Compacted surface and subsurface soil layers, frequently encountered within urban environments (due to foot traffic, vehicular traffic, or otherwise), are restrictive to root spread and compromise the free movement of air and water (Jim, 1998). All soil types will reach a bulk density where moisture and air content is reduced; urban areas more readily meet such bulk density levels, as was demonstrated by a study in Copenhagen where soil bulk density was found to be 2g/cm³ (Buhler et al., 2007) – soil is considered to be compacted at 1.6g/cm³ (Bassett et al., 2005).
Because roots grow principally through macro-pores in the soil, the lack of aeration and increase in soil shear strength as a result can limit the potential root growth rate. The micro-pores that remain are not large enough to provide for root penetration and growth; the shear strength increase impedes upon root elongation (Jim, 1993; Roberts et al., 2006). Roots are essentially unable to enter diameters smaller than the root cap of a growing root, unless they can exert sufficient pressure to displace soil – this task is incredibly difficult, in such compacted urban soils (Day & Bassuk, 1994; Roberts et al., 2006).
Compacted soils are also unable to provide moisture drainage (lower hydraulic conductivity) on the level of a non-compacted soil, given the decrease in porosity on the surface and just below. Therefore, moisture availability may be reduced within the rooting environment of a tree growing in compacted soils, whilst surface run-off increases (Day & Bassuk, 1994; Millward et al., 2011; Roberts et al., 2006).
Such lack of availability of both moisture and feasible rooting environment ultimately serves to reduce the tree’s ability to uptake water and nutrients, and in turn impacts its ability to sustain and generate additional mass from the energy it produces through photosynthesis (Jim, 1993; Millward et al., 2011). If compaction occurs after a tree has grown to a considerable size, it may even be forced to reduce its mass as the new soil conditions are unable to offer the water and nutrients required for simple mass maintenance.
Sources:
Bassett, I., Simcock, R., & Mitchell, N. (2005) Consequences of soil compaction for seedling establishment: Implications for natural regeneration and restoration. Austral Ecology. 30 (8). p827-833.
Buhler, O., Kristoffersen, P., & Larsen, S. (2007) Growth of street trees in Copenhagen with emphasis on the effect of different establishment concepts. Arboriculture & Urban Forestry. 33 (5). p330-337.
Day, S. & Bassuk, N. (1994) A review of the effects of soil compaction and amelioration treatments on landscape trees. Journal of Arboriculture. 20 (1). p9-17.
Jim, C. (2001) Managing urban trees and their soil envelopes in a contiguously developed city environment. Environmental Management. 28 (6). p819-832.
Millward, A., Paudel, K., & Briggs, S. (2011) Naturalization as a strategy for improving soil physical characteristics in a forested urban park. Urban Ecosystems. 14 (2). p261-278.
Roberts, J., Jackson, N., & Smith, M. (2006) Tree Roots in the Built Environment (Research for Amenity Trees 8). UK: The Arboricultural Association.
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