Fire Seasonality, Seasonal Temperature Cues, Dormancy Cycling, and Moisture Availability Mediate Post-fire Germination of Species With Physiological Dormancy

Abstract

Fire seasonality (the time of year of fire occurrence) has important implications for a wide range of demographic processes in plants, including seedling recruitment. However, the underlying mechanisms of fire-driven recruitment of species with physiological seed dormancy remain poorly understood, limiting effective fire and conservation management, with insights hampered by common methodological practices and complex dormancy and germination requirements. We sought to identify the mechanisms that regulate germination of physiologically dormant species in nature and assess their sensitivity to changes in fire seasonality. We employed a combination of laboratory-based germination trials and burial-retrieval trials in natural populations of seven species of Boronia (Rutaceae) to characterize seasonal patterns in dormancy and fire-stimulated germination over a 2-year period and synthesized the observed patterns into a conceptual model of fire seasonality effects on germination. The timing and magnitude of seedling emergence was mediated by seasonal dormancy cycling and seasonal temperature cues, and their interactions with fire seasonality, the degree of soil heating expected during a fire, and the duration of imbibition. Primary dormancy was overcome within 4–10 months’ burial and cycled seasonally. Fire-associated heat and smoke stimulated germination once dormancy was alleviated, with both cues required in combination by some species. For some species, germination was restricted to summer temperatures (a strict seasonal requirement), while others germinated over a broader seasonal range of temperatures but exhibited seasonal preferences through greater responses at warmer or cooler temperatures. The impacts of fires in different seasons on germination can vary in strength and direction, even between sympatric congeners, and are strongly influenced by moisture availability (both the timing of post-fire rainfall and the duration soils stay moist enough for germination). Thus, fire seasonality and fire severity (via its effect on soil heating) are expected to significantly influence post-fire emergence patterns in these species and others with physiological dormancy, often leading to “germination interval squeeze.” Integration of these concepts into current fire management frameworks is urgently required to ensure best-practice conservation. This is especially pertinent given major, ongoing shifts in fire seasonality and rainfall patterns across the globe due to climate change and increasing anthropogenic ignitions.

Seed functional ecology in Brazilian rock outcrop vegetation: an integrative synthesis

“…Smoke did not affect germination percentage but consistently reduced germination time. This result agrees with the role of smoke-derived compounds as germination stimulants of non-dormant seeds or those where dormancy has been alleviated (Mackenzie et al . 2021).…”

Seed dormancy alleviation by warm stratification progressively widens the germination window in Mediterranean climate Rutaceae

“…A lack of smoke response for fresh seed of the two study species does not neatly align with previous records for Australian Rutaceae. Significant germination responses have been noted following the application of heat, smoke or karrikinolide (KAR1) (Commander et al 2009;Mackenzie et al 2016), or the passage of fire (Roche et al 1997;, and soil stored seeds may respond differently than those studied here following the effects of seasonal afterripening and stratification (Mackenzie et al 2021). It does however demonstrate the inconsistency of smoke response between species of Rutaceae and suggests that dormancy alleviation may reduce the requirement for smoke stimulated germination in the inter-fire period.…”

Factors influencing the germination, establishment and distribution of

“…This supports the interpretation of the stem-length size-class data indicating continuous recruitment. However, the emergence of seedlings may peak after fire, and fire, in conjunction with seasonal temperatures (Mackenzie et al 2021), could promote emergence in areas containing a soil seedbank (Read et al 2000).…”