Post-fire regrowth and survival of the invasive tropical shrubs Cryptostegia grandiflora and Ziziphus mauritiana

Exotic shrubs are significant invaders of the tropical woodlands of northern Australia. They cause problems for pastoral industries and are likely to induce significant change in woodland communities. A single fire in the middle of the dry season affected the survival and vegetative growth of two important shrub species, Cryptostegia grandiflora and Ziziphus mauritiana. The fire killed about 96% of small plants (height<100 cm), 80% of medium-sized plants and 45% of large plants (height>200 cm) of C. grandiflora. However, only about 10% of small Z. mauritiana were killed. Most plants of Z. mauritiana resprouted vigorously within three months of the fire. Effects on the vegetative phenology of C, grandiflora persisted until 10 months after the fire, but by four months after the fire, burnt and unburnt Z, mauritiana were similar as regards the distribution of individuals between phenological categories. Changed fire regimes may partially explain the colonization of northern and northeastern Queensland by C. grandiflora but probably had little effect on Z. mauritiana. Fire has significant potential as a tool in the management of C. grandiflora wherever adequate grass fuel can be attained. The use of fire will be particularly valuable for preventing range expansion, for situations where the species is in the early stages of invasion and in parts of the landscape where densities are relatively low.