Woody Plant Roots Fail to Penetrate a Clay-Lined Landfill: Management Implications GEORGE R. ROBINSON1, * STEVEN N. HANDEL Department of Biological Sciences Rutgers University Piscataway, New Jersey 08855-1059, USA ABSTRACT / In many locations, regulatory agencies do not permit tree planting above landfills that are sealed with a capping clay, because roots might penetrate the clay barrier and expose landfill contents to leaching. We find, however, no empirical or theoretical basis for this restriction, and instead hypothesize that plant roots of any kind are incapable of penetrating the dense clays used to Seal landfills. As a test, we excavated 30 trees and shrubs, of 12 species, growing over a clay-lined municipal sanitary landfill on Staten Island, New York. The landfill had been closed for seven years, and featured a very shallow (10 tO 30-cm) soil layer over a 45-cm layer of compacted grey marl (Woodbury series) clay. The test plants had invaded naturally from nearby forests. All plants examined--including trees as tall as 6 m--had extremely shallow root plates, with deformed tap roots that grew entirely above and parallel to the clay layer. Only occasional stubby feeder roots were found in the top 1 cm of clay, and in clay cracks at depths to 6 cm, indicating that the primary impediment to root growth was physical, although both clay and the overlying soil were highly acidic. These results, if confirmed by experimental research should lead to increased options for the end use of many closed sanitary landfills. Restrictions on Use of Woody Plants on Closed Landfills Modern landfill technology includes methods for isolating landfill contents, largely to prevent wetting of the contents and subsequent pulses of leachate that might contaminate surrounding lands and waters. This is accomplished by sealing the top of a completed landfill with an impermeable liner, using one of two methods. Either a thick layer of dense clay is spread Over the top and sides of the mounded trash, or the mound is carpeted with a synthetic waterproof fabric (a goetextile). Both types of linear are covered with a layer of soil, which is designed to function as a combi- nation barrier protection layer, drainage channel, and growth medium. Both systems are engineered to function for several decades, during which time land- fill contents are expected to slowly decompose anaer- obically (Anonymous 1980, Lutton 1982, Oweis 1989, Miller 1988, Woodward 1989). Given their constant shifting and settling, closed landfills are often unsuitable for building construc- tion, and options are limited to their end use (e.g., KEY WORDS: Clay liner; Environmentalregulation; Restorationecol- ogy; Root penetration Sanitary landfills; Woody p ants ~Currentaddress: Department of Biological Sciences,State Unlver- Slty of New York, Albany, New York 12222, USA. *Author to whom correspondenceshould be addressed. Aplet and Conn 1977). Therefore, the main defining feature of many closed landfills, other than shape and size, will be their vegetative cover. Although the soil materials used for final cover, including surface lay- ers, are designed primarily for containment, most sites can accommodate a variety of plant communities, if provided sufficient soil cover (Carnell and Insley 1982, Bradshaw 1984). Typically, however, the vege- tation is engineered to match the site, rather than the reverse. Part of the reason for this approach lies with fears that some types of vegetation might interfere with containment. In cases where final cover includes synthetic geotextiles, that concern has been somewhat alleviated by tests demonstrating that those materials are resistant to penetration by tree roots (Landreth 1991, Dobson and Moffat 1993). However, on clay caps, landscaping materials are often restricted by law to herbaceous plants (e.g., grasses and wildflower mixes), out of concern for potential damage to clay barriers posed by woody plant roots. The origin of those concerns is not clear, although they are expressed in regulations and technical guide- lines (e.g., Anonymous 1989, 1991, 1992, citations in Dobson and Moffat 1993). It is not even clear that herbaceous plants should be any less threatening than trees and shrubs. For example, roots of native bunch grasses from the Great Basin of western North Amer- ica are known to reach depths of several meters (Weaver 1920) in their native soils. Indeed, studies of clay-capped landfills in Wisconsin, USA, indicate that Environmental Management Vol. 19, No. 1, pp. 57-64 9 1995 Springer-Verlag New York Inc.