Implications for Assessment of Sustainability

4.5 Implications for Assessment of Sustainability

At the levels of variability encountered, it appeared that a mean change of at least 15% was required to register a statistically significant change in any given property. This is compatible with the proposed interim indicator of 20% change, and in that sense the proposed indicator is meaningful.

It appears that the subjectively oriented, randomly positioned line-intercept transects gave the more accurate estimate of the features associated with logging. The grid-point intercept method, on the other hand, appeared to be better suited to obtaining an accurate measure of the least abundant site classes (e.g., log dumps). However, with the ease of use, accuracy, and low cost of small hand-held GPS equipment, log dumps and access tracks are probably better suited to being mapped manually as recommended in the procedure proposed by Rab (1999). The data to date suggest that these features, together with major snig tracks, which can also easily be mapped, are the only classes that significantly threaten site productivity. Hence, one possibility for assessing sustainability could be to simply identify the spatial extent (and percent of logged area) of these classes using GPS-based methods. If the positional accuracy was relatively high, at least to within 10 m, this would also offer the additional advantage of creating digital disturbance maps that could be stored in a GIS database for management of subsequent logging operations.

The issue concerning the area of coupe to survey is worth noting. The interim indicator refers to the area and percent of soil affected. Just what total area we are reporting a percentage of is left undefined. At the broadest level, it is natural to assume that this would mean percentage of the area managed for timber production. However, a significant part of that area is reserved from logging for functions such as stream, wildlife habitat and flora protection. For surveys in the southeast NSW integrated harvesting coupes, this presents a dilemna, because much of the unlogged area exists within the coupe boundaries. Furthermore, the percentage of the coupe in such reserved areas varies widely from coupe to coupe. A more accurate and meaningful basis for data to be surveyed and reported would be the net logged area of coupes, i.e., the area available for logging exclusive of filter strips, wildlife corridors or any other management imposed exclusions. It is assumed that such exclusions would remain static through successive cutting cycles, and hence the net logged area is what would ultimately represent the productive land base on which sustainability must be assessed.

Although the interim indicator logically suggests that the proposed change relates to the top 300 mm, it is logistically difficult and time consuming, and hence costly, to obtain samples from this depth. It may be feasible to extract disturbed cores for bulk density assessment, but the additional care needed to extract undisturbed cores for aeration porosity assessment greatly increases the time required. We suggest that assessment methods for indicators that involve any soil sampling should

be limited to the top 10 cm as a concession to practicality. An indicator that requires significant change to be detectable to 30 cm (e.g. >20% increase in bulk density) could only serve to reduce the area affected as compared with a 0-10 cm based indicator. Further, it would appear from the data of compartment 394, that if changes to soil physical properties at 10 cm are enough to lead to a growth impact, they will almost certainly be detected at greater depth as well. Change at depth is not independent of change at the surface; hence, it would be sensible to try to identify the degree of change at the surface that is associated with unacceptable productivity loss. Either that, or methods that reduce the time required to obtain reliable measurements will need to be pursued. (Such methods could include non-destructive measures such as gamma probes, ground penetrating radar, or powered coring equipment, none of which is well suited to use in forest environments.)

Whilst the extraction and processing of soil cores for bulk density and porosity at depth is laborious and time consuming, the reverse is true for penetration resistance. Three hundred to four hundred individual sampling points can be assessed in two or three days, with data to 40 cm generally acquired with ease. The potential for using penetration resistance is often dismissed due to its temporal instability with changing moisture content. We would argue, however, that the technique Whilst the extraction and processing of soil cores for bulk density and porosity at depth is laborious and time consuming, the reverse is true for penetration resistance. Three hundred to four hundred individual sampling points can be assessed in two or three days, with data to 40 cm generally acquired with ease. The potential for using penetration resistance is often dismissed due to its temporal instability with changing moisture content. We would argue, however, that the technique