Managing high threat weeds

Why is weed control important along the upper Murrumbidgee?

The upper Murrumbidgee River is an important corridor connecting the landscape of the catchment and providing a sheltered niche in our sub-alpine climate.   The instream and riparian (riverside) vegetation reflects this and the diversity of habitat types the corridor traverses on its travels. 

The upper Murrumbidgee’s headwaters are characterised by swampy meadows and River tussock lands.  It’s steep sided, rocky, inaccessible gorges have shrubby woodlands, including quiet stands of Cypress pine and instream shrublands where bottlebrushes can be seen with trunks as thick as ones thigh, holding together the cobble based beds during times of high flow.  Where possible instream reeds from wetland complexes and shallower backwaters contain large beds of submergent ribbon weed, water milfoil and pondweed.    The wide open floodplains of the river would have had floodplain forests- tall ribbon gums upstream of the ACT and large whispering River she-oaks downstream, signalling the change as one moves through from the upland (above 600m ASL) to the lowland areas (below 600m) of the river.  This rich matrix of habitat and vegetation supports the ecological processes of the river and the species which rely on it.   

Unlike many rivers around the world the upper Murrumbidgee is fortunate that many areas of high quality native vegetation remains, found primarily in the steeper, rockier gorge sections.  The more open, alluvial floodplains however have been extensively cleared due to historical agricultural practices and little native vegetation remains, leaving the riparian and instream ecosystems open to be taken over by weeds- and in many cases this is what has occurred. 

Woody weeds such as willows, box elder and blackberry are a problem for both high quality and degraded areas where native vegetation has been removed.  Firstly, the fact native vegetation is missing disrupts the essential processes and functions which this vegetation provides and these altered conditions allow woody weeds to take hold.  Once established weed infestations have their own impacts on a site which can affect the health of the river ecosystem.  Woody weeds can also infest areas of high quality native vegetation with similar impacts.    

One woody weeds form dense stands, these outcompete native vegetation, or prevent native vegetation from regenerating where it has been cleared.  Dense willow infestations also shade out reeds and instream vegetation, simplifying habitat on which aquatic species rely.  Willow roots colonise instream habitat, trapping sediment and reducing instream habitat availability.  Leaf fall from willows and box elder affects water quality as it degrades and change the trophic food-webs characteristic of upland streams, in turn affecting river health and habitat quality. 

Managing woody weeds is an important issue.  However it must be carried out in a strategic way to ensure that the ecosystem processes and functions (such as shading, vegetation complexity and habitat) of a site are supported while native vegetation recovers.  At a site where there is existing native vegetation, the priority is to control woody weeds so that native vegetation health can improve.  At a site where there is little native vegetation present a staged approach may be taken to control and removal of weed infestations and establishment of native vegetation and habitat.  Site specific species assessments and monitoring should be part of every control program.