The largest root of this street tree in Marrickville has been severed.

The largest root of this street tree in Marrickville has been severed.

By accident I came across this Letter to the Editor in the Adelaide Advertiser written on Tuesday 23rd April 1901.   http://trove.nla.gov.au/ndp/del/article/4833731

It makes for very interesting reading if you are interested in history as well as soil, footpaths & trees as I have become.  What makes this letter especially worthy is that 112-years has passed; yet we still encounter the same problems & the same attitudes towards trees & tree roots.

THE FOOTPATH AND THE TREES.

To the Editor.

Sir.  For many years it has been an article of municipal creed that the cracks in tar-paved and tar-dressed footpaths are due solely to the roots of trees, and untold numbers of the trees on or along such footpaths have fallen victims to this belief.

For several years, distinguished by great drought, I have collected observations on this subject, especially during the present season, and I have arrived at the conclusion that, excepting a single species in common use, the trees and their roots have nothing or very little to do with the cracking of the tar-paving or tar-dressing, that is, directly and indirectly, no more than gas or water pipes or any extended resisting object near the surface.

The real cause is, on the contrary, to be found in the physical properties of the soil and the material employed, and this is so obvious now that anyone who cares to see may do so on his way to and from town.

Let me explain first the physical properties of materials.  The soil is largely or chiefly made up or clay.  Now, clay expands and swells when wet, more when warm and moist, than when cold and moist and contracts when drying in the opposite way.  Cracks are then found wherever there is least cohesion.  Straws, dry rootlets, pieces of wire, in fact anything notchy, operates in the same way.  The materials used for tar paving form, on the other hand, a kind of sandstone more or less impervious to moisture. Consequently they form a rigid implastic sheet, entirely subject to contraction by cold, expansion by heat, and in direct proportion to the variation of temperature, viz., from near freezing point to near 200°F.

The difference in length with the great extension is, therefore, many inches, perhaps some feet, between kerbing and kerbing. What are the results? Nay, what must they be? Such footpaths are formed usually when the surface of the ground is moist.  In this state it is leveled and compressed, and the soft, plastic material spread over and leveled by  heavy rollers.  As soon as the tar-sandstone sets it becomes a rigid mass, pressing against the kerbing along one side and both ends, and on the other side against the walls, fences, or merely the soil of the opposite side, and in all cases has to overcome, in its diurnal and seasonal motion, the friction between the surface covering and the soil, but it must move somewhere, and does with irresistible force.  It occurs in this way between the two forces acting on it.

As the clay yields in drying beneath the impervious surface, the latter is depressed and forced down upon it, and slight depressions are formed where the soil is softest-the surface becomes puckered as it were.   By the longitudinal strain slight transverse bulges are formed, by the transverse strains between kerbing and walls each in the longitudinal direction.  Wherever any pipe, board, wire within a few inches of the surface, occurs there is an extended obstruction to compression.

At that place an elevation will remain, and consequently, upward bulging is promoted so long as the surface remains plastic to some extent and the ground beneath firm.  As the heat and drought increase the bulges become larger, but the clay soil beneath gradually becomes dry, and the traffic above, be it vehicle or human feet, forces the surface covering down upon contracting clay, pulverising it, while the now rigidly-set tar-paving expands and contracts daily more and more.  In its motions the bulges are raised by the strains and compressed by the tread of the passengers until they crack at the weakest spots to relieve the former.

Later on the rains set in, the clay soil swells with the moisture imbibed from the ground alongside, or ground and water tables, for moisture must diffuse equally below, and a difference of distance only means a difference in time.  The ground not only slips below the rigidly-sat paving or dressing (this the cause of it always being found as loose rubble beneath separated pieces), but also swells most at the bulges, thus exaggerating constantly the elevated spots, these being places of least resistance.  The foundation being gone after some time, and the material weak and unyielding in itself, the latter become broken up, and kicked aside by the passers-by.

That this is the true explanation, and that the roots of trees are only very indirect causes, one may see most plainly where the tar-paving, crosses bridges and culverts, as in Pirie street, Kent Town, where it is cracked and destroyed, just in the selfsame way as elsewhere, without the possibility of “roots” thicker than a cord being present.

If such are supposed to cause the cracks it supposes a magical and superstitious power in them indeed!

The above illustrates again the saying of one of our writers, that the generally accepted theories are usually those requiring revision most urgently and it is hoped that in future the trees and shrubs will be spared destruction and ill-treatment for damage of which they are wholly innocent.

The fig trees, which furnished the excuse at first, possess the quality of many trees habitually growing in wet or flood-exposed localities, of raising themselves on their trunk-roots above the wet surface.  As such are of course, wholly unfit for street or garden planting, but eminently suited for the park lands, where, however, they are rarely found, thanks to the wisdom of the ruling powers.

However, of whatever kind, the green foliage of trees and shrubs is probably of far more important than all the footpaths put together for town and country in general, on account of the cooling, air-purifying, and soil-disinfecting properties which it possesses and exercises, not withstanding the superstitious beliefs to the contrary.

J.G.O.T. Norwood,  April 20, 1901.

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