Seed germination from deposited sediments during high winter flow in riparian areas

Abstract

In this study we examine how seed richness, diversity and composition from winter deposited sediment vary at increasing distances from a river channel. The objectives were to (1) describe the abundance, diversity, species composition and indicator species of seed deposited in a riparian wetland at different distances from the stream, (2) determine the range of variability in seed deposition at two spatial scales namely within distances and between distances from the river, and (3) determine the temporal patterns in seed germination at different distances from the river. A transect was established perpendicular to the river channel extending 101m into the river valley. Samples of winter deposited sediment were collected in early spring in 20cm×20cm plots with three replicates positioned 2m, 16m, 23m, 41m, 70m and 101m from the river. Germination from the sediment seed pool was followed for six weeks under, respectively, moist and wet conditions in a greenhouse with a natural light regime and a mean temperature of 20°C. Overall, we found that winter high flow deposited a substantial amount of viable seeds in the inundated area. Germination was most successful under moist conditions where the number of seedlings ranged from 1050 to 3817m−2. Species richness (10.7±1.5 species), Shannon diversity (2.13±0.13) and evenness (0.90±0.03) peaked in samples taken 16m from the river channel. Variation in vegetation parameters was very high between samples, and 36% of the observations showed higher variation within replicates from the same distance than between distances reflecting high variability on a small spatial scale. The higher diversity of seedlings 16m from the river coincided with a confluence zone emerging within the floodplain due to the complex flow arising during inundations, causing higher sediment deposition. Most seeds and most species germinated within the first three weeks, indicating that species commonly deposited with winter high flow in riparian wetlands show early germination, giving them a competitive advantage when vegetation establishes in early spring.