TOLERANCE TO SOIL TEMPERATURES EXPERIENCED DURING AND AFTER THE PASSAGE OF FIRE IN SEEDS OF ACACIA-KARROO, ACACIA-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 de grees C) of various durations (1, 3, 5 minutes) on seed viability, ger mination and early seedling growth of Acacia karroo, A. tortilis and C hromolaena odorata were determined in laboratory 'simulations'. Seed v iability 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 w ith length of exposure in all species. Seeds of C. odorata did not sur vive any exposure to temperatures of 100 degrees C or above. The acaci as 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 tre atments did not show any abnormal growth patterns, as indicated by vis ual appearance and tissue allocation patterns relative to unheated con trol seedlings. The tolerance of A. karroo to high soil temperatures e xperienced during and after the passage of fire may be a factor that c ontributes to its ability to encroach in disturbed areas. The sensitiv ity of C. odorata seeds to high temperatures may be useful in the cont rol of this aggressive alien invasive plant.