Seed bank dynamics of two exotic grass species in Australia's northern savanna

The control of two exotic grasses, Andropogon gayanus (gamba grass) and Pennisetum polystachion (mission grass) represent a major management challenge in Australia's tropical savannas. Understanding the size and longevity of the soil seed bank can assist in understanding the management requirements for these species, such as on-going control requirements. This study assessed the seed longevity of mission and gamba grasses, and the seed bank in sites densely invaded by either gamba or by mission grass, and compared them with uninvaded sites. The germinable seed bank of the exotic grasses declines markedly from the dry season to the early wet season, whereas the germinable native seed bank increases during this time. Longevity trials suggest that the proportion of exotic seeds carried over is low, ~2.3 and 0.1% for gamba and mission grass, respectively. The carryover of the exotic seed bank demonstrates the importance of ongoing control. The seed bank of native species in the invaded sites was higher than the exotic seed bank, and it represents a natural source of seed for rehabilitation following control.