Physical dormancy in legume seed is typically broken due to heat shocks in fire-prone ecosystems. Daibes et al. applied heat shocks to 46 legume species from a fire-vegetation gradient (grassy savannas, woody savannas and forest) in a tropical savanna (Cerrado) of central and south-eastern Brazil.
They found Heat shocks had little effect on germination, but seed mortality was variable across treatments and species. Seed mortality was lowest under the 100 °C 1 min treatment, and significantly higher under 100 °C 3 min and 200 °C 1 min; larger seed mass decreased seed mortality, especially at 200 °C. Tree species in Detarioideae had the largest seeds and were unaffected by heat. Small-seeded species (mostly shrubs from grassy savannas) were relatively sensitive to the hottest treatment. Nevertheless, the presence of physical dormancy helped to avoid seed mortality in small-seeded species under the hottest treatment. Trees had larger seeds (phylogenetically clustered), avoiding seed mortality under the hottest temperature.
The authors conclude physical dormancy-break is not tied to fire in the Cerrado mosaic. Heat tolerance appears in both forest and savanna species and is predicted by seed traits (seed mass and physical dormancy), which might have helped forest lineages to colonize the savannas. The results show seed fire responses are better explained by historical than ecological factors in the Cerrado, contrasting with different fire-prone ecosystems throughout the world.