Effect of heat shock on germination of 23 plant species in pine-oak and montane cloud forests in western Mexico

We determined the effect of heat shock on seed germination of 23 species growing in fire-prone areas covered by pine-oak and montane cloud forests. Seeds of each species were exposed to 60, 80, 100 or 120 degrees C for 5 or 60 min in dry and moist sand. Seeds of nine species were incubated for 15 h in moist sand and exposed to the temperature that best promoted germination. Eighteen species required heat shock for germination, and five tolerated it. Dry heat shock for 5 min promoted germination of these species; heat shock in moist sand at 100 degrees C induced germination of 10 species. Exposure for 60 min to heat shock in dry and moist sand improved germination of 14 and 5 species respectively. Logistic models indicate that germination probabilities of most species are increased by temperatures of 70-120 degrees C. Depending on substrate and seed hydration, heat shock increased or left unchanged the germination of most pine-oak and montane cloud forest species. Thus, montane cloud forest seeds can survive low-intensity surface fires. Most species with seeds hydrated before heat shock showed little or no germination. Prescribed burns at the end of the rainy season may therefore be harmful to pine-oak and montane cloud forest species.