Post-fire recruitment for many plant species from wet and dry habitats in southeastern Australia occurs from soil seed banks.  Seeds of a number of such species are water-permeable, and thus may be in a variable state of hydration when heated by the fire.  As the upper layer of soil in dry habitats is generally dry at the time of burning, dry habitat seeds would mostly be air-dry.  In wet habitats however, the upper layer of soil may be wet when a fire passes, and the seeds thus at or near full hydration when heated.  As the heat tolerance of organisms is greatly reduced with increasing hydration, there is the potential for differential mortality of soil-stored seeds in wet and dry habitats during fire.  Thus the contemporary selective pressure to withstand heat whilst hydrated differs between the two habitats.  Whether this has led to differential upper thermal tolerance of hydrated seeds from wet and dry habitats is unknown: if such differences exist, they could explain the current observed habitat segregation of species from wet and dry habitats.  The hypothesis of different upper thermal tolerance for hydrated seeds from wet and dry habitats was tested by examining the germination of seeds that ranged in hydration state from air-dry to fully imbibed when heated at temperatures between 20°C and 180°C.   Seeds from replicate populations of congeneric shrub species in Epacris (Ericaceae) and Baeckea (Myrtaceae) that regenerate after fire from soil-stored seeds in wet or dry habitat were used.

Results/Conclusions

The median upper thermal tolerance of air-dry seeds heated for 5 minutes was 140 ^oC; partial hydration of seeds reduced this to 120 or 140 ^oC for Epacris, or to 80 ^oC for Baeckea (depending on length of hydration).  Whether seeds were from wet or dry habitat did not affect this pattern.  Fully hydrated seeds heated at 80 ^oC for 5 minutes all showed reasonable germination; if heated at 100 ^oC for 1 – 3 minutes, lethal effects became apparent for the longer time exposures.  Fully hydrated seeds of some wet habitat Ericaceae still showed reasonable germination after 3 minutes exposure.  In general, there was no support for the hypothesis of differential tolerance of heat for hydrated seeds from wet or dry habitats. Since contemporary selective pressure did not explain the patterns observed, alternative explanations such as historic selective pressure were examined leading to predictions of heat tolerance for hydrated soil-stored seeds across fire-prone regions.