Weeds - Phase 1 - Research Highlights

Seed banks of weed-invaded wetlands

Seed banks of weed-invaded wetlands: implications for biodiversity and restoration

What the report is about

This report examines the consequences of Para grass invading a wetland in the Townsville Town Common Conservation Park and its impact on wetland vegetation and bird communities.

To date, understanding of the regenerative potential of wetlands affected by invasive species has been limited, particularly in northern Australia. Such wetlands are diverse and environmentally, socially and economically important.

This study focused on understanding the restoration of a wetland which has had a long history of invasion by para grass - the Townsville Town Common Conservation Park. This project examined the effect of the invasive macrophytes on wetland soil seed banks. The effects of management techniques to remove para grass and their subsequent impact on soil seed banks were also examined.

What geographical area is affected?

This project was carried out in the wetland area of the Townsville Town Common Conservation Park (TTCCP), north-east of Townsville in north Queensland, and was focused on the freshwater wetlands of the park.

These results also have implications for the management of other invasive species in wetlands in northern Australia.

Aims/objectives

The study asked the questions:

• What is the impact on the soil seed bank in long invaded areas, short invaded areas and areas where para grass has been removed?
• What is the impact on the soil seed bank in areas managed by fire or fire and grazing?
• What is the impact of hydrology on the soil seed bank?
• What are the implications for the recovery of wetlands following the removal of invasive species?

Methods used

All sample sites were within an area of the TTCCP that had previously been used as an experimental area to investigate the use of fire and grazing to rehabilitate weed-invaded wetlands. Sampling was restricted to four paddocks, with historical treatments of burnt and grazed and burnt and ungrazed combinations.

To investigate differences in the seed bank response with time since the invasion of para grass, the study area was stratified into three zones:

1. Areas of long invasion (invaded by para grass prior to 1982)
2. Areas of recent invasion (invaded by para grass after 2000)
3. Recovered (areas previously invaded by para grass but where para grass was absent as a result of previous experimental treatments in 2006).

The length of para grass invasion was determined using high-resolution digital photography of the Town Common area taken in 1982, 2000 and 2006.

Results/key findings

Results indicate the influence of invasion history of soil seed bank species richness. In areas where para grass has been removed, native species richness was found to increase. In comparison, areas which had been invaded by para grass for the longest had a simplified seed bank dominated by one or two species.

Native rather than non-native species dominated the soil seed bank, indicating that management via fire or fire and grazing may be a useful strategy for removing invasive species while maintaining the native soil seed bank. Para grass (the dominant invasive species in this study) was found to have only two seeds represented in the soil seed bank, suggesting that regeneration from seed is unlikely to be a major factor in these wetlands. Management is likely to be complex, expensive and longer term when dealing with other invasive macrophytes, such as Hymenachne amplexicaulis, which has a long‑term seed bank that would continue to be a major management problem.

Within areas of the wetland which are exposed to wet/dry periods, restoration from the soil seed bank will be greater, in comparison to areas constantly inundated by water. In the deeper areas, restoration is likely to be influenced by surrounding species which can regenerate from vegetative mechanisms. Results from this study will be integrated into the current management of para grass in the TTCCP. These results also have implications for the management of other invasive species in wetlands in northern Australia.

Implications for relevant stakeholders:

This study concludes that the use of fire or fire and grazing for the removal of para grass in the TTCCP has resulted in a germinable seed bank dominated by native species. In addition, in areas where para grass has been removed, species richness in the soil seed bank increases. In comparison, in areas where para grass has been established the longest, there is a simplified soil seed bank which has lower species richness but high seed density, dominated by one or two species. That native soil seed bank species can increase following the removal of para grass is useful information for managers seeking to understand the restoration potential of such areas.