Arboreal Lichen Distribution on Native and Introduced Trees along a Climate Gradient in South Australia

Author: Van Vuong Le

Le, Van Vuong, 2019 Arboreal Lichen Distribution on Native and Introduced Trees along a Climate Gradient in South Australia, Flinders University, College of Science and Engineering

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Lichens are under-researched organisms in Australia, and ecological studies that have examined lichens in the context of environmental parameters are rare in all states with the exception of Tasmania. In South Australia, there have been extremely few ecological studies of lichens, none of which have focused on epiphytic lichens on trees. The paucity of research on lichens in Australia poses not only ecological questions but raises conservation concerns.

This study address some of the knowledge gaps related to the issues outlined above through three research questions and a further line of investigation. The questions are: 1) what are the relationships between epiphytic lichens and trees as substrates in the Adelaide Hills, Mid North and Southern Flinders regions of South Australia? 2) what are the relationships between epiphytic lichens and climate in the Adelaide Hills, Mid North and Southern Flinders regions of South Australia? and 3) Does the occurrence of lichens differ between native and introduced trees? The additional line of investigation analysed the threats to, and conservation status of, lichens in Australia, with specific reference to South Australia.

Five species of corticolous macrolichens — Xanthoria parietina, the Caloplaca holocarpa group, Physica aipolia, Lecidella elaeochroma and Chrysothrix xanthina – were found on 621 trees in 37 study sites situated along climate gradients in the Adelaide Hills, Mid North and Southern Flinders Ranges regions of South Australia: 254 had corticolous trunk lichens. Lichen species richness per tree ranged from one to three. No follicolous lichens were found.

Statistically significant evidence (Yates χ2 and Fisher’s Exact tests applied to contingency tables) showed that the probability of lichens growing on introduced trees is higher than it is for lichens growing on species native to the Australian mainland. This aspect of lichen occurrence has not been investigated before to the author’s knowledge and is a potentially very important finding that needs further research.

Lichens appeared to be relatively independent of tree diameter and height, with the resulting inference being that lichen occurrence is independent of tree age for all tree species sampled. This was also the case for lichens growing on trees of the Callistemon and Fraxinus genera and Eucalyptus torquata.

A weakly positive but statistically significant correlation indicates lichen cover increased as bark became less acidic. Lichens were rare on bark with pH <4.5. The association between lichen occurrence and bark roughness was weakly negative, but again statistically significant. Bark shedding was strongly related to lichen occurrence as indicated by Yates χ2 and Fisher’s Exact tests. There was a clear preference for lichens to establish on stable bark surfaces.

Statistically significant differences in lichen cover were found between the north, east, south and west aspects. Post-hoc Dunn’s tests showed that the most frequently occurring pair of significantly different aspects was north and south. This stresses the importance of incident solar radiation and the relative humidity of the trunk microclimate in relation to lichen growth.

The occurrences of trunk and canopy lichens were significantly different. Approximately three-quarters of trees either had lichens on their trunks and in their canopies, or no canopy or trunk lichens. But 14.8% of trees only had canopy lichens; these trees mainly occurred at intermediate rainfall sites. The inference drawn was that lichens begin to establish at an atmospheric humidity threshold that is lower than that required for trunk lichens on the same trees.

In terms of regional-scale climate-lichen relationships, X. parietina frequency on all trees, was found to increase with increasing annual, summer and winter precipitation; decreasing annual, summer and winter temperatures; and increasing annual, summer and winter evaporation. These results are intuitive and generally fit existing ecological theory.

The most important threat to lichens in the study area is climate change, with changes in habitat, fire and air pollution being of secondary or local importance. Schedules 7, 8 and 9 of the South Australia National Parks and Wildlife Act do not list lichens in their conservation categories. However, the long-term conservation strategy for the state provides for non-vascular plant conservation. To bring the lichen element of this strategy to fruition, more lichen research needs to be conducted in the state and awareness of lichens needs to be raised through formal and non-formal education.

Keywords: corticolous lichens, gradient, climate gradient

Subject: Biodiversity and Conservation thesis, Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Professor Andrew Millington