South Sister St. Marys, Tasmania
SUMMARY
This paper considers the balance between erosion and formation of soil under natural and clearfell deforestation (with subsequent plantation establishment) on certain baserocks. It concludes that the soil will be an unrecoverable resource - and hence also the trees that rely on it for their growth - within a relatively short period of time under current clearfell and plantation practice.
Much emphasis is placed today on renewable and sustainable energy, agriculture, and forestry (inter alia). But despite intensive studies of the nutritional status of the various soils, little or no attention seems to have been directed to measuring the renewable capability of a soil. The balance between erosion and formation has been overlooked, though this can easily lead to the denudation of hillslopes and thus a loss of productive land.
No geomorphologist would contradict the proposition that hillslopes under their natural vegetation are in delicate equilibrium with their climatic environment, and that any change in that vegetation cover will result in loss of soil from the higher slopes, either by sheet, gully, or slip erosion. What we do not know precisely is how fast that loss is replaced naturally by weathering of the bedrock; or the relationship between slope angle and volume loss in specific cases. This paper seeks to pinpoint such unanswered problems, to make initial estimates of the effect on a soil of logging upper slopes, and to encourage the consideration of soil as an extremely valuable natural resource which needs to be protected and used sustainably.
Weathering of rock proceeds by thermal shock which induces microfractures (cf. Currie et al., 1974) or in the course of wetting and drying cycles (ibid) or simply by mineral solubility of components in the groundmass of the parent rock. Microfractures which arise from the cooling of a magma also provide in the fullness of time a means for the entry of fine tree roots which, by their expansion, are able further to split surface rocks and thus expose new surfaces to be basic processes of thermal shock or wet-dry cycles ( process especially notable in alpine regions).
Reducing a rock mass to the fine, insoluble fragments (mostly quartz, feldspars, micas and clays) that we call soil - particles less than 60 mm in diameter - takes time which depends on the mineral composition of the parent rock, the ambient climatic conditions, and the vegetation cover (or lack thereof). Some indicative figures would be as follows (Table 1), but because of the strong influence of climate (hot-cold, wet-dry cycles) rates can vary by as much as two orders of magnitudes.
Road engineers have devised methods of measurement for rock weathering based on comparative accelerated tests such as Los Angles Abrasion, Texas Ball Mill, Sulphate Soundness, etc. (AS 1141 - 1974 et seq.); but these do not appear to have been adapted as yet to addressing the rate of soil formation on forested hillslopes. Such an accelerated test is urgently needed, and a microfracturing index established by means such as those used by Currie et al. seems to be an appropriate methodology.
| Rock Type | Rate | Reference | Remarks |
|---|---|---|---|
| Chert | 0.02 ~ 0.2 | 1 | |
| Diabase | 0.007 ~ 0.03 | 3 | also known as dolerite |
| Granite | 0.01 ~ 20 | 1, 2, 3 | wide range due in part to mineral grain size (fine or coarse) |
| Limestone | 0.009 ~ 1.3 | 1, 3 | wide range may be due to acid v. neutral water |
| Sandstone | 0.02 ~ 0.5 | 1, 2, 3 | |
| Schist | 0.01~ 1.0 | 2, 3 | |
| Shale | 0.2 ~ 13 | 1, 2 |
References:
(1) Bell, F. G. (1987)
(2) Young (1972)
(3) Pearce & Elson (1973)
It is readily seen from Table 1 that granite has the highest weathering potential and dolertie the lowest, but in all cases an expectation of as much as 1 mm per year would be unusual. Clearly, to form as much as 1 m depth of soil is a process requiring something in the order of 1000 years. No surprise, therefore, that the denuded hills of Queenstown remain exactly that after barely one century.
Soil, once formed, is held on a slope by just two things - friction and natural "soil anchor" i.e. tree roots. Friction is greatly reduced (about halved) by one natural lubricant - water. Naturally stable slope angles for various soils are well known (cf. Ingles & Clark, 1975), but the highly variable effects of vegetational cover, climate and microclimate mean that all slopes in their natural state are in a delicate equilibrium. Removing ground cover, or adding water load to the soil, will inevitably result in soil loss from upper to lower slopes. The breakeven point between erosion and deposition generally lies somewhere between 12° and 20° depending on the coarseness of the soil (the finer the grain size, the lower the angle): although some soils will erode at angles as low as 7° and others, stabilised by roots, cling precariously to slopes of 35°.
