Seed dispersal and population genetic variation in the context of fire - A case study on two plant species in fire-prone mallee in South Australia

Author: Bianca Frieda Dunker

Dunker, Bianca Frieda, 2015 Seed dispersal and population genetic variation in the context of fire - A case study on two plant species in fire-prone mallee in South Australia, Flinders University, School of Biological Sciences

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Fire is and has been a common occurrence in many vegetation types across the world, impacting on the structure and function of those fire-prone ecosystems. Human influence and a changing climate are causing changes in fire regimes, particularly intervals between fires and the seasonal timing of fires. The frequencies of fires are increasing and fire seasons are tending to shift from hot and dry seasons (natural fires) to cooler and wetter seasons (prescribed burns). Clearing for farmland or urban settlements, in which fire-prone vegetation occurs demands new knowledge to understand the interactions between fire regimes and biodiversity.

In this study I examined the effects of fire on two plant species, Triodia irritans, and Callitris verrucosa. Both species are important features of mallee vegetation and have primarily wind mediated seed dispersal but show different life history traits. The study was carries out at Hincks Wilderness Protection Area on the Eyre Peninsula, South Australia, aiming to assess seed dispersal and population genetic variation under different fire regimes, with a view to making inferences about their resilience to changing fire regimes.

I collected empirical data on primary seed dispersal by physically tracking seeds and measured wind conditions to simulate primary seed dispersal with a trajectory model. Both methods were repeated during different seasons and at different sites. For the genetic assessment I collected plant material from T. irritans and C. verrucosa at sites with different fire histories to determine population genetic diversity, genetic structure and the distance seeds are dispersed.

Primary seed dispersal by wind for T. irritans was found to cover distances of less than 10 m under strong wind conditions. However, historical gene flow between sites was high and rare seed dispersal events of up to 3 km were detected which suggests that secondary seed dispersal and possibly pollen dispersal play an important role in the overall genetic connectivity of T. irritans populations. Triodia irritans population genetic diversity and structure was similar under various fire regimes. I found no evidence for clonality within this species but high levels of polyploidy.

Estimates of primary seed dispersal by wind for Callitris verrucosa were up to 40 m under strong wind conditions while rare dispersal events of up to 8 km were identified. Population genetic diversity and structure in C. verrucosa seems to be independent of fire frequency and the species appeared to be panmictic. Population genetic diversity was very low at all sites. Fire season was found to influence seed dispersal distance in C. verrucosa with longer seed dispersal distances in the natural fire season of summer compared to spring and autumn (prescribed burning).

Findings of this study suggest that both species are well adapted to current fire regimes, focusing on fire frequencies and size of area burned. However C. verrucosa is likely to profit from longer intervals without fire to increase its seed dispersal distances. This information is intended to improve present management of mallee vegetation on the Eyre Peninsula and can potentially be applied to other Mediterranean type ecosystems.

Keywords: Mallee, Callitris verrucosa, Triodia irritans, seed dispersal, population genetics, fire season, fire frequency

Subject: Biological Sciences thesis, Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Biological Sciences
Supervisor: Michael Bull