Relationships betw een changes in soil physical properties and subsequent forest grow th

1.2 Relationships betw een changes in soil physical properties and subsequent forest grow th

As a result of the studies reported herein, and the recent related study by Pennington et al. (2002) in Tasmania, it is becoming clear that there is a level of soil physical property change that can be associated with reduced timber volume in the next rotation. However, a common experience to all studies is that despite the evidence that exists in the form of greatly altered class means, the data are highly variable. “Proof” of change through classical statistical procedures is always going to be very difficult to achieve in these highly variable native forest environments. Thus, in the NSW study, the weight of evidence, as opposed to the statistical proof, was that access tracks and major snig tracks (major snig tracks and landing snig track classes) supported lower tree stem volumes compared to the lightly disturbed class. Most other areas, including rehabilitated log landings, were either clearly not impacted, or there was not enough evidence to speculate that they were. In the Victorian study, a similar conclusion could be reached, except that there seemed to be stronger evidence that most disturbance classes had been impacted. Although only the primary snig tracks were “proven” to support lower stem volume at 10 years, the trend in the data was that trees on all classes except S1 disturbance were displaying lower levels of growth than those on the S0, undisturbed, class.

It was also interesting that in both the NSW and Victorian study we were able to obtain a continuous relationship between tree growth parameters (stem volume for both and also DBH and height in Victoria) and individual soil physical property values. The weakness in this relationship for the NSW (soil penetration resistance) study was that the relationship did not exist at the individual plot level, but only when the data were aggregated to the class level. At the very least, this makes it difficult to have any confidence over the form of the relationship as it could change considerably with a change in just one or two class means.

The relationship between bulk density and stem volume for the

E. regnans data suggests a rather dramatic reduction in stem volume as bulk density increased from 0.7 to 0.8 Mg m -3 . However, the form of this relationship was dependent on just three values. The real problem with these data, as with any relationship between bulk density and plant growth, is that it is not a property which, of itself, influences plant root function. In other words, even if this relationship can be verified as being robust and causally linked for this forest system, it will not be transportable to other soil types without verification. Relationships based on soil strength, aeration and water relation properties would be much more robust. Relationships between tree growth and soil properties such as strength, aeration or LLWR clearly need to be developed, and these in turn clearly linked to various types E. regnans data suggests a rather dramatic reduction in stem volume as bulk density increased from 0.7 to 0.8 Mg m -3 . However, the form of this relationship was dependent on just three values. The real problem with these data, as with any relationship between bulk density and plant growth, is that it is not a property which, of itself, influences plant root function. In other words, even if this relationship can be verified as being robust and causally linked for this forest system, it will not be transportable to other soil types without verification. Relationships based on soil strength, aeration and water relation properties would be much more robust. Relationships between tree growth and soil properties such as strength, aeration or LLWR clearly need to be developed, and these in turn clearly linked to various types

In conclusion, the study has enhanced our empirical database on relationships between disturbance and forest regeneration. It provides us with a basis to draw some tentative working conclusions and hypotheses. However, it must also be said that the starting base was very low, and there is still much to be learned, especially with respect to the specific relationships between soil properties and tree growth, and interactions between soil properties and other environmental factors.