Workshop Report: Poplar and willow planting for Land Overlay 3A, Gisborne, East Coast region
Poplar and willow planting spacings for erosion control
Variations in workshop results, and in some cases a lack of a clear preference for one particular treatment, occur because combinations of treatment options were selected by most participants to give a more robust soil erosion outcome in the context of continued pastoral farming on Land Overlay 3A land.
The majority of respondents’ comments to the draft workshop report focused on the issue of planting density/spacing for poplars and willows. There was strong support by respondents for the concept of devising recommended planting spacing (cf. planting densities, that is, stems per hectare) in recognition that poles should be sited in locations where they are most likely to survive and where they are most needed to effect control of the erosion feature in question. This requires acceptance that poles will inevitably be unevenly distributed across an eroding feature with a greater concentration planted on the unstable part.
A range of planting spacing criteria for poplar and willow plantings, advocated by workshop participants to achieve a successful erosion control outcome on moderate and severe earthflows, slumps and for linear gullies respectively within Land Overlay 3A is presented in Table 1. The sites at which each pole is placed would be decided at the time of work plan development. The auditing of treated areas may involve a count of surviving poles. The responsibility of replacing trees that had died in the interim rests with the landowner.
Consensus on the required spacing to achieve adequate erosion control for moderate earthflows and slumps (10–12 metres after thinning) equates approximately to the equivalent of 100–70 spha. For severe earthflows and slumps planted at 7–10 metre spacing but not thinned the final density is equivalent to ~200–100 spha for the unstable parts of these features (Table 1). This is backed up to some extent by evidence from field surveys showing the relationship between disturbed ground and poplar plantings which clearly demonstrate that to provide adequate erosion control for moderate earthflows and slumps the final stand density needs to be between 50–100 spha and that initial planting densities in excess of 100 spha helps achieve stability at severely eroding sites (Hicks 2007).
Hick’s prescription for pole spacing rather than planting densities as the most appropriate means of prescribing a treatment for any given area or feature met with general acceptance from other participants. That is, in developing a work plan for an area to be treated using poles, the specification would require a given number of surviving poles within a given boundary.
With regard to the planting of wands, there was general agreement