High summer temperatures do not interact with fire to promote germination among seeds of Cistaceae: A reinterpretation of Luna (2020) with extra data on wet/dry conditions*

In a unique study, Luna (Luna, Sci Rep 10:1–10, 2020) examined the viability and germination of 12 hard-seeded Cistaceae in the Mediterranean Basin by alternating a prolonged summer-type temperature (50/20 °C at 12 h cycles) treatment with a fire-type heat pulse. A re-analysis of their data shows that the summer treatment applied before the heat pulse was superfluous as similar high levels of germination under ambient conditions were attained with the heat pulse only. Additional tests using the hard seeds of Acacia showed that the water gap opened at once in the presence of dry heat such that contact with moisture is not required to complete the process of softening. The abundance of hard seeds remaining when the summer treatment was applied after the heat pulse is better explained by ungerminated seeds having become hard again under such dry conditions rather than remaining dormant, i.e., acquiring secondary physical dormancy, and thus becoming ‘desensitized’ to their environment. While this response may be adaptive, such a retarding effect will be limited in practice as most fires are expected in autumn, at least historically, and are thus close to the start of optimal winter conditions for germination. Future studies should concentrate on the fate of the water gap plug during such alternating treatments and also ensure that realistic summer temperature regimes are used.