Street trees have an incredibly tough life

The average life span of a tree is very short in many of the big cities. Some cities have an average replacement cycle of 10 to 11 years. Some cities it’s 14 years, but it’s very, very short considering that the natural lifespan of some of those tree species is close to a hundred years in the natural environment. This continuing replacement cycle constitutes a massive cost, as well as a huge lost opportunity cost.

Trees growing in typical urban ‘tree boxes’ are usually surrounded by compacted soil. This often leads to the roots seeking out the space between the compacted soil and the overlying pavement, where air and water are present, which then causes footpath heaving.

If the tree roots cannot expand into the surrounding soil, they continue to grow until they have filled up the available space.

When the tree’s needs for nutrients, air and water can no longer be met, the health of the tree will begin to decline and it will eventually die. Trees grown in these conditions rarely reach their full growth potential and cannot provide a wide range of benefits that mature, healthy trees have to offer.

Let’s get to the root of this problem

There are many parts to a tree. They’re a complex and finely balanced living organism, which all needs to be taken into consideration as part of the design process. So we cannot ignore the below-ground part of a tree when designing for its long-term requirements In its natural environment, a tree has a large horizontal root structure supporting the canopy.

The analogy of a wine glass on a large flat plate has been used, with the large flat base beneath the stem, with the structure above. Via the roots, trees obtain nutrients from the soil, but the roots also need the oxygen and water that occupy voids between soil particles. In uncompacted soil, voids are abundant.

The availability of space for tree roots to develop is crucial to the tree’s ability to grow and stay healthy. In the natural environment, the roots of a growing tree will extend far into the surrounding soil to more than twice the width of the mature tree’s canopy. Everybody has some experience of a neighbour’s tree roots impacting service or foundation, a long way from the tree itself

The conflict between tree growth, and built structures

However, typically, the engineering demands of paved structures make it impossible to grow trees in cities. Pavements require structural support, which historically is concrete, or crushed stone, or road base compacted efficiently, and the small opening in the pavement for the tree is not sufficient to support that large root plate (similar to wine glass on a flat plate)

For trees in hard-surfaced areas, a fundamental conflict exists between maximising the soil volume available for tree rooting while also providing a stable base for roads and pavements. If soil is treated as a structural material and required to bear the load of pedestrians, building and roadways, it will be consolidated to the point that air and water are excluded and insufficient space is available for roots to grow.

Trees planted using best practise, growing successfully in Melbourne Lonsdale Street, Dandenong, 2018

Trees planted using best practise, growing successfully in Melbourne Lonsdale Street, Dandenong, 2018

 Trees planted correctly 7 years ago, growing healthily in New South Wales | Laman Street, Newcastle, 2020.

Trees planted correctly 7 years ago, growing healthily in New South Wales | Laman Street, Newcastle, 2020.

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