This has previously been posted as:  11 December 09 – much ado about clay soils

What concerns me about the removal of street trees when they have been accused of causing damage to property is the following:

1. We have just experienced a long & protracted drought & more than 3/4s of NSW is still regarded as drought-affected.  The long-range weather forecasters say we have entered into another phase of drought. The Inner West has been affected, as has the whole of the Sydney metropolitan region.  This drought has resulted in severe water restrictions & this has affected our soils to a great degree.  Trees are stressed & quite a few have died within our local LGA.  Many of the saplings Council plants do not survive the summer because they are not watered.  This must result in significant financial loss to Council.

2. Even though we had quite a bit of rain over winter, we are still on water restrictions.  The park trees were able to capture much of the water when it rained, but our street trees have only a slight chance at getting a proper drink.  Most of the street trees in our LGA have been given very little ability to capture rainwater or storm water run-off.  Most have

Newtown street tree

a small opening left open amidst the cement or bitumen. I have only seen 4-5 street trees planted in cement where I feel they have been given adequate soil space around them to give them a fighting chance to capture rainwater.  Two were in a back street of Newtown & I doubt Council did this, as the kerb had been removed.  Both these trees had a lot of soil around their trunk.  The other example is new plantings bordered by raised brickwork on Unwins Bridge Road Tempe.  I think it would be wonderful if all our street trees that are in cemented areas could be planted in this way.  I would also very much like to see Council give funding for the watering of young street trees as they do in other Councils.

3. Much of the Inner West is built on clay soils.  Clay soils are quite volatile as they shrink when they do not get sufficient water & expand when there is too much water.  Both these extremes affect the foundations of buildings & footpaths.  The term Seasonal Fluctuations describes the normal variations of the moisture content of clay soils through the seasons.  When clay soil gets a lot of water it is called Field Capacity.  When the soil is dry due to evaporation, heat  & lack of rain during drought it is called Desiccated & suffering from Soil Moisture Deficit.  If a drought is lengthy & the soil does not return to Field Capacity after some rain & then enters into another dry period, it is called Persistent or Permanent Moisture Deficit.  This is likely to have already occurred in the Inner West or will do during the next period of drought in which we are now entering.

4. Soil movements cause what is termed Subsistence & Heave.  With Subsistence, the soil moves downwards.  With Heave, there is an upwards or lateral move of building foundations or footpaths.  While the clay soils go through Seasonal Fluctuations foundations of buildings don’t move much, though that depends on how much clay soil is directly under the foundations.  When the clay soil suffers from Persistent Moisture Deficit, foundations of buildings move & walls start to show cracks, paint peels & doors & windows are suddenly off balance.  Just last week our front door over night became hard to open & stiff on its swing.  It is recommended that you don’t fill in the cracks or shave wood off door or window surrounds when the ground is dry, but wait until the soils have returned to normal moisture content before doing repairs.  Most times the doors & windows will realign & the cracks will come closer together though paint work will need repairing.  If you do fill cracks while the soil is dry, the space will not be available for the building to move back into place & new cracks will appear, generally on the other side of the building.

5. The experts suggest occasional watering of the exterior of the house as this will help keep the clay soil moist so that your foundations don’t move during a drought.  In my case, I have been keeping the water up to the back garden, but to save water, I have left the front garden manage as best it can.  Here is the reason why our front door has moved. Also, it is our hallway & the front room that are suffering the cracks & peeling paint.

6. My research has shown that trees can increase the seasonal movements of clay soil, however, engineers also stress there are too many factors to consider before one blames the presence of a tree for causing the damage.  One of the problems is, if a mature tree is removed, this has a major impact on clay soil moisture & the property can actually sustain more damage as the soil fills with water & starts a process of Heave.  It is hard to stop Heave because there is no longer a tree to be taking up the excess water.  Because it is so difficult to categorically say that tree roots cause damage even if the roots do go towards & under the house, many engineers recommend using water to restore the water levels to clay soils rather than removing the tree.  I have read that a tree is not able to lift a house because of the weight of the house. A footpath is a different story.

Trees ... we need them

There are things we can do to minimize or prevent further cracking or movement of buildings that do not involve removing trees.  According to eminent scientists, we have entered a dangerous period of existence with CO2 levels actually threatening life on this planet.  All agree on the supreme value of trees.

Just today in Copenhagen, the Food & Agriculture Organisation of the United Nations & Brazil’s National Institute for Space Research signed an agreement to work together in the field of emissions measuring & reporting by monitoring forests & calculating CO2 sequestration.  Essentially they are going to map & monitor the green canopy of forests of the world.  This clearly demonstrates the value of trees in dealing with climate change.  We should only be removing them after all options have been exhausted.

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