Effects of fire on germination and viability of rubber vine (Cryptostegia grandiflora) seeds

Abstract

Studies were undertaken at 2 sites in northern Queensland (Wrotham Park and Ten Mile Creek) to quantify the effects of fire on germination and viability of both loose rubber vine seeds and seeds held within pods. Seed lots were placed at a range of positions within the vertical profile of rubber vine infestations; 0.5 cm below ground (loose seeds only), on bare ground, below fuel, above fuel and on crowns of free-standing rubber vine. The influence of seed age (0 and 1 year old) and seed plumage (with and without plumes) was also examined for loosely held seeds. Late season fires were imposed at both Wrotham Park and Ten Mile Creek, with an additional early season fire undertaken at Ten Mile Creek to enable comparison between seasonal fires. Maximum fire temperatures reached during burning averaged 853, 827 and 871˚C for Wrotham Park, Ten Mile Creek early and Ten Mile Creek late season fires, respectively. The temperatures that seeds were exposed to depended on their location within the vertical profile, with temperatures above fuel > below fuel > on rubber vine crown > bare ground > 0.5 cm below ground. Seed viability after burning ranged from 0 to 100%, with seeds generally most affected if located above or below the fuel, and least affected if buried below ground. The age of seed and the presence of seed plumes interacted with seed placement so that seed germination and viability of loose seeds were affected. Some seeds remained viable, but were inhibited from germinating, particularly if located 0.5 cm below ground. Positive, non-linear relationships occurred between seed mortality and maximum temperature and between seed mortality and the duration of exposure. These results suggest that fire may inhibit germination and kill a substantial proportion of rubber vine seeds.