Alien and native plants of urban river corridors: a study of riparian plant propagule dynamics along the river Brent, Greater London

Abstract

This thesis investigates the dynamics of alien and native plant propagules in relation to the standing vegetation of the urban riparian corridor of the River Brent, London and also experimental results regarding the effectiveness of physical management of Impatiens glandulifera, one of the most common riparian alien invasive species in the United Kingdom. The study has shown that viable plant propagules are well-distributed within the top 10 cm of urban riparian soils, with no significant difference in propagule abundance or species richness between 0-5 and 5-10 cm layers or with distance (0-1, 1-2, 2-3 m) from the low flow channel. Urban riparian propagule banks are as species rich as those in more rural situations, but they contain a greater proportion of alien species (>20%). Soil propagule abundance was greater in the autumn than in the spring but species richness varied little. The propagule bank species composition varied seasonally, more unique alien species being recorded in the spring than in the autumn. Using artificial turf traps placed within the riparian zone, the deposition of fluvial sediment and viable propagules was investigated. Significant correlations between sediment weight and propagule abundance and richness indicated the important role of hydrochory in delivering viable propagules to the riparian zone, particularly during winter. Lower sediment weight and propagule species richness of summer samples and weaker correlation between sediment weight and propagule abundance and richness indicated the importance of local seed-rain in summer. Comparison of species composition of the propagule bank and standing vegetation demonstrated little relationship between the two, with far greater abundance of alien species in the propagule bank, the majority of which were not found in the local vegetation. The propagule bank also contained more species with long-lived persistent seeds, than was observed in the standing vegetation. Impatiens glandulifera was the most frequently occurring species in the standing vegetation, while Buddleja davidii was the most frequently occurring species in the propagule bank. Investigating experimental plots, the study found a strong negative relationship between percent cover of Impatiens glandulifera and of other species. Experimental pruning and removal of Impatiens glandulifera at six-week intervals over two years had a marked positive effect upon vegetation species richness and the percent cover of other species, particularly on heavily invaded plots, with removal showing a stronger effect than pruning