A study of riverine plant communities in Tasmania, with especial reference to central East Coast rivers

Plants growing in and along rivers have received little ecological and conceptual attention. Current deterministic and stochastic lotic community concepts are examined, along with concepts in plant ecology (individualistic and organismic concepts, plant succession, disturbance, physiographic plant geography) and their application to lotic plant communities. This theoretical framework is used to investigate the spatial and temporal variability of riverine plant communities in the island state of Tasmania and to establish relationships between this variation and major environmental factors. In order to establish a suitable strategy with which to sample lotic plant communities, levels of hydrological and chemical disturbance were investigated along Tasmanian rivers. Annual flow records, monthly flow records, peak and low flow records were used to derive an objective hydrological regionalization for 77 rivers. Four contiguous groups were determined with the south east region of the island exhibiting hydrological regimes similar to those of the drier areas of mainland Australia. The wettest areas, in the south and west, have regimes with no analogue elsewhere within mainland Australia. The water chemistry of Tasmanian rivers displays similar contrasts, though variability tends to be greater temporally than spatially. To provide data on aquatic vegetation dynamics following disturbance, 14 permanent plots were set up along the Swan and Apsley Rivers, eastern Tasmania, and monitored over a 28 month period. Records were made of plant species composition and cover, as well as measurements of water chemistry and hydrology. Between-site (spatial) as well as within-site (temporal) variability of aquatic plant communities along the study rivers is pronounced. Species cover, diversity, richness and turnover show significant changes after high and low hydrological events, but less so between seasons. Spatial variability is highly influenced by the water chemistry, and within the brackish and freshwater components of the rivers, hydrological variables are the major influence. This evidence on site-specific succession does not support an organismic interpretation, and lotic aquatic plant communities appear to have a low resistance to disturbance and low resilience after disturbance. Disturbance tends to favour opportunistic species which recover rapidly vegetatively, and tends to allow the coexistence of a relatively high number of species which display polydominance. To provide data on the spatial distributions of riverine communities, riparian and aquatic plants were intensively sampled along the length and breadth of the Swan and Apsley Rivers. Riparian plant communities vary longitudinally along the river in response to geology and gradient index (which combines altitude and distance from the drainage divide). Lateral variations away from the river are a response to a flooding gradient and substratum stability. Dense overhanging riparian vegetation excludes growth of aquatic species, though this situation is nullified where the canopy has been cleared for agriculture or for roads. A geographical survey of aquatic plants along rivers in Tasmania indicates that the plant communities are predominantly influenced by filterable residue and colour and to a lesser extent by substratum and hydrology.