Soil compaction is not the only cause of poor moisture availability to the roots. Many urban surfaces, such as blocked paving, are largely impervious and therefore can significantly limit soil moisture – impervious surfaces typically have the wettest soil in the top 10cm, whereas in field conditions the soil usually progressively accumulates more moisture at lower depths and only then followed by the top 10cm. Much like soil compaction, surface run-off and evaporation are high, whilst percolation into the soil is limited (Morgenroth & Buchan, 2009). Drainage systems in urban areas can exacerbate such low soil moisture levels further, because moisture is essentially ‘locked away’.
Existing trees that have grown in open conditions that are suddenly paved around may suffer most, as water availability suddenly is reduced and vigour is impacted as a result. As a result of a loss of vigour, photosynthetic ability is impacted (as water is required in the process), in turn causing potential mass reduction in the longer-term unless the tree can find a source of water. Further to this, water is required for transpiration as a means of cooling the tree, so if water is not in such abundance that cooling can occur at a necessary rate, leaves may be damaged by heat during very warm weather periods – this can also impact upon the tree’s photosynthetic ability, thereby reducing energy levels within the system.
Morgenroth, J. & Buchan, G. (2009) Soil moisture and aeration beneath pervious and impervious pavements. Journal of Arboriculture. 35 (3). p135-141.