Soil disturbance by transect surveys

3.1.3 Soil disturbance by transect surveys

To the extent that they were comparable, the disturbance results from transect surveys (Table 5) were generally consistent with the GPS data (Table 4). The proportion of the coupe that was logged, and the proportion that remained undisturbed, was more accurately estimated by the line-intercept method (taking the GPS measure to be accurate). The proportion consisting of snig tracks was similar for both survey methods, but both estimates were almost double that of the tracking study (sum of all traffic intensity classes). This suggests either that the trafficking study under-estimated the areas of tracks or that the survey methods over-estimated them. In the absence of full, detailed ground mapping, it is difficult to say which is the case.

Track usage 0 - 10 11 - 30 30 - 49 7 8 50 - 200

0 100 200 Metres

15 Coupe Boundary

RDD GI S SFN SW August 2001 20 7

Figure 10. Distribution of individual tracks identified by GPS tracking data. Numbers represent the estimated number of passes and line thickness is proportional to the number of passes (track usage classes).

Table 5. Percent of coupe occupied by three types of ground classification estimated from two different survey methods and applying Equation 2 (rounded to nearest whole percent).

Grid Point-Intercept Method

Line Intercept Method

Percent Percent

Operation category

Access road

1 Access Road

Log landings

1 Log Landings

Major snig

3 Snig

Moderate snig

Minor snig

Harvested area

24 Harvested Area

Undisturbed area

57 Undisturbed Area

Soil disturbance class

S0 63 S0 55 S1 12 S1 17 S2 16 S2 25 S3 9 S3 3

Dominant Horizon

A 17 A 27 B9 B3

None of the transects of the line intercept method intercepted any of the log landings, a consequence of the small percentage and compact spatial configuration of this category. The grid- point intercept method, however, recorded an almost identical proportion as measured by ground mapping. This might indicate one drawback of using subjectively oriented transects rather than a method that uses full randomisation and disregards the non-random nature of logging disturbance.

The percent area occupied by different soil disturbance classes saw reasonable agreement between the two survey methods. There was, however, a very low estimate of the heaviest disturbance class, S3, by the line intercept method compared with the grid-point intercept method. It is possible that there was some systematic difference in classification by the operator between the two surveys, although this is considered unlikely as the same field staff conducted both surveys.

Differences between the two methods were most pronounced in the dominant horizon results. There was close agreement in the Class 1 and Class 2 estimates, but the allocation of remaining points between the A and B classes differed (Table 5). This result would be partly due to the difficulty in identifying the actual horizon exposed in this soil type. For both methods, there was a close correlation between the soil disturbance class and dominant horizon data: not surprising since both are effectively a reflection of the depth of soil displacement.

The breakdown of soil disturbance severity by operation class shows a clear pattern, with access road, log landings and major snig tracks having very high percentages of severely disturbed soil and the balance being moderately disturbed (Figure 11). On the other hand, the proportions of less severe disturbance classes progressively increased as we moved to the less intense operational categories. There is no great revelation in this result as it is either intuitive, in the case of access road, landings and undisturbed area, or reflects the role of disturbance severity in the original basis of classification. That is, the snig track sub-categories and harvest areas were largely classified as a result of their degree of disturbance. The high proportion of severe disturbance in the major snig tracks class should be considered along with the overall distribution of this class (Table 4). Overall, The breakdown of soil disturbance severity by operation class shows a clear pattern, with access road, log landings and major snig tracks having very high percentages of severely disturbed soil and the balance being moderately disturbed (Figure 11). On the other hand, the proportions of less severe disturbance classes progressively increased as we moved to the less intense operational categories. There is no great revelation in this result as it is either intuitive, in the case of access road, landings and undisturbed area, or reflects the role of disturbance severity in the original basis of classification. That is, the snig track sub-categories and harvest areas were largely classified as a result of their degree of disturbance. The high proportion of severe disturbance in the major snig tracks class should be considered along with the overall distribution of this class (Table 4). Overall,

Severely disturbed 100 97.6

Undisturbed

Lightly disturbed

Moderately disturbed

Points in disturbance class (%) oad

cess g Landings

H arves

Undisturbed Ar

Figure 11. Breakdown of operation categories by disturbance severity for the line intercept survey data.