Nelson Evening Mail. MONDAY, MARCH 10, 1913. FORESTS ON WATERSHEDS.

Nelson Evening Mail, Volume XLVIII, Issue XLVIII, 10 March 1913, Page 4

Nelson Evening Mail. MONDAY, MARCH 10, 1913. FORESTS ON WATERSHEDS.

ON several occasions recently we have dealt with the great question of deforestation. and a correspondent, "H. C!. Kingsland," has placed some interesting facts before readers concerning the ch a nges taking place in the Wairoa river due to the destruction of bush in the higher regions. It is interesting on this occasion to observe what an Australian authority, R. B. Bonsfield, 8.A., has to sa.y 011 the matter. He first of all reminds his readers that it is one of those stern relentless laws, which neither Mede

nor Persian can alter, that "Nature maketh no allowance for ignorance in the doling out of her penalties against the offenders." Tire trouble -'in this matter is that it is not generally realised that the volume of water which feeds rivers and streams is not dependent upon rainfall alone, but also upon' the trees which surround and shade their sources. He gives Iwo simple experiments which anybody can perform with the aid of the kitchen scales and a couple of flowerpots to reveal the underlying principles which this fact conceals. Let the reader in imagination, he says, iili one of these.' pots with earth taken from a. depth of several feet—that is, what is termed "mineral earth,'' composed of rock particles. The other we will till with vegetable matter, consisting of dead leaves, rotting wood, and decayed matter, all well crushed up together. Both these pots may then be weighed upon scales, and afterwards filled with water to a point of saturation, and left to drain. When only the water which each will hold back is left, the pots should be weighed again, and by a process of simple substraotion it will be found, that the vegetable matter has retained from two to nearly four times the quantity of water held back by the rock soil. Rut this is very far from all. If we now place the pots in the sunshine and leave them to dry out, it will be found that the vegetable earth still holds the larger part of its water, long after the rock earth has become completely dried ont and sun-caked. Thus it is found that. 1/his vegetable earth, or "humus," as it is scientifically called, not only soaks up and holds back from washing away by flooding, from twice to nearly quadruple the quantity of water to that retained by the other, but it clings on to it in the face of heat and evaporation, for many times as long again. If we were now to repeat the experiment with the rock soil in the hot sunshine, and the humus in the cool cellar, we should have, he points out, attained very nearly the conditions under which rain is received upon the bared mountain, and the cool depths of the primaeval forest. It is upon these facte that the whole secret of Nature's mountain reservoirs depend whether we are dealing with the sources of the mighty ' Amazon, the devastating floods of the American rivers, for which the lumberman is so largely answerable, or the tiny volume of the perennial tricklet which courses its way the slopes of mountains. The serviceable rivers rise amid the nibble and decay of centuries. trickling slowly down through the wet, spongy earth to the rockbed beneath ; and oozing thence from hunI dreds of yards around, to issue forth., into the gullies as clear and sparkling streams, to swell the volume of the rivers. At the sources of rivers that are still serviceable all the year round, there is the decayed accumulated matter of centuries formed over the top of the non-holding rock soil, a vast natural reservoir for retaining the rainfall; and letting it off at "half-cock'' into the streams, so that no I waste may occur through evaporation or i drainage. Nature's system allows of no i burst dams and deluging cluices, and a • minimum of waste by evaporation which 'defies every invention of man to emulate. | But what, it will be asked, has become I of this reservoir upon the cleared land? ; A simple but somewhat lengthy experiment is given in explanation. If we throw an amount of this vegetable decay, dead leaves, etc., into a stone basement, and leave it to the action of the hot | sun, we shall find; a- few years, that practically nothing remains. A fine dust, consisting only of rock particles and mineral earth, equal to about one-ninth of the original deposit. The vegetable mould has gone ! Precisely the same effect would have 'been produced if we I > had burned it. Burning is merely a I process by which the non-mineral irat- ' ter is taken up by the oxygen, and carried off in flame. We expose ;i vegetable soil to the unshaded heat of the sun. the vegetable matter combines with the oxygen of the air, and disappears •by a process of slow burning, until nothing but the unburnable rock soil reImams. The other eight-ninths, taken by the plant's leaves from the air, to form the hundreds of tons of live and decaying forest growth, have returned whence they came as surely as by a bush fire. When we expose the watersheds by deforestation, and clear away the protecting shade, to form pastures, not only does the forest humus dwindle away, but I the hot sunshine, admitted to the ground, ' is enabled to steam off the little water | which t.he impoverished soil is still capable of holding. Nor is this all. [ While the forest lasted upon the water- | shed the gradual loss of humus soil on the surface, through wind, and heat, was constantly checked 'by fresh falls from above—layer upon layer upon the old, covering it up from, the - action of the air. The water-holding soil on top had a constant tendency to increase its depth, and the amount of rain which it would hold back from flooding and evaporation. The litter of brushwood on top acted as 'what is called "a mulch." That is to |ay, the loose litter on top would dry out, and while admitting of- the free entrance of fresh rain from above, acted as a check to the rise of water by "capiiiary action" from below, and to the consequent loss of water by continual evaporation, as it reached the surface. But loss does not end here. One of the greatest sources of evaporation is the wind, which catches the water from the ground., in the same way as from a wet sheet hung upon c clothes-line..' Tons of vegetable waterholding soil, also, are carried by wind and water down to the valleys, from their carpeting over the mineral bed; soil which 'before was held back against' the flerc.est deluge, by the network of roots and tree trunks to which they formerly clung.