Abstract:
Riparian rehabilitation is the dominant strategy in river management, however current schemes are nowhere near as successful as they should be. Hydrochory, or seed dispersal by water, may have a significant influence on the structure of riparian vegetation and seed banks, however little is understood of these processes. This study examines the connections between hydrochory, geomorphology and flow regime in a small stream in the Waitakere Ranges, west Auckland, as a step towards an ecosystem based approach to riparian rehabilitation. Seed simulation experiments and soil seed bank analyses were used to assess seed transport, deposition and propagation across different geomorphic units in a representative reach in Glen Esk stream. Deposited seeds were located, counted and mapped in terms of their geomorphic position. The experiment was repeated 5 times in different flow. Soil seed bank samples were collected across gravel bars, benches, banks and floodplains, and germinated for identification. Soil characteristics and inundation frequency of different geomorphic units were analysed to examine potential for plant growth. Seeds deposited in areas of high geomorphic diversity, supporting the need to consider diverse geomorphic structure to promote seed deposition. Deposition patterns were most evident at the hydraulic unit scale. As expected, seeds deposited in low flow units (< 0.05 ms- 1) along channel margins and were transported in higher flow units (> 0.05 ms-1), such as runs and riffles. Seed deposition was greatest during base flow conditions; a range of 263 - 895 seeds were deposited in flows between 0.04 m3s-1 - 0.11 m3s-1, compared with 3 seeds deposited in flows of 1 m3s-1, suggesting that low flows are important for seed deposition in small streams. Deposition sites were poorly suited to plant growth with poor soil and disturbance characteristics however site suitability is prone to change. The results have important implications for rehabilitation of impacted streams that are often characterised by low flows, disconnected floodplains and high sediment loads. Incorporation of these processes in current management will ensure more effective rehabilitation results.
