Effect of alien plant invasion on riparian seed bank assembly rules

Abstract

Globally, riparian areas are particularly susceptible to invasion. Once invaded, riparian systems play a major role in the spread of invasive species throughout the landscape; particularly those species with high seed loads that are long-lived and able to build up in the soil to form extensive seed banks. Knowledge of seed bank composition assists in predicting the initial post-recruitment vegetation following a disturbance, be it natural (flood) or man-made (alien clearing) and can bear vital information for effective restoration of invaded rivers. Assembly rules are often used as tools for understanding the patterns that shape ecological communities. We investigated the impact of alien plant invasion on the composition of riparian seed banks and looked specifically at how their assembly rules become altered after invasion. Sampling was done along four main river systems in the south-western Cape, along different moisture gradients (dry, wet and transitional banks), slopes (mountain stream and foothill) and within different vegetation states (reference or invaded). All species that germinated from the soil seed bank samples were recorded and identified as close to species level as possible. Correspondence analyses were computed for the 20 most frequently occurring species on three different scales; landscape scale (between rivers), reach scale (between mountain stream and foothills sections) and habitat scale (between wet, dry and transitional bank zones). A clear pattern is evident at all scales showing close grouping within reference plots and greater species richness. Reference plots were tightly assembled and more closely related to each other than the scattered groupings shown in the invaded plots. Within reference plots the seed bank assembly rules are less affected by variables such as river, slope or zone. This study illustrates that once a riparian area has become invaded, the assembly rules are significantly altered as additional variability is introduced to the seed bank.