Tolerance to soil temperatures experienced during and after the passage of fire in seeds of Acacia karroo, A-tortilis and Chromolaena odorata: a laboratory study

The effects of high daily soil surface temperatures (50 degrees and 70 degrees C) of various durations (1, 2, 3, 4 weeks), and temperatures experienced in the soil during fires (100 degrees, 150 degrees, 200 degrees C) of various durations (1, 3, 5 minutes) on seed viability, germination and early seedling growth of Acacia karroo, A. tortilis and Chromolaena odorata were determined in laboratory 'simulations'. Seed viability declined with increasing duration of exposure at 50 degrees C in A. tortilis and C. odorata, but only slightly in A. karroo, which was stimulated to germinate. At 70 degrees C seed viability declined with length of exposure in all species. Seeds of C. odorata did not survive any exposure to temperatures of 100 degrees C or above. The acacias survived temperatures of 100 degrees and 150 degrees C, with a few A, karroo surviving 200 degrees C (< 20% survived after one minute of heating). The emergent seedlings exposed to any of the temperature treatments did not show any abnormal growth patterns, as indicated by visual appearance and tissue allocation patterns relative to unheated control seedlings. The tolerance of A. karroo to high soil temperatures experienced during and after the passage of fire may be a factor that contributes to its ability to encroach in disturbed areas. The sensitivity of C. odorata seeds to high temperatures may be useful in the control of this aggressive alien invasive plant.