Volume by Track Sub-plots

3.2.3 Volume by Track Sub-plots

The track plots were examined in more detail to investigate the relative roles of the tracks, edges and outer plot areas. Box plots of tree volume per hectare in 1-m increments moving out from the track centre were prepared to investigate any tendency for an increase in volume within the plot edge zone compared to the inner and outer sub-plots. We consider the edge zone to generally occur in the 3-m and 4-m increments. These box plots, shown in Figure 21, give a clear suggestion of an edge effect in the minor snig class, major snig class and access track class. The effect appears to be greatest in the minor snig class, but the data are also the most variable for that category resulting from a few particularly large values.

With all track disturbance classes pooled, analysis of variance on the mean plot volume with all trees in the plot (P<0.001) revealed a significant difference between edge (47.3 m 3 ha -1 ) and track (14.1 m 3 ha -1 ) sub-plots. However, neither the edge nor the track component was significantly different to the outer plot component (31.1 m 3 ha -1 ). If only the five largest trees of each sub-plot

3 were used, the edge plots (44.3 m -1 ha ) had a significantly higher mean volume than both the outer and track plots (19.7 and 12.1 m 3 ha -1 respectively). The latter two were not significantly different.

Major snig e (m

3 ha Minor snig

tree volum

Sub plot

Sub plot

Access road e (m

Landing snig

tree volum

plot tr

Sub

Sub plot

Sub plot distance from center of plot (m)

Figure 21. Box plots of plot-level tree volume in 1 m increments from the plot centre for each of the track disturbance classes. All trees used.

The edge plot tree volumes of the minor snig, major snig and access track classes were all larger than the respective track and outer plots. However, the difference was only statistically significant The edge plot tree volumes of the minor snig, major snig and access track classes were all larger than the respective track and outer plots. However, the difference was only statistically significant

As the track, edge and outer plot data are amalgamated, there is a tendency towards lower spread in the means (Table 9, Figure 22). This move toward homogeneity is confounded to some extent by the very high mean volume of the edge sub-plot of the minor snig class. The minor snig class was still significantly higher than the other classes when track and edge data were combined, but no differences between means were found after outer plot data were amalgamated.

Table 9. Mean tree volume and results of analysis of variance multiple range test for various components of the track-type plots, all trees used. Means followed by the same letter, within the same column, are not significantly different.

Mean volume (m 3 ha -1 )

Track Class N

Track data only

Edge plots only

Track + edge Track + edge + outer

40.0 A Major snig

Minor snig

29.7 A Access track

26.0 A Landing snig

A particularly instructive result can be obtained if we put aside variability and statistical tests, and assume that the means are indicative of a real signal rather than just part of the noise. In so doing, we see that tree volume of the track + edge is markedly lower than that of the outer plot alone for the access track and landing snig classes (Figure 22). This is instructive only in that it corroborates the results of the simple analysis of variance data where these two classes were found to have significantly lower mean volumes than the classes with the highest means.

Track plot

Edge+Track plot

Edge plot

3 ha Outer plot

Access road

Landing snig

Major snig

Minor snig

Figure 22. Average tree volume by disturbance class and sub-plot for the four track-type disturbance classes in compartment 394. Five largest trees used.