using the International Union for the Conservation of Nature criteria. Although within the protected area estate, the Mount Lewis subpopulation is considered ‘vulnerable’ due to a very restricted range, small number of mature trees, ongoing threats from fire, environmental weeds and the low likelihood a stochastic event will result in a population decline >50% over 100 years.
The northern subpopulation of A. bidwillii favours open sites over rainforest for seedling establishment. This finding is consistent with recruitment strategies of other Araucaria in South America, New Guinea and New Caledonia where vegetation dynamics influence population structures. At Mount Lewis the recruitment of A. bidwillii may be partly dependent on a long-term temporal interaction between tall open forest and rainforest. Beginning as a seedling in tall open forest and if able to escape the seedling bank, A. bidwillii may outlive surrounding eucalypts to eventually become a structural emergent in the subsequent rainforest over several hundred years.
Tall open forest is an important habitat for A. bidwillii, which may have implications for the ongoing use of fire as a management tool intended to restrict rainforest expansion. Ensuring the ecological processes that have allowed A. bidwillii to persist within the landscape can continue to operate is likely to be the most effective conservation strategy for this important subpopulation of a species of great significance.
Find the full thesis here at JCU: http://researchonline.jcu.edu.au/44651/