A REEVALUATION OF OBJECTIONS TO TREE PLANTING ON CONTAINMENT LANDFILLS

Legislation from developed countries indicates that planting trees on containment landfills is generally forbidden. Concerns centre on the s upposition that tree roots can penetrate into and through capping mate rials, and will thus compromise control of water ingress into waste, a nd allow the escape of landfill gas. An associated anxiety is that if roots penetrate a clay cap they could cause desiccation and cracking o f the clay through excessive moisture abstraction. It is also consider ed that trees growing on the relatively shallow soil above a landfill cap could be especially prone to uprooting. However, a review of the w orld literature indicates that maximum depths achieved by tree roots a re usually between 1-2 m. Almost 90% of a tree's roots may be found in the upper 0.6 m of soil. Tree roots are highly sensitive to environme ntal conditions and their downward penetration can be restricted by a number of soil factors including compaction, poor aeration and inferti lity. A detailed study of these factors indicates that the materials u sed for capping landfill sites, such as HDPE (high density polyethylen e) and compacted clays, can provide an effective barrier to downward r oot growth. The available information also suggests that tree roots ar e extremely unlikely to be a primary cause of desiccation cracking in a clay cap owing to their inability to extract more than about one-qua rter of the total moisture held in a clay of the density required to e nsure a permeability of 1 x 10(-9) m s(-1). Trees growing on landfill sites with a rootable soil depth of at least 1.5 m should be at no gre ater risk of windthrow than most forest trees on undisturbed sites. Me thods are available to assess the likelihood of windthrow. In any even t, windthrow should not cause disruption of a cap, due to the inabilit y of tree roots to penetrate HDPE, or mineral materials compacted to a bulk density of 1.8 g cm(-3). (C) 1995 ISWA