Temporal changes in soil physical properties

1.3 Temporal changes in soil physical properties

These studies, particularly Study B of Part II, have shown that significant compaction persists, even to 30 cm, for at least 10 years on the more heavily impacted soil disturbance categories. This in itself is not new, and persistence for much longer has been demonstrated previously (e.g. Jakobsen, 1983), but the difference is that this study has provided some definition of the actual degree of recovery and classes that it occurs in. It appears that moderate levels of compaction, which may be recognised as significant and potentially threatening to productivity, undergo a significant degree of recovery over ten years. The regeneration studies showed some evidence that the original changes may have had some negative influence on growth performance over the ten years on the moderately disturbed classes. However, there would certainly be some prospect that something approaching full recovery may be achieved in these classes (i.e. all except primary snig tracks and S3 disturbance) by the end of a rotation at age 40+ years.

The soil strength measures in compartment 394, Yambulla State Forest, also presented clear evidence of elevated soil strength on the major snig tracks seven years after harvesting. For reasons discussed in Part I, mainly to do with soil fertility, this had a less pronounced influence on growth than might have been expected. Nevertheless, this persistence in impacted soil physical properties needs to be further examined for any future recovery before the next rotation.

The key consideration for temporal change in detrimentally impacted soil properties is to what extent detrimental change from one operation still exists at the time of the next operation. If all areas have recovered to be indistinguishable from never-disturbed soil, then each operation may be considered as a discrete event. But if, as the evidence presently indicates, there are areas retaining some degree of detrimental condition, then the potential for cumulative impact exists. Sustainability indicator measurement will clearly need to consider this fact when dealing with previously logged compartments. On the evidence currently available, we would have to assume that there is a high probability that major (primary) snig tracks, and access tracks will not recover within a normal rotation in either the NSW or Victorian type forests. Secondary and tertiary tracks, provided they have not experienced S3 level disturbance, are more likely to recover, at least to the point that if they receive no further impact they would probably not affect growth rates in the subsequent regenerated stand. If this can be confirmed, only the access/primary snig tracks and S3 disturbance of a previous operation would need to be accounted for when assessing a previously logged coupe at the end of the second rotation.

This presents a clear rational basis for not only assessing and reporting identifiable categories of soil disturbance that directly influence sustainability, but options for managing them. The current practice of ripping log landings, for example, if extended to the primary snig track network may reduce the area of detrimental impact to negligible levels. If extending it is not feasible, and there are many reasons why it might not be (increased erosion hazard, cost), an alternative approach would be to explicitly regard the primary access track/primary snig tracks as having been dedicated to forest management, much like the road network, rather than being part of the productive area. If this ensures that the potential for cumulative soil physical property impact is removed, then it would be an acceptable approach. The concept of dedicated snig tracks has been around for some time (Froehlich, 1984), and has parallels with dedicated traffic lanes in agriculture, but has never been seriously considered or trialed in Australia to the authors’ knowledge.

In conclusion, we have obtained some important data points on temporal changes in soil physical properties. Robust relationships over complete rotation periods may not yet exist, but our working assumptions now have a more solid basis, and considerable insight has been gained into the differential response pattern of varying categories of disturbance severity.