Wetlands Australia 33: Passive regeneration shapes ecological values at Jerrabomberra Wetlands

Author: Jane Roberts, Member of Jerrabomberra Wetlands Advisory Panel

Jerrabomberra Wetlands, at the eastern end of Lake Burley Griffin in the heart of Canberra, is just over 50 years old. Despite being a young ecosystem, it provides interesting lessons and case histories about wetland vegetation and wetland plants. This article describes the pleasing – and not so pleasing – outcomes of an unplanned and passive approach to wetland vegetation establishment, as revealed by vegetation mapping in October 2013.

Planting is a well-accepted strategy for wetland restoration, also known as assisted or active regeneration. In contrast, unassisted or passive regeneration is when no planting is done, and re-vegetation is by plants self-establishing from propagules already in the wetland seed bank, or by dispersal.

But what happens when a wetland is newly formed, and there are no wetland propagules? Will the wetland become a mess of opportunistic, introduced species, blown in by the wind? One answer, revealed when the vegetation of Jerrabomberra Wetlands Nature Reserve, ACT was mapped in October 2013, is: Not necessarily! Be prepared for surprises!

These wetlands developed after Lake Burley Griffin filled, just over 50 years ago, on land that had been dairy country, and prior to that had been grazed, cropped and cultivated. Some planting has been done, but not into wetland areas.

Unexpectedly, only 4 of the 10 wetland-vegetation mapping units were dominated by invasive species. Three were commonly-occurring native species (Typha spp., Phragmites australis, Carex appressa) – so no surprises there – but the fourth was an unusual exotic weed (Yellow Water Lily Nymphaea mexicana). This lily is viviparous, meaning the seed germinates, then develops roots and leaves while still on the parent plant. When the seedling eventually floats away, it is well-advanced. The likely source of seedlings reaching Jerrabomberra Wetlands is the public gardens further down Lake Burley Griffin, where Yellow Water Lily is an ornamental.

Yellow Water Lily (Nymphaea Mexicana) showing its bulky leaves and rhizome-roots, Wetland 1, Jerrabomberra Wetlands, March 2014. Photo: Jane Roberts.

A big surprise was that the species composition of two units was analogous to ‘natural’ plant communities of the surrounding South Eastern Highlands bioregion: Eleocharis acuta – Amphibromus nervosus community, and Puccinellia - Schoenoplectus pungens community. How this happened is not understood, but both these areas have an oscillating wet-dry water regime, and attract mud-foraging waterbirds.

Six species had distribution records of interest, being either rarely or not previously recorded in the ACT. This was pleasing, but not surprising, as wetland plants are frequently under-collected.

The expectation that reserve vegetation would be mainly introduced species was supported. Of the 298 species recorded in October 2013, only 68 were native. However, wetland plants were disproportionately represented, accounting for 39 of the 68 natives, even though wetland vegetation covers only 11% of the reserve.

Overall, and fortuitously, the passive approach has been positive for this reserve, adding to its diversity although not completely free of management headaches such as eliminating Yellow Water Lily.

Further information

Eco Logical Australia (2014). Jerrabomberra Wetlands Nature Reserve. Vegetation Mapping 2014. Prepared for ACT Parks and Conservation Service, Territory and Municipal Services, and the Jerrabomberra Wetlands Board of Management.

Schoenoplectus pungens sedgeland near the Peninsula, Jerrabomberra Wetlands, October 2020. Photo: Jane Roberts.