Loss of soil - soil erosion - from slopes exceeding 7° ~ 12° occurs by three principal mechanisms: sheet erosion, rill or gully (incision) erosion, and mass (landslip) erosion. All may occur on the same slope angle. Erosion, transportation, and deposition of soil particles by flowing water has been carefully investigated by Hjulström (1935 - see Figure 1), and loss is maximum at a grain size of about 0.2 mm. The greater the clay content, or the coarser the gravel, the more resistant is the soil to erosion. I find it curious that Forestry Tasmania have chosen an erodibility test based on the internationally nonstandard size of 0.25 mm, virtually identical with the Hjulström maximum erodibility size, thus ensuring that all the samples tested will appear to be less and less erodible (cf. Laffan, 2003).
It is also true that vegetation protects a soil against erosion in two ways: firstly, the understory protects the surface against velocity erosion (i.e. sheet and gully types), whereas the top story (tall trees) protect the deeper profile by lowering the water content of the soil through evapotranspiration (which in turn raises the friction angle of the soil, protecting it from mass movements). Laffan himself writes: "Following clear-cutting, the frequency of landsliding has generally found to be the highest between the time of decomposition of the root system of felled trees and the re-establishment of stabilising roots from reafforestation." (p. 33). He also notes that "removal of a forest cover by clear-cutting can significantly increase the risk of landsliding".
I am concerned that Table 5 of the Forest Practices Code allows for the harvest on high erodibility soils on slopes up to 19°, because should this be done by clearfelling, the subsequent soil losses will be severe. It would be interesting to know how this rather precise angle of 19° was decided, as it may have ignored sheet and gully erosion and been based solely on mass movement considerations, which in turn are influenced not merely by logging but especially by roading (cf. Table 2) where unsealed surfaces and inadequate drainage provisions cause ongoing soil losses throughout the whole working life of a forested slope whether plantation or regrowth (see, for example, Heiken (1997)).
| Site Factors | Mass Movement events, % | Total area % | No. of events/1000 acres |
|---|---|---|---|
| Undisturbed | 10.6 | 84.6 | 0.4 |
| Logging | 17.0 | 13.6 | 3.9 |
| Road construction | 72.4 | 1.8 | 125.9 |
Table 3 following sets out some various measurements of the soil loss rate in higher rainfall areas and on various soil types. It is easily seen that clearing and cultivation greatly increases soil loss off hillslopes; and that some baserocks generate much more highly erodible soils than others - granite and various sedimentary rocks being especially bad. I have personally noted a loss averaging about 3 mm/year from 11° farmed slopes in the Northern Midlands. The high erodibility of granite soils has been previously noted in various Tasmanian Government publications (Pinkard, 1980, Kiernan, 1990).
| (a) Vegetation influence (Kirby, 1969) | (b) Parent rock influence (Various sources) |
|---|---|
| Forest 0.008 | Granites 1 ~ 17 |
| Pasture 0.03 | Sedimentaries 50 |
| Woodlots 0.10 | Shales 4.4 ~ 6.3 |
| Cultivated on contours 10.6 | Volcanics 0.5 |
| Bare (unworked) land 24.4 | Haul Roads 11 |
| Cultivated downslope 29.8 |
Though very variable, we might summarise these figures by saying that the natural loss rate is increased at least one hundredfold and, with careless soil protection, as much as one thousandfold by human intervention with the natural slope vegetation. Erosion protection is due in very large measure to the binding effect of vegetation roots in the forest understory, and any disturbance of those roots or removal of that understory leads to soil losses at least an order of magnitude greater than soil formation rates.
Thus a soil which has taken a thousand years to form can be destroyed in less than a century by poor conservation practices; and so clearfelling in upper catchments requires justification by prior research into soil loss rates - especially from plantations, and in the intervals between rotations - if future denudation is to be averted. The bare hills of Queenstown, which still persist after some 100 years, should be a warning to other districts in Tasmania. In South West Tasmania, after soil loss, the revegetation of many slopes is likely to need some 500 years based on personal site inspection of road cuttings.
The economic damage is not restricted to soil loss from upper slopes. As Walker (1977) has pointed out "excessive sediment output from eroding slopes becomes excessive sediment input to drainage networks. Accelerated soil erosion can therefore destroy recreational resources along rivers, reduce the quality of town and farm water, and cause siltation of dams, navigable channels and coastal harbours." We are seeing examples of this in many parts of Northern Tasmania at the present time where, for example, sediments accruing in the Upper Tamar from poor catchment practices must be dredged out at considerable cost to the local citizens.
