Elucidating relationships between disturbance and invasion in riparian zones

Citation

Land & Water Australia. 2008. Elucidating relationships between disturbance and invasion in riparian zones. [Online] (Updated April 28th, 2009)
Available at: http://lwa.gov.au/node/2581 [Accessed Tuesday 26th of April 2011 03:56:53 PM ].

Summary

This project sought to develop improved riparian weed management strategies by analysing the linkages between disturbance from flood events and both weed invasion and tree recruitment in riparian areas. Analysis of an existing dataset containing assessments of vegetation variables from 23 rivers across Victoria resulted in the development of predictive models relating the number of eucalypt and acacia seedlings at a site to the number of parent trees, the amount of bare ground, the catchment management region and the cover of native sedges (eucalypt seedlings only).

Aims

By elucidating the relationships between the disturbance caused by flooding and weed invasion, and tree recruitment, this project will be able to provide land-managers with guidelines so their weed management activities can optimise tree recruitment and the creation of these bare areas through flooding, it is highly probable that the period immediately after a flood event is critical to tree seeding establishment. Understanding the dynamics of weed invasion and establishment during this recruitment window will allow the development of appropriate weed management strategies. It is possible that these strategies may differ, not only depending on the mix of weeds present, but as a consequence of river type.

The objectives of this project are to develop improved riparian weed management strategies for land managers, incorporating analysis of the linkages between:

Disturbance from flood events and weed invasion in riparian zones;
Disturbance from flood events and the recruitment of native tree species.

The outputs from this project are threefold:

Riparian weed management guidelines;
Weed manager training;
Scientific paper.

Outcomes

The cover of exotic life forms and number of exotic taxa did not affect tree seedling numbers, indicating that weeds did not inhibit tree recruitment at the landscape scale in the sites studied. A new dataset containing hydrological variables for the rivers under study was compiled and initial regression of these variables against the vegetation variables was undertaken. The cover of various exotic life forms was not affected by flood frequency or the total, average or longest duration of floods, while the total cover of weeds decreased as the number of flood events increased. Sites with more frequent and longer duration floods had a higher proportion of native to exotic taxa. The number of days that the riparian area was under water was negatively related to the number of eucalypt and acacia seedlings found at a site. Riparian weed management guidelines have been developed, in association with on-ground managers, which provide information and advice which can be incorporated into weed management programs seeking to enhance the condition of riparian areas.

Background

Riparian zones are highly dynamic landscape components, forming the interface between aquatic and terrestrial ecosystems. Flood events are integral to the functioning of riparian zones, by periodically redistributing energy and materials. This results in heterogenous ecosystems providing numerous microsites in which different plants can establish. In both Australia and overseas it has been found that a wide range of native and exotic species exploit these microsites, making riparian zones some of the most biodiverse communities in the landscape.

This, however, results in sites with multiple weed species, increasing the complexity of weed management. This is further compounded by the limitations on chemical control methods which can be applied near waterways, while the presence of some level of native vegetation in the riparian zone also limits control options. This is particularly important in sites where the vegetation community depends on natural regeneration processes as some weed control strategies can destroy entire cohorts of young native plants.