VERTICAL GROWTH AND MYCORRHIZAL INFECTION OF WOODY PLANT-ROOTS AS POTENTIAL LIMITS TO THE RESTORATION OF WOODLANDS ON LANDFILLS

We assessed the vertical growth and mycorrhizal infection of woody pla nt roots on a closed landfill, using tree and shrub clusters that had been previously installed in patches of increasing size to establish p rotocols for woodland restoration. The density of the fine roots of sh rubs, which had poor-to-moderate mycorrhizal infection, decreased stro ngly with increasing depth. Oak (Quercus) seedlings planted within and outside patches were assessed for ectomycorrhizal infection. Oak root systems were mycorrhizal, but root-tip proliferation was improved and ectomycorrhizal composition was influenced by woody debris in the min eral soil. Most surviving oaks were found within patches, but all seed lings showed poor growth: most taproots were deflected horizontally ab ove the boundary between surface soil and subsoil layers (greater than or equal to-15 cm). Abrupt decreases in pH between surface and subsur face horizons (6.9 versus 5.3), together with poor drainage and aerati on of the latter soil, were probably responsible for poor root growth. Root growth of greenhouse-grown pine and maple seedlings was similarl y restricted in pots packed with top-soil over subsoil material. Our r esults suggest that many current specifications for the cover of close d landfills will not permit restoration of native woody plant communit ies because of physical limitations to root growth and infectivity. Th e structure of the engineered soil must address basic plant growth req uirements as well as traditional concerns of drainage and barrier prot ection.