CAN THE REGENERATION OF VEGETATION FROM RIPARIAN SEED BANKS SUPPORT BIOGEOMORPHIC SUCCESSION AND THE GEOMORPHIC RECOVERY OF DEGRADED RIVER CHANNELS?

摘要

For rivers degraded by erosion and channel widening, the re-establishment of riparian vegetation is essential. We assess the potential for riparian seed banks to facilitate natural channel contraction through the regeneration of plants involved in the biogeomorphic succession of three discrete geomorphic units of increasing age and height above the channel bed: bars, benches and floodplain. Standing vegetation upon each unit type was surveyed for four river reaches in the Hunter catchment of eastern Australia. Seed bank composition was determined using seedling emergence techniques on sediment sampled from the units. We compared species richness and composition, and longevity, growth form and seed dispersal mechanisms between the standing vegetation and seed bank species. The seed bank was similar across bars, benches and floodplain, containing mostly perennial pioneer herbs, sedges and rushes, dispersed by wind and hydrochory (water transport). While bar vegetation was similar to the seed bank, bench and floodplain vegetation included later successional species such as shrubs and trees, significantly more grasses and vines (benches: X_(5, N=402)~2= 102.033, p＜0.001; floodplain: x_(5,N=792)~2 = 30.324, p＜0.001) and higher proportions of unassisted and animal-dispersed seeds (benches: X_(5,N=352)~2, = 89.409, p＜ 0.001; floodplain: X_(5,N=338)~2=56.026, p＜0.001). The results suggest that seed banks may support early stages of biogeomorphic succession, via regeneration of pioneer plants. However, plants, such as shrubs and trees that are observed upon units of increasing age and height above the channel bed (i.e. benches and floodplain), are likely sourced from transient seeds produced by local vegetation, rather than seed banks.