Walker also notes that "The immediate costs of soil erosion are decreased agricultural productivity, increased management costs, and loss of agricultural land. Because arable soils take many thousands of years to develop, soil stripped by erosion is effectively lost forever." I am unaware of any study by Forestry or Agricultural experts in Tasmania which has been directed to determining precise rates of soil loss, such that it might be fairly estimated just how rapidly we are depleting the soil resources from which a future generation must win its living and indeed its food.
Surely the collection of natural and human intervention soil erosion data is both critical and urgent, and for world heritage and contingent areas (also likely to be affected) ought to be sought by international rather than by local bodies. With modern surveying techniques able to detect very precise changes in a surface level, and by assessing those values statistically, it should be possible to achieve a major improvement in the determination of erosion rates under various conditions such as those shown in Tables 2 and 3.
In regions where the average depth of soil is reported to be less than or equal to half a metre, such determinations are surely of critical urgency. Streamside reserves are not a sufficient protection where clearfelling and haulroad crossings occur. Granitic soils of the East and North East are at particular risk (see Walker's account of such soils in New South Wales).
The economic losses arising from a loss of soil will be lost productivity barely one or two generations hence, together with a loss of water both in quality and quantity; and the costs of remedial action against massive siltation in lower reaches of the waterways must be borne by the present and next generation. There needs to be a comprehensive economic analysis of clearfell practice in any water catchment area so that the present timber value is properly balanced against the monetary losses imposed upon the wider and future community. Is there a net social gain, especially if the resource being sacrificed (soil) is more valuable - because irreplaceable - than that being won (woodchip) which latter is renewable only provided it still has soil from which to grow!
Timber is indeed a valuable resource. But is it currently being harvested in a manner which protects our other unrenewable resource, the soil in which it grows? In seeking to answer that question, I have chosen to rely on reputable international sources rather than citing any associated with the local industry, so that these conclusions may justly be seen to be unprejudiced and deserving of serious consideration for the protection of a future generation.
Australian Standard 1141-1974, "Sampling and Testing
Aggregates" S.A.A.. tests 20-24
Bell, F. G. (1987), "Ground Engineers Reference Book",
Butterworths, London, (at p.2/8)
Chorley, R. J. (1969), "Water, Earth and Man", Methuen,
London
Currie, D. T., Ingles, O. G., Williams, A. F. & Barton,
C. M. (1974), "Evaluation of a Fine Grained Sedimentary
Rock", Proc. 7th Conf. ARRB, Vol.7, 90-103
Dryness, C.T. (1967), "Mass Soil Movements in the
H. J. Andrews Experimental Forest", USDA Forest Service
Research Paper PNW-42
Heiken, D. (1997), "Landslides and Clearcuts - What does The
Science Really Say?", Umpqua Watersheds: Landslide Studies, 8
PP.
Hjulström, F. (1935), "Studies of the morphological
activity of rivers as illustrated by the river Fyris",
Uni. Uppsala Geol. Inst. Bull., 25, 221-557
Ingles, O. G. & Grant, K. (1975), "The effect of
compaction on various properties of coarse-grained
sediments", Chap. 6 in Compaction of Coarse-Grained
Sediments by Chilingarian & Wolf, Elsevier, Amsterdam
Kiernan, K (1990), "Geomorphology Manual", Forestry
Commission, Tasmania
Kirby, M. J. (1969), "Erosion by Water on Hillslopes",
in Chorley, R. J (loc. cit.) at p. 235
Laffan, M. (2003), "An Introduction to Tasmanian Forest
Soils", Forestry Tas. Tech. Rep. 09/03
Pearce, A. J. & Eison, J. A. (1973), "Postglacial rates
of denudation by soil movement, free face retreat, and fluvial
erosion, Mont St. Hilaire, Quebec", Can. J. Earth Sciences,
10, 91-101 (at p. 100)
Pinkard, G. J. (1980), "Land Systems of Tasmania, Region
4", Tas. Dept. Agriculture
Walker, G. (1977), "Systems and Soil Erosion", The
Australian, 7 June, p. 18
Young, A. (1972), Slopes, Oliver and Boyd, London, (at
p. 122)
Owen G. Ingles, B.A., M.Sc., Ph.D., F.R.S.C., F.I.E.Aust.,
M.I.E., M.A.I.E., C.Eng., C.Chem., C.P.Eng.
Owen Ingles P/L,
Soil Engineering and Risk Management Consultants,
Swan Point, Tasmania
[Republished with permission of the author and editors of Upper Catchment Issues, which first published this in Vol.2#2 Supplement, ISSN 1444-9560.]
50419-6517 (1, 5, 11, 128)