Abstract
Dormancy and germination requirements determine the timing and magnitude of seedling emergence, with important consequences for seedling survival and growth. Physiological dormancy is the most widespread form of dormancy in flowering plants, yet the seed ecology of species with this dormancy type is poorly understood in fire-prone vegetation. The role of seasonal temperatures as germination cues in these habitats is often overlooked due to a focus on direct fire cues such as heat shock and smoke, and little is known about the combined effects of multiple fire-related cues and environmental cues as these are seldom assessed in combination. We aimed to improve understanding of the germination requirements of species with physiological dormancy in fire-prone floras by investigating germination responses across members of the Rutaceae from south eastern Australia. We used a fully factorial experimental design to quantify the individual and combined effects of heat shock, smoke and seasonal ambient temperatures on germination of freshly dispersed seeds of seven species of Boronia, a large and difficult-to-germinate genus. Germination syndromes were highly variable but correlated with broad patterns in seed morphology and phylogenetic relationships between species. Seasonal temperatures influenced the rate and/or magnitude of germination responses in six species, and interacted with fire cues in complex ways. The combined effects of heat shock and smoke ranged from neutral to additive, synergistic, unitive or negative and varied with species, seasonal temperatures and duration of incubation. These responses could not be reliably predicted from the effect of the application of single cues. Based on these findings, fire season and fire intensity are predicted to affect both the magnitude and timing of seedling emergence in wild populations of species with physiological dormancy, with important implications for current fire management practices and for population persistence under climate change.
Highlights
Many plant species in fire-prone regions worldwide rely on recruitment from soil seed banks for population persistence in the face of recurring fires [1,2,3]
Different maturation times between species and logistical constraints meant that the time elapsed between seed collection and commencement of experiments ranged from 5 weeks (B. anemonifolia, B. fraseri, B. pinnata, B. serrulata) to 9 weeks (B. floribunda, B. ledifolia, B. thujona), during which seeds were stored in envelopes under ambient temperatures varying between c. 20–25°C
We found that heat shock was essential for germination of B. fraseri and B. ledifolia, and that it enhanced the speed and magnitude of germination of B. anemonifolia in the presence of smoke (Fig 2)
Summary
Many plant species in fire-prone regions worldwide rely on recruitment from soil seed banks for population persistence in the face of recurring fires [1,2,3]. The first step is to characterise species’ dormancy and germination requirements as these control the timing of seedling emergence [5] which has important consequences for seedling survival and growth [6,7,8,9], and flow on effects for population dynamics and community composition [10]. A specific seasonal temperature is the most common primary cue and this ensures that seedling emergence coincides with the time of year when conditions are most favourable for successful establishment [5]. In fire-prone ecosystems, seeds with PD may require additional fire-related stimuli such as heat shock and/or smoke to promote significant levels of germination [13,14,15,16], resulting in a seasonal pulse of post-fire seedling emergence [17,18]
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