A Re-Evaluation of Objections To Tree Planting On Containment Landfills

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Abstract
Legislation from developed countries indicates that planting trees on containment landfills is generally forbidden. Concerns centre on the supposition that tree roots can penetrate into and through capping materials, and will thus compromise control of water ingress into waste, and allow the escape of landfill gas. An associated anxiety is that if roots penetrate a clay cap they could cause desiccation and cracking of the clay through excessive moisture abstraction. It is also considered that trees growing on the relatively shallow soil above a landfill cap could be especially prone to uprooting. However, a review of the world literature indicates that maximum depths achieved by tree roots are 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 environmental conditions and their downward penetration can be restricted by a number of soil factors including compaction, poor aeration and infertility. A detailed study of these factors indicates that the materials used for capping landfill sites, such as HDPE (high density polyethylene) and compacted clays, can provide an effective barrier to downward root growth. The available information also suggests that tree roots are extremely unlikely to be a primary cause of desiccation cracking in a clay cap owing to their inability to extract more than about one-quarter of the total moisture held in a clay of the density required to ensure a permeability of 1 ×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 greater risk of windthrow than most forest trees on undisturbed sites. Methods are available to assess the likelihood of windthrow. In any event, windthrow should not cause disruption of a cap, due to the inability of tree roots to penetrate HDPE, or mineral materials compacted to a bulk density of 1.8 g cm-3. © 1995 ISWA