Weeds - Phase 1 - Research Highlights

Sagittaria platyphylla - clonal propagation

 

What the report is about

Aquatic weeds threaten many Australian riverine systems, adversely affecting native biodiversity, hydrological flows, water quality and availability.

The changes aquatic weeds can cause include restricting water flow and increasing sedimentation, displacing native plant species and limiting opportunities for recruitment, reducing habitat for aquatic species such as fish and tortoises, and limiting recreational activity.

Despite these threats, the demography and dispersal dynamics of many aquatic weeds are not well understood in Australian riverine systems.

This report quantifies clonal propagation in the invasive aquatic weed Sagittaria platyphylla by determining its corm production and regenerative capacity under controlled herbicide treatment and growth environments.

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Background

Some 20 species of Sagittaria, or arrowhead, exist worldwide, most of them occurring naturally in North and South America. Several species are edible; others are used for the aquarium trade, which has led to their introduction into many new environments. The genus has become increasingly invasive in Australia in the past two to three decades, and two species-S. platyphylla and S. montevidensis-are now recognised as aquatic weeds, there being infestations in most parts of Australia.

The genus is especially prevalent in irrigation areas and natural river systems of the central Murray-Darling district. In Victoria S. platyphylla is invasive throughout natural waterways and the Goulburn-Murray Water irrigation district, infesting wetlands and streams that are tributaries of the River Murray-among them the lower Ovens River and the Goulburn River and throughout the Ramsar-listed Barmah National Park wetlands.

Aims/objectives

The aim of the research was to investigate the relative contribution of seed and vegetative propagules to the invasion front in order to help refine management strategies by doing the following:

• identifying which propagules are most likely to disperse and successfully establish in new environments
• identifying high-risk source populations requiring more strategic management
• evaluating the efficacy of current strategies for controlling Sagittaria.

This information will be used to improve the means and the cost-effectiveness of control, particularly for interested parties with large and ongoing investments in Sagittaria control.

The relative contribution of clonal reproduction to dispersal and invasive spread was assessed by evaluating the spatial genetic structure of S. platyphylla in the core region of its current distribution in Victorian and NSW waterways.

Methods used

This project used genetic and demographic approaches in order to understand Sagittaria platyphylla dispersal in the Barmah National Park wetlands in northern Victoria and adjoining Millewa Forest in New South Wales. In this ecosystem, S. platyphylla appears to be at a dynamic invasion stage, with many small patches ranging from clustered to highly dispersed and relatively fewer large, continuous stands (50 to 1000 square metres), this offers the opportunity to gain an understanding of dispersal and recruitment dynamics at relatively fine scales.

Six wetlands on tributaries of the River Murray in Victoria and New South Wales were chosen as study sites. The sites were representative of the core geographic range of S. platyphylla infestations in natural waterways during the 2008-09 growing season. During March 2009 leaf material was randomly sampled for genetic analysis from 20 to 60 plants at each site, depending on the size of the infestation. Five replicate soil cores (25 square centimetres and 10 centimetres deep) were also randomly collected at sites 1 to 5 for germination experiments. Whole plants for growth trials (30 per site) were collected at least 10 metres apart so as to minimise the chance of sampling replicate genotypes from each site.

The leaves, soil and plants collected were used to generate information about the following:

• the genetic composition and relatedness of plants within and among the sampled sites
• the persistence of the soil seed bank
• the capacity of vegetative fragments to establish and thrive, including following herbicide application.

Results/key findings

The main findings of the investigation are as follows:

• Seeds, rather than clonal propagules, are the main dispersal unit in S.platyphylla. Seed propagation is a primary leverage point in the weed's life cycle for improving effective control.
• Production of viable vegetative propagules (corms) is prolific under conditions conducive to growth. Glyphosate applied at field rates translocates to corms and substantially reduces regeneration potential. Desiccation over seven to 30 days reduces, but does not void, corm viability. Combining herbicide application and desiccation might be a useful control for established infestations.
• Genotypic diversity varied between the assessed localities. Shared genotypic groups were detected among localities, suggesting a high degree of connectivity. The genetic distribution among streams is consistent with multi-directional range expansion and long-distance seed dispersal (tens of kilometres). These results suggest that the increasing invasion of S. platyphylla has been facilitated by the movement of genotypes within and among streams under the prevailing habitat and flow regimes.

Implications for relevant stakeholders

Seed appears to be the main dispersal mechanism for S. platyphylla within and among natural streams, suggesting that seed propagation is an important leverage point for species management.

Current herbicide application rates in natural waterways have the capacity to reduce Sagittaria biomass and limit the recruitment ability from corms. Corms can, however, resprout vigorously when detached from the parent plant and when placed in water. Desiccation for seven to 30 days substantially reduced, but did not completely void, the corms' ability to regenerate. This suggests that herbicide and drying in combination could be used to control vegetative expansion of Sagittaria biomass.

The spatial genetic distribution of S. platyphylla reflects the movement of genotypes (genes) among streams in the river floodplain ecosystem, consistent with multi-directional range expansion. Genetic connectivity among localities is consistent with long-distance dispersal via seed (tens of kilometres).

No evidence for a persistent soil seed bank was found, suggesting that seedlings are derived predominantly from seasonally reproduced seed crops, rather than persistent seed. This is consistent with the rapid germination rates and the lack of dormancy mechanisms observed for S. platyphylla seed under trial conditions.