Spot Mounding for cutover rehabilitation (in poorly drained Clay Soil)
- Ngaumu Forest

Figure 1 - Trial at age three, untreated site on left showing poor growth, ripped and mounded site on right showing good growth

A trial to look at the effectiveness of spot ripping and mounding as a means for improving establishment on a failed cutover re-establishment site was planted in 1996.   It was initially unclear as to why the trees had failed to survive or grow satisfactorily in the first attempt at re-planting as a good crop of trees had been harvested from the area.   However, it was found that despite much of the site being close to gullies the area was not well drained, with a water table at approximately 10 to 15 cm below ground level.

Table 1 - summary of results for annual measurement - 2000 at age 4

Note; results in a column (Table 1) followed by the same letter are not significantly different, results in a column followed by different letters are significantly different (P 0.05).  Health and Form scores; these scores are based on a subjective assessment of each tree on a scale of 1 to 5, with 1 being an optimum score and 5 being the lowest.  A lower average score for a treatment indicates a superior result.

There were significant differences in DBH and DBH increment (Table 1), with the two mounding treatments giving significantly better growth than the un-mounded treatments.   There were significant differences between the mounded and un-mounded plots for height and height increment, but no significant differences between the fertilisation and no fertilisation.

The mounding has significantly improved the health and form of the trees, the fertilisation had no significant effects.  The difference in health score is indicative that the trends for divergence seen in height and diameter will continue, as the less healthy trees in the un-mounded treatments are unlikely to grow as well as those in the mounded plots in the next year at least.

There were significant differences in survival with both mounding treatments giving survivals of over 90%.  The mounding treatments were not different to each other but were both significantly better than the un-mounded sites.

Figure 2 - Basal area and volume per hectare at age 4

When the differences from diameter, survival and height are accumulated in basal area and volume the effects are substantial, as well as significant (Figure 2).   The un-mounded treatments have only a small percentage of the basal area and volume growth of the mounded treatments.  The differences being that the un-mounded plots have 10% of the basal area of the mounded plots and only 5% of the volume.

The cultivation system used (Wilco spot cultivator) creates a hole next to the mound, as this is where the soil to make the mound comes from.  In many instances these holes had standing water in them at the time of measurement, indicating that the water table on this site is still at a level of 10 to 15 cm.  The planting of the trees on the top of the mound lifts the trees above this.

On this site the spot mounding with the Wilco spot cultivator has produced significant and substantial differences in DBH, height and survivals.  When these were converted to basal area and volume, the result was very large differences in growth .

The growth from the un-mounded plots would be regarded as unacceptably low, with the survivals alone being so poor as count these plots an establishment failure.  The trial has shown that the spot ripping-mounding can alleviate this problem and produce a viable crop.