Workshop Report: Poplar and willow planting for Land Overlay 3A, Gisborne, East Coast region
Results
Although a wealth of data was collected during the desktop exercise, the results presented here are restricted to determining consensus on:
- treating Land Overlay 3A only versus 3A plus additional land surrounding the erosion feature in question but within the same watershed (Figure 1);
- pole/wand planting versus forestry as a primary treatment (Figure 2);
- pole/wand planting densities of 200–500 or 75–200 or less than 75 spha (Figure 3).
Note: The following analyses are not claimed to be statistically defensible, given there are just 16 participants, that is, 16 observations for each parameter, but nonetheless show basic trends/patterns of participant’s perception of treatment options required to provide effective erosion control of three of the most common erosion types associated with Land Overlay 3A: gullies, earthflows and slips and slumps. Where results showed a clear preference between 3A only, 3A + under 50 percent of watershed, or 3A + over 50 percent of watershed these are commented on.
Land Overlay 3A area only versus 3A area and adjacent land
Gullies
For 6 of the 10 gully examples there was a definite preference for including additional land to Land Overlay 3A (Figure 1A) in their treatment and that treatment includes more than 50 percent of the watershed. Sites 1 and 3 were the exceptions where participants considered that the planting of the area defined as Land Overlay 3A alone would provide effective erosion control. For site 1 the presence of a considerable number of existing poles within the area designated as Land Overlay 3A and the presence of gentler terrain surrounding this gully likely influenced their choice. For site 3, justification for planting the Land Overlay 3A area only was possibly influenced by the presence of considerable regenerating scrub, existing pole plantings and the benign appearance of this gully as shown on the aerial and ground-based images provided to the workshop participants.
Earthflows
For about half the earthflow sites, opinions were evenly split. At sites 5, 7, and 9 it was deemed that treatment of Land Overlay 3A alone would suffice (Figure 1B) while at sites 3 and 11 it was considered that treatment would be more effective if it included land outside the designated 3A area. At site 3, visited the previous day, it was clear to participants that the entire slope had already been planted and at site 11 participants could see from the photographic imagery provided that this site was particularly wet on account of drainage from earthflows feeding into an existing gully that also required treatment.
Slips and slumps
For slips and slumps there was no clear domination of one option over the other (Figure 1C) but where it was deemed necessary to include additional land outside Land Overlay 3A for treatment, many chose to include greater than 50 percent of the watershed. At sites 1 and 3 the erosion feature lies within and only occupies a small proportion of the area designated as Land Overlay 3A, thereby justifying the planting of just the 3A area. Site 4 was clearly deemed to require additional planting outside the 3A area because a considerable portion of the slump lay outside the designated 3A polygon. For site 5, the presence of considerable reversion both within and upslope of the designated 3A polygon may have influenced a few participants towards the retirement option and to include poor quality land adjacent to the 3A area.
There was confusion over the location of Site 6, as shown in the oblique photograph provided to the workshop participants, relative to the Land Overlay 3A, and this may have influenced the participants’ choice. It turns out that the bulk of the area portrayed in the oblique photograph lies outside the now notified Land Overlay 3A area. Realistically, for half of the six sites (sites 2, 5 and 6) participants were evenly divided over the decision on whether or not additional land outside of the 3A polygon would need to be treated to ensure effective treatment of this erosion feature.
Figure 1: The proportion of participants who considered that treatment would be effective if confined to Land Overlay 3A only versus Land Overlay 3A and additional surrounding land in gullies, earthflows and slips and slumps
Figure 1A - Land Overlay 3A only versus Land Overlay 3A and additional surrounding land: gullies
Figure 1B - Land Overlay 3A only versus Land Overlay 3A and additional surrounding land: earthflows
Figure 1C - Land Overlay 3A only versus Land Overlay 3A and additional surrounding land: slips and slumps
Forestry treatment versus planting of poles/wands
Gullies
Based on the results of the workshop there was general consensus about choosing the most appropriate treatment option for large and active gullies (8 of the 10 examples) where reforestation was considered to be the most practical option (Figure 2A). Afforestation was clearly favoured for gully systems in an “advanced” (4) and “mid” (4) stage of development. Both foresters and soil conservators appreciated that such features often require additional treatment of the channel through the use of wands or the allowance for setbacks if indigenous reversion is already present. Similarly, when deciding on an appropriate treatment option for smaller and perhaps less active gullies (examples were not well represented at the workshop except as sub-catchment elements of larger gully complexes) there is unlikely to be disagreement over treatment options as it is generally accepted that the planting of poplars and/or willows along the channel and banks of narrow, linear gullies would be sufficient to prevent further downcutting and/or lateral bank erosion at the majority of sites. Reversion, if sufficiently advanced, is also an accepted alternative option. However, although not well represented in the workshop examples, it is anticipated that gullies in the mid size range (~2–5 hectares) on pastoral land will be the most contentious when agreement is sought on their most effective treatment. Unless a set of criteria can be agreed on to define the limitations of pole/wand treatment of the different erosion types, perhaps based on type of feature, size of feature, degree of activity, aspect, slope etc, the choice of treatment will be partly subjective and open to criticism by one party or the other. These differences will need to be resolved at the time of developing a site plan or during subsequent site inspections.
Earthflows, slips and slumps
The desktop exercise results indicate a strong preference for pole/wand treatment on earthflows (9 of 11 examples, Figure 2B). There was a split decision between poles and forestry for slips and slumps (3 of 6 examples, Figure 2C). For poles, the combination of site factors at each locality will often determine the success or otherwise of these plantings. There are slope treatment options often used in conjunction with plantings for example, drainage of ponded areas and/or debris dams that can significantly enhance the chance of success of planting.
Figure 2: The proportion of participants who considered forestry versus pole planting most likely to provide the best erosion control outcomes for gullies, earthflows and slips and slumps
Figure 2A - Forestry versus pole planting: gullies
Figure 2B - Forestry versus pole planting: earthflows
Figure 2C - Forestry versus pole planting: slips and slumps
Pole/wand planting densities for earthflows and slips and slumps
Earthflows
For all 11 earthflow sites, the density range 75–200 spha was a popular choice but at only two sites (1 and 2) was it the dominant choice (Figure 3A). For sites 7 and 8 the choice of planting density was split between 75–200 spha and the heavier stocking density of 200–500 spha and for the remainder of sites the trend was towards lighter stocking rates with <75 spha showing an even ranking with 75–200 spha.
Slips and slumps
For slips and slumps, the 75–200 spha option dominated at sites 2 and 6 (Figure 3B). To achieve an effective erosion control outcome some participants recognised the need for higher densities of poles on the larger and more active slumps (sites 1 and 4) and stocking evenly split on erosion features with existing poles (site 3) and advanced reversion (site 5).
Gullies
Pole planting was chosen as the primary treatment for only two of the 10 gullies presented in this exercise. For site one the preference was for 200–500 spha and for site 3 it was 75–200 spha. There was insufficient data to graph. The gully elements and sub-catchment size is expected to influence the density chosen.
Figure 3: Pole/wand planting densities selected by participants as most likely to provide the best erosion control outcome for earthflows and slips and slumps
Figure 3A - Forestry versus pole planting: earthflows
Figure 3B - Forestry versus pole planting: slips and slumps
Contact for Enquiries
East Coast Forestry Project
Ministry of Agriculture and Forestry
Cnr Grey St & Childers Rd
1st Floor ZGFM Building
PO Box 2122
Gisborne 4040
Tel: +64 6 986 8691
Fax: +64 6 986 8697
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