Water stress in trees within the urban landscape

Trees within the urban environment may more frequently suffer from water stress when compared to their rural counterparts. This is due to urban environments being heavily covered with impermeable surfaces, the limited rooting environment of many urban trees, and the fact that stormwater management systems swiftly remove run-off (within sealed pipe networks) during periods of heavy rainfall. Ironically, whilst surrounded by water, urban trees are unable to access what they may very well require. Therefore, in times of marked drought, urban trees may very well begin to suffer greatly.

The Great Drought of 1976, which was the driest summer for over 200 years, was one of the more distinct examples of how urban trees suffered as a result of drought conditions. Young and newly-planted trees displayed symptoms of stress, and ultimately died, within the very same growing season as the drought – particularly where they received no aftercare by local authorities. It was only because of the successful plea by such authorities to members of the public to use their less contaminated waste water (from bathing and washing up) that mortality of young trees was not incredibly damaging.

However, it was not just the young trees that suffered. Mature trees, whilst not necessarily showing symptoms during the year of the drought, at times did not re-flush, or flushed weakly, in the following growing season. For many, Johnston profoundly states, it would be their last.

How would an avenue of mature trees fare during a drought similar to that of 1976, when their rooting environments are covered almost entirely by impermeable surfaces?

Such a brief account of the drought of 1976 is a good example of how precariously-balanced even mature urban trees may be, with regards to life and death. Granted, the drought was significant and an outlier, but it should serve as a reminder to those within the industry (and associated industries, such as developers and engineers) that urban environments are not accommodating for trees and their water demands, even at the best of times. This is discounting the huge array of other biotic and abiotic stressors placed upon our urban trees, such as pollution of the air and soil, ground compaction, mechanical damage, and vandalism.

Source: Johnston, M. (2015) Trees in Towns and Cities – A History of British Urban Arboriculture. UK: Windgather Press.

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