A song of smoke and seeds: post-fire seedling emergence in grasslands and grassy woodlands of south-eastern Australia

Joshua Hodges

Research output: ThesisDoctoral Thesis

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Abstract

FIRES are common globally. Although fires are famed for their destructive power, many species have evolved to not only tolerate, but benefit from fire. Fire benefits species by removing biomass and dead litter, reducing competition for light and resources which allows smaller species to establish and flower.
Species have evolved different mechanisms to take advantage of favourable conditions in the post-fire environment, such as re-sprouting from buds-and-tubers, post-fire flowering, thick bark, seperated canopies, post-fire seed dispersal, thermal insulation and post-fire seedling emergence in response to smoke and heat cues.
In critically endangered native grasslands and grassy woodlands of south-eastern Australia disturbances such as fire are needed to maintain diversity of native species. Disturbances maintain diversity by reducing competition for light and gaps. In the absence of disturbance, small wildflowers (also known as
‘forbs’) and grasses are unable to establish and larger grasses can be killed under the weight of their own accumulated dead litter. Given the important role of fire in maintaining diversity, it stands to reason that species may have evolved to germinate in response to smoke and heat cues. However, based on 20+ years
of research it is typically thought that few species in these ecosystems germinate in response to smoke and heat cues.The supposed absence of fire-cued germination in these ecosystems may be because: a) germination trials to
date have focused on a small number of common species, b) field studies have attributed post-fire germination to a reduction in biomass without considering smoke and heat and c) grassland and grassy woodland species are seed
limited (i.e., little viable seed is present in the seed bank). That is, it is not that species do not respond to fire it is just that the species are not there. This would mean no seed was present to respond to a fire, and may account for no
seedling emergence observed post-fire in observational studies. Based on these three lines of evidence, I expected that the absence of evidence of post-fire seedling emergence among grassland and grassy woodland species was
not evidence of absence. This thesis tests the hypothesis that fire (via smoke
and heat cues) increases germination and seedling emergence of many species common in grasslands and grassy woodlands of south-eastern Australia. The experiments presented in this thesis were designed to test the effect of fire on different aspects of germination (percent germination, germination speed and germination timing) in the laboratory and seedling emergence in the field.
First, I conducted an experiment on 55 common native forbs and grasses to determine if they germinate in response to smoke and heat cues. I found that smoke and heat cues increased both percent germination (the number of seedlings which germinated) and germination speed (how quickly after being watered they germinated) when data of all species was pooled (Chapter 2), and for 44 of the 55 species (Chapter 3). Whether a species responded to smoke or heat was explained by water-permeability of the seed coat. Hard-seeded
species responded to heat alone and species with water-permeable seed coats responded to either smoke or smoke and heat combined (Chapter 2). Secondly, I examined if smoke and heat effects germination timing or if the timing of application of smoke would affect percent germination. I found that fire does not typically alter germination timing and that most species responded to smoke regardless of when it was applied (Chapter 4). Although, four species did not germinate when exposed to spring smoke. I concluded that fire-cues can increase how much and how fast germination occurs, but does not effect when germination occurs. Lastly, I conducted a field experiment to determine if seedling emergence was linked to a reduction in biomass alone, or if fire was needed to increase emergence. Despite biomass removal and fire having similar
affects on light and space availability, only fire increased emergence of native species (Chapter 5). I conclude that fire, not simply biomass removal, increases seedling emergence of native grassy ecosystem species. In contrast with more than two decades of research—the combined results of the field and lab experiments presented in this thesis demonstrate that fire (via smoke and heat cues) increases seedling emergence of common grassy ecosystem species. This
is interesting ecologically and provides important information for grassland and grassy woodland land managers by highlighting the role of fire in maintaining diversity of native forbs and grasses by increasing emergence. It is hoped that the findings presented here encourages grassy ecosystem land managers to utilise fire to maintain diversity in critically endangered grassy ecosystems of
south-eastern Australia.
This thesis has broadened our understanding of how grassland and grassy woodland species respond to fire and has important implications for management of these critically endangered ecosystems. However, there remains several unanswered questions:
1) Why are species common in native grassy ecosystems of south-eastern Australia seed limited? Is it because of poor dispersal, rapid germination shortly following dispersal, short seed longevity, poor seed defence from
predators or another reason?
2) Seed addition can be both expensive and labour intensive, so what methods and technologies can we develop to improve the success of seed addition and ensure that seed is available to respond to fire?
3) How do frequent spring fires effect emergence of species which appear to be sensitive to seasonal timing of exposure to fire-cues?
4) What drives the apparent within-species variation in response to smoke and heat cues observed when comparing different grassy ecosystem germination studies?
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Charles Sturt University
Supervisors/Advisors
  • Price, Jodi, Principal Supervisor
  • Guja, Lydia K., Principal Supervisor, External person
  • Nicotra, Adrienne B., Principal Supervisor, External person
Award date19 Dec 2023
Place of PublicationAustralia
Publisher
Publication statusPublished - 19 Dec 2023

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