Soil Disturbance and Physical Property Changes
4.1 Soil Disturbance and Physical Property Changes
The overall level of disturbance was relatively low. Of the three independent methods of measuring undisturbed area, the lowest was 42%, whilst somewhere between 24 and 35% consisted of relatively lightly disturbed harvested area (i.e. harvested area excluding snig tracks, access tracks and log landings). Both survey methods agreed that the area of snig tracks was a little under 20% of the coupe.
The soil disturbance classes rather than operation category probably give the best overall snapshot of the level of disturbance, with around 60% undisturbed (S0). The proportion of the coupe in the other three categories (S1-3) varied between the two survey methods, but in respect of soil physical The soil disturbance classes rather than operation category probably give the best overall snapshot of the level of disturbance, with around 60% undisturbed (S0). The proportion of the coupe in the other three categories (S1-3) varied between the two survey methods, but in respect of soil physical
be low. This category was most closely associated with log landings, access tracks and major snig tracks, although about half of the area of moderate snig tracks was also S3 level disturbance (Figure 11). This would explain why the moderate snig tracks had elevated, but not significantly, bulk density and 25-100 mm soil strength compared with undisturbed areas.
It is important to note, that there was a reasonably close association between significant differences at 0-10 cm, and significant differences at greater depths. The undisturbed core data, for example, did not show a significant depth by category interaction for aeration porosity, yet the mean of the major snig track class was significantly lower than the other three classes (Fig. 12). Compared to the least disturbed categories, soil strength of the log dump and major snig track (and presumably access track) categories was significantly higher to a depth of around 30 cm (deeper for log landings, Fig. 13).
Surveys of soil disturbance in 20 coupes in the Victorian Central Highlands (Rab and Dignan, Study A, Pt II of this report) yielded a mean of 17% and range from to 12 to 24% occupied by snig tracks. The proportion of the coupe remaining undisturbed, was, however, much lower in the coupes reported by Rab and Dignan. They were generally much smaller coupes as well (mostly <20 ha), indicative of fundamental differences in harvesting between the harvesting systems of the two different forest types. The close match in the proportion of coupe under snig tracks, log landing, and access track operational categories is therefore somewhat surprising. The set of coupes measured by Rab and Dignan included coupes with up to around 35% of the coupe in these categories. Nevertheless, it would appear that most operations in south eastern Australia have between 15 and 25% of the coupe area in these operational categories. This has some implications for sustainabilty assessment as discussed further below.
It is likely that the relatively low level of soil changes is a result of the soil type and soil moisture status during logging. In both the earlier study and the current study, logging was performed whilst soil moisture was very low. This is unlikely to have affected the extent of disturbance, except perhaps the relative proportions of moderate and severe disturbance, but may explain the smaller increases in bulk density and penetration resistance and smaller reductions in aeration porosity. Logging in forests with finer textured soil in higher rainfall environments may lead to higher levels of soil property changes, particularly on all classes of snig track. However, Aust et al. (1995) also demonstrated that although logging on wet soil can bring about more changes than on dry soil, the difference is not as much as might be suggested by visual assessment alone. This is because saturated soil, whilst quite susceptible to churning, mixing, and structural degradation, is resistant to compaction because the soil pores are full, and thus unable to be compressed.