You step outside. The newly built piazza with young trees looks promising. There’s a sense of a wonderful new green refuge growing in your city.
After the first season, the trees haven’t grown much. Their canopies are thin, the shade is slight, and the air still feels hot.
Another year on, most of the trees are struggling to survive; some have already been removed. The promise of a green refuge suddenly feels empty. There is no effective shade, you feel the heat, and you wonder why the trees have not grown.
The answer is right under your nose! Actually…. beneath your feet.
In nature, roots travel through a continuous layer of soil, spreading wide.
In Gulf cities almost all landscapes are constructed landscapes. Even in constructed landscapes, trees still require the soil and water they would find in a natural environment.
Everyone understands trees need water. Though, weirdly, it seems not many people connect the dots between water, soil volumes and root mass. Here’s all you need to know.
More soil = more feeder root mass.
More feeder root mass = more available water.
Enough available water = healthy large trees!
Lets look at this another way……
Less soil = less feeder root mass.
Less feeder root mass = less available water.
Not enough water = sick or dead tree!
The key to large healthy trees is providing enough soil. But, soil is not soil is not soil. Tree roots seek both water and oxygen, and that is concentrated in the upper layer of soil. So, soil volumes are best wide and shallow (with enough depth of anchorage).
Designing urban forests needs thinking in terms of the landscape below ground. Generally, give each tree at least a four-metre radius of soil, and fill the space beneath the surface to create connected soil volumes. This sub-surface landscape looks nothing like what you see above ground: below, roots extend well beyond the edges of surface level planting beds. Too often though, planting beds are the limit of soil, and the result is stunted trees. To make sure enough soil is provided, a sub-surface soil volume plan should be a required part of any construction drawing package.
This is perhaps the most important drawing, and yet, we have never been asked to make one!
Now imagine the same space designed following the natureculture way. Instead of small boxes, the soil beneath the paving is connected and wide. Roots inhabit the generous soil volume, even below hard paving, unseen but essential. Above, the canopies expand until they meet, weaving a green canopy overhead. Shade cools the air, birdsong fills the branches, and you, you feel like a forest bather on holiday.
We all love the cool green landscapes we see and feel. Keep in mind it is the below ground landscape that makes these living systems possible.
