Viable soil seed banks in disturbed lowland tropical rainforest sites in North Queensland

Four 0.125 m² soil samples, 50 mm deep, from each of eight tropical lowland rainforest sites ranging from primary and secondary forests to degraded Imperata grasslands were exposed to shade-house conditions. Germinations of soil seeds were monitored over 28 weeks in April-October. The viable seed banks in the primary forest (592 m^-2) and the secondary forest sites resulting from one cycle of clearing and abandonment (1300–1400 m^-2) were composed predominantly of secondary species. The viable seed banks were small in Imperata grassland sites that had been burnt regularly (200–400 m^-2) and very large (2000–3000 m^-2) in sites that had not been burnt for five years or more. The seed banks of the burnt and unburnt Imperata grassland sites were composed mostly of agricultural weeds whilst seeds of secondary rainforest trees were rare or absent. Similarities between the species composition of soil seed banks and regrowth vegetation types suggest that soil seed composition is important in determining the initial floristic composition following disturbance and that lack of seed of secondary rainforest trees is one factor which restricts re-growth and secondary succession on degraded Imperata grasslands. Species such as Eucalyptus intermedia R. T. Bak., E. pellita F. Muell. and Imperata cylindrica (L.)Beauv. which can establish and deflect the normal rainforest secondary succession were not present in the soil seed bank, and these species appear to rely on fresh seed inputs, lignotubers or rhizomes to colonize and regenerate after disturbance.