Seed production, dispersal and germination in Cryptostegia grandiflora and Ziziphus mauritiana , two invasive shrubs in tropical woodlands of northern Australia

Cryptostegia grandiflora and Ziziphus mauritiana are exotic shrubs that are invading tropical woodlands of northern Australia. Although they are widespread, they currently occupy only small proportions of their respective potential ranges. Large C. grandiflora can produce more than 8000 wind‐dispersed seeds in a single reproductive episode and can set seed at least twice per year. More than 90% of seeds will germinate within 10 days of moisture becoming available. Few, if any, seeds survive for more than 12 months in the soil. Large Z. mauritiana can produce more than 5000 fruits per year. The fruit is a drupe that contains a single seed (sometimes two) enclosed in a woody endocarp. Less than 10% of fresh seeds will germinate without removal of the endocarp. Removal of the endocarp increased germination to 56%. Less than 10% of seeds of Z. mauritiana remain viable after burial in the soil for 12 months. Seeds of Z. mauritiana are dispersed by several mammalian vectors, including wallabies and feral pigs, but domestic cattle are probably the major means by which large numbers of seeds reach new sites. Propagules pass intact through the digestive tract of cattle, feral pigs and wallabies and contain viable seeds that germinate more readily than seed in fresh intact fruits. Controlling movement of cattle that have had recent access to mature fruit of Z. mauritiana could significantly reduce the likelihood of new infestations. For both species, management that reduces seed production will be important for containing spread.