Tree Volume in Relation to Disturbance

4.3 Tree Volume in Relation to Disturbance

The most overwhelming signal to come through the generally high variation (noise) of the regeneration survey data, was that the access track and landing snig track classes supported significantly and dramatically lower volumes of regeneration than some of the other classes. It is impossible to conclude that any of the other classes have lower regeneration volumes as a result of disturbance and changes to soil physical properties. In the case of major snig tracks, this result is important, since this class still displayed significantly higher soil strength at 7 y after logging.

Factors such as spatial variability in the post-logging burn, soil moisture distribution, or solar radiation (aspect, slope position) could all potentially contribute to variations in seedling growth. Some of these we were able to measure, and in one case, aspect, we found a strong association with growth. This did not improve our ability to see a disturbance effect. Another source of variation is that regeneration can also stem from either lignotubers or seed, whilst some young trees that we measured as regeneration may in fact have been small saplings that survived logging. These two sources of variation may have explained the high edge sub-plot values of the minor snig track class. It is difficult to know without baseline assessments what the mechanism of regeneration was for an individual stem. Even coppice from old stumps may be difficult to identify if the stump is covered by accumulated litter and humus in the intervening years since logging.

The fact that relationships between regeneration volume and all the factors measured were so weak suggests that we were either dealing with high levels of measurement error (quite possible) or that another source or sources of influence were more important. The most likely unmeasured factor was nutrition. Overall, we know that the soil has a low nutrient status and low water holding capacity (Ryan, 1993). These are probably the major limiting factors on this soil, and trees therefore do not show strong responses to minor variations in factors such as light, competition, or moisture supply. The occasional spikes in tree size corresponding to plot edges may have been due to local concentrations of displaced topsoil, thicker ash-beds in this zone, or accumulation of nutrients by rain-wash. Although we must also acknowledge the lignotuber/prior regeneration theory that has already been raised to explain the high variability in this zone.

The importance of the role of site fertility in causing machine-induced changes to soil physical properties (or other soil changes) is supported by other recent studies. Pennington et al. (2002) and Rab and Kellie (part II of the current report) recorded much stronger negative associations between major snig tracks and tree growth than we measured. In both these studies, the sites were high productivity native eucalypt forests (E. obliqua and E. regnans respectively) on soils with good The importance of the role of site fertility in causing machine-induced changes to soil physical properties (or other soil changes) is supported by other recent studies. Pennington et al. (2002) and Rab and Kellie (part II of the current report) recorded much stronger negative associations between major snig tracks and tree growth than we measured. In both these studies, the sites were high productivity native eucalypt forests (E. obliqua and E. regnans respectively) on soils with good

In summary, whilst the access tracks and landing snig tracks seem to have been heavily impacted, resulting in severely suppressed tree growth, it is difficult to be conclusive about any of the other disturbance classes. It would simply appear that with a soil environment already very limiting to growth, quite major changes in soil physical properties may be needed before they become the most limiting factor. In such a complex and heterogeneous environment as a logged native eucalypt forest, it would appear that the task of defining relationships between soil physical property change and soil productivity is a very complex one indeed.