WOODY PLANT-ROOTS FAIL TO PENETRATE A CLAY-LINED LANDFILL - MANAGEMENT IMPLICATIONS

In many locations, regulatory agencies do not permit tree planting abo ve landfills that are sealed with a capping clay, because roots might penetrate the clay barrier and expose landfill contents to leaching. W e find, however, no empirical or theoretical basis for this restrictio n, and instead hypothesize that plant roots of any kind are incapable of penetrating the dense clays used to seal landfills. As a test, we e xcavated 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 se ries) clay. The test plants had invaded naturally from nearby forests. All plants examined-including trees as tall as 6 m-had extremely shal low root plates, with deformed tap roots that grew entirely above and parallel to the clay layer. Only occasional stubby feeder roots were f ound in the top 1 cm of clay, and in clay cracks at depths to 6 cm, in dicating that the primary impediment to root growth was physical, alth ough both clay and the overlying soil were highly acidic. These result s, if confirmed by experimental research should lead to increased opti ons for the end use of many closed sanitary landfills.