School of Earth & Environmental Sciences
Natale, Sophie, The role of fire in the population dynamics of the threatened plant species Pomaderris walshii and Pomaderris adnata, BEnviSc Hons, School of Earth & Environmental Sciences, University of Wollongong, 2016.
Fire is a common occurrence across the Australian landscape, hence it is important to understand the effect that fire has on vegetation. In fire prone habitats, fire regimes have shaped the evolution, demography and life history traits of the associated vegetation. The fire regime is composed of a number of characteristics, such as frequency, severity, intensity and seasonality, all of which can contribute to shaping population persistence and response.
This study looks at the effect of fire severity on dormancy, germination and recruitment, with particular emphasis on the impact it can have on temperatures reached within the soil profile. Soil temperature has a direct effect on seeds with physical dormancy and the focus of this study is on two physically dormant and endangered species, Pomaderris walshii and Pomaderris adnata (Rhamnaceae). This study aims to gain an understanding of how these two species respond to fire by:
(1) Determining the dormancy breaking temperature thresholds, as well as the initial seed viability, for both species
(2) Estimating whether there is a germination response from seeds stored within the soil seed bank for P. walshii, and
(3) Evaluating the post-fire life history of P. adnata through tracking seedling emergence and adult resprouting response.
Dormancy breaking temperature thresholds of seeds were determined for both species by applying three heat-shock treatments (60, 80 and 100°C) and following germination response compared against an unheated control. As expected, the germination occurred after application of heat shock treatments, with the amount of germination increasing with increasing temperatures for both species. The greatest germination response occurred after the 100 C treatment, indicating that both species had very high dormancy-breaking temperature thresholds compared to many other physically dormant species in the region.
For P. walshii, I applied a fire treatment to soil seed bank samples collected from underneath adult plants. Different amounts of leaf litter were used to simulate different severities of fire, placed on top of replicate soil samples and then burnt. Seedling emergence was monitored over time. All replicates which were subjected to heat treatments were able to produce at least one P. walshii seedling. However, there was no significant effect of the experimental fire severity, which failed to produce significantly different soil temperatures. While a low level of emergence (60.69 ± 26.64 seedlings per square metre) occurred in response to heat, a lack of a greater response was presumably due to higher soil temperatures not being reached.
Post-fire response and seedling emergence of P. adnata, followed after a section of its population had been burnt, allowed some understanding of the post-fire dynamics of this species. A major finding of this study was that P. adnata was found to resprout post-fire, although it was previously thought to have no resprouting capabilities. A near significant relationship was found for diameter at breast height (DBH) and resprouting, with increasing DBH resulting in a decrease in adults which are found to be resprouting. Following the initial flush of P. adnata seedlings, seedling survival remained high, with the mean number of seedlings per square meter (12 ± 4.6 seedlings per square meter) remaining relatively unchanged throughout the duration of the census. This initial large flush was not followed by subsequent pulses of emergence, despite several large rainfall events. This suggests that all dormant seeds had been exhausted from the seed bank.
The outcomes of this study have enabled a much better understanding of the ecology of these species, particularly in their response to fire. The results of this study lead to the conclusion that both species respond to fire and that seedling recruitment is cued to the post-fire environment. Importantly, the high temperatures required to break dormancy suggest that a higher severity fire would produce the greatest germination response by ensuring higher soil temperatures. This should be considered when implementing fire as a management tool, to achieve high levels of recruitment.