RESTORATION OF WOODY-PLANTS TO CAPPED LANDFILLS - ROOT DYNAMICS IN ANENGINEERED SOIL

Closed or abandoned landfills represent significant land areas, often in or near urban centers, that are potential sites for ecological rest oration of native woodlands. But current guidelines in many jurisdicti ons do not allow for the installation of trees or shrubs above landfil l clay caps, although these plants have many environmental, functional , and aesthetic advantages, including a rapid start to community succe ssion. Typical closure procedures for capped landfills include only a grass cover to control moisture infiltration and impede soil erosion. The main concern that limits the application of a woody cover to a clo sed landfill is that roots may penetrate and weaken the clay cap. As p art of a comprehensive experimental program on woodland restoration, w e installed 22 tree and shrub species on Staten Island, New York (the Fresh Kills Sanitary Landfill). We found no evidence that roots of the transplanted woody plants penetrate caps used on these landfills. Roo t growth requirements and dynamics stop penetration of these materials . Anoxic and acidic conditions were found in the sandy subsoil above t he cap, as indicated by corrosion patterns on steel test rods. Also, t he intensity of mycorrhizal infection on the experimental plants was h igh in the surface soil and decreased progressively with increasing so il depth. The potential vertical rooting depth during this time period was greater than that occurring over the clay cap. This was shown fro m data collected on a nearby control site, where seven of the species were installed on an engineered soil lacking a clay barrier layer, and roots of all seven species penetrated deeper than on the landfill. Th e engineered landfill soils are poor growth media for roots, and below ground constraints that limit restoration on these sites must be addr essed.