Influence of heat shock on seed germination of plants from regularly burnt savanna woodlands and grasslands in Ethiopia

The effect of heat shock on the germination of seeds of 21 plant species from fire-prone wooded savanna ecosystems in western Ethiopia was analysed in order to examine the possible implications of fire upon plant regeneration after this disturbance. Seeds were subjected to 6 different heat intensities (20, 60, 90, 120, 150 and 200 degreesC) for 1 or 5 minutes, in order to simulate the situation in the upper soil layers or on the soil surface during fires. Germination tests were carried out in pots in a greenhouse over 20 weeks. After 9 weeks no more seedlings emerged. There was wide interspecific variation in the responses of seeds to the different treatments. In all species, germination was significantly affected by the temperature treatment level. Short exposure of seeds to high temperatures generally stimulated germination whereas prolonged exposure reduced seed germination. However, some species even tolerated 5 min treatment at 200 degreesC. Seed heat resistance was positively correlated with seed length and mass among the species. Hence, production of large seeds with protective tissues promotes survival in fire-prone savanna areas. Also, the seeds of some species showed both a low and a high temperature optimum which ensures that at least some seeds germinate in the absence of fire, but also that viable seeds still remain if subsequent late fires kill emerging seedlings. Frequent and light burning in wooded savanna grasslands seems to stimulate and enhance germination of most of the studied plant species.