Tasmania's native riparian vegetation

Riparian vegetation has significant environmental, social and economic values that are intimately linked to its roles and functions in the terrestrial and aquatic environments. Yet, riparian vegetation research is in its infancy relative to studies of other terrestrial vegetation. Riparian plant communities have not been included in national or statewide vegetation mapping projects and there is a general lack of knowledge about individual native species and floristic assemblages that inhabit the riparian zone. In order to address some of the knowledge gaps associated with riparian vegetation, a rapid survey methodology was developed and used to document the native species composition of approximately 50,000 km2 of mainland Tasmania. No native riparian vegetation could be found in 6 000 km2 of the survey area. Structural attributes of the riparian vegetation, and key environmental factors associated with riparian soil, substrate, landform, channel and bank characteristics, were also measured or described during the field survey. Climatic data, altitude, aspect, adjoining land use, visible disturbances and the vegetation structure of riparian vegetation and adjoining vegetation were also recorded. The number of native vascular plant species recorded in the riparian zone of 460 sites was 860. Of these species, only 8 were recorded in at least 50% of sites; the majority of native vascular species in the riparian zone of Tasmania occur in less than 10% of sites. Only 2 native vascular species are considered to be possibly obligate riparian species. At least 46 species listed under the Tasmanian Threatened Species Protection Act 1995 were located in the riparian zone. In addition, populations of at least 4 undescribed species were found during the survey. From the statewide reference data set, 21 riparian floristic communities were described and their distributions mapped. Altitude, rainfall and temperature, hydrologic and geomorphologic factors, and the composition of the vegetation itself, all contributed significantly to an explanation of floristic variation in riparian vegetation. At present, assessments of river condition at the state and regional scales are predominantly made using the AUSRIVAS modelling approach. An AUSRIVAS-style model was developed and used to assess the condition of native riparian vegetation in Tasmania. As well as providing an interpretation of riparian vegetation condition based on species composition relative to a reference data set, the model is able to generate a predictive list of species for any site in the survey area. While the model was considered to be suitable for assessing the condition of riparian vegetation at the statewide scale, the large discrepancies between observed and predicted species lists from the preliminary trial raised considerable doubts as to its suitability for use for revegetation purposes or for ecological classification of sites into communities at this stage. There is a strong case to support the view that all remaining native riparian vegetation should be conserved or protected because of its natural significance, its significant functions and roles as part of freshwater ecosystems and its high economic, cultural and social values. However, the reality is that only a part of the landscape can be managed primarily for conservation, and this is usually a relatively small part. In order to provide decision-makers and natural resource managers with an objective scientific process to facilitate the conservation of riparian floristic communities that are poorly-reserved or unreserved, a 5- stage planning process was developed to illustrate how priority riparian reaches could be selected from an extensive reference dataset. If riparian vegetation is to retain high environmental, social and economic values, considerable strategic planning for long-term management of all components of freshwater ecosystems needs to be undertaken at the local, catchment and state levels. The riparian zone is an area where the preservation of what remains that is native is by far the most cost effective strategy for management.