Seed size-mediated dormancy thresholds: a case for the selective pressure of fire on physically dormant species.

In physically dormant species, fire-generated heat breaks seed dormancy, with dormancy-breaking temperature thresholds varying among species. Fire-related heating decreases with depth, due to the insulating effects of soil. Concurrently, smaller-seeded species are restricted to germinating near the surface because of limited reserves within such seeds. We hypothesized that dormancy-breaking temperature thresholds of physically dormant species would be higher in smaller-seeded species, to ensure emergence is restricted to shallower soil depths, and that seed survivorship would follow the same pattern. This was tested experimentally for 14 species from south-eastern Australia, and by using a larger data set of species compiled from the literature to assess if any patterns hold across a broader group. Seed size was negatively related to dormancy-breaking temperature thresholds. Mortality at 100 °C showed a positive relationship with seed size. Our findings suggest that small-seeded species are subject to firerelated selection pressure that results in higher dormancy-breaking temperature thresholds and resistant to hotter temperatures, which may act as a depth detection mechanism. By using a broader range of species, we highlight that this relationship is strong and representative of species across a number of phylogenetic groups. [ABSTRACT FROM AUTHOR]