Simple Comparison of Tree Volume by Disturbance Class

3.2.2 Simple Comparison of Tree Volume by Disturbance Class

To compare all eight classes with one another, the track data were standardised to a five-largest-tree basis, the same as the area plot data. For the track plots, only the central track component of the plots was examined in this initial analysis.

Mean plot tree volumes were much lower for the access track and landing/snig track classes than for all other classes (Figure 20). However, data variability was high and the analysis of variance model was on the margins of significance (P=0.06). The mean plot volume of the minor snig tracks class was the highest of all disturbance classes, but was only significantly different to the means of the access track and landing/snig track classes. There were no other significant differences amongst classes. Nevertheless, the ranking of the means for the track-type classes was in line with the inferred relationship between disturbance severity and soil physical properties (Figure 20). However, the pronounced difference in mean volume between minor and major snig tracks was not statistically significant.

For the polygon plots, there were no significant differences between any of the classes (Figure 20). However, it was interesting to note that mean volume of the minor disturbance class was a little lower than that of the other three classes. The fact that the result for landings was similar to the other classes is also noteworthy. The landings had obviously been ripped (rip furrows still visible), but they commonly appeared to the naked eye to be less well stocked than other areas. This was possibly due to fewer and smaller ground cover and shrubs rather than trees. As sampling proceeded it became evident that stocking of the log landings was not as low as first thought, and the resulting data bear this out. The relatively low mean of the minor disturbance class is also noteworthy, as is the higher mean for light disturbance, which is what we assumed to be the closest class to a control. Yet, there is no statistical basis to believe that the differences between these polygon classes is anything other than random, or some other source, of variation.

Major disturbance Major snig Minor disturbance Landing snig Minor snig

Light disturbance

Landings

Access road

Figure 20. Mean of plot tree volume for the five largest trees for all disturbance categories. Snig plots include track area only. Classes with the same letter are not significantly different (Duncan’s multiple range test). Light disturbance is considered the least disturbed and closest to a control.

On the basis that only two or three of the larger trees in each plot are likely to become dominant, or form part of a commercial stand, we examined the mean stem volume for the largest, two largest and three largest trees of each plot. The relative differences between disturbance classes was almost identical for each of these three variables, hence we present only the data for the two largest trees per plot (Table 8).

Table 8. Mean stem volume (m 3 ha -1 ) and variability by disturbance class for the two largest trees of each plot. Means followed by the same letter are not significantly different.

Duncan’s comparison Major dist

Track Class n

Std deviation

Coefficient of

Light dist

Landings 12 0.016 76.558 0.021BA Minor snig

0.017BAC Minor dist

0.014BAC Major snig

0.010BC Landing snig

0.006BC Access road

There were some differences between the analysis based on volume per hectare and that based on mean volume of the two largest trees. Firstly, the order of the first six variables changed, with major disturbance and light disturbance having the largest trees. This is not unusual given that the variability is relatively high and there were not large differences separating the means. Major and light disturbance were significantly greater than the trees from the major snig track class in addition to the landing snig and access track classes. There was also a greater degree of segregation of the There were some differences between the analysis based on volume per hectare and that based on mean volume of the two largest trees. Firstly, the order of the first six variables changed, with major disturbance and light disturbance having the largest trees. This is not unusual given that the variability is relatively high and there were not large differences separating the means. Major and light disturbance were significantly greater than the trees from the major snig track class in addition to the landing snig and access track classes. There was also a greater degree of segregation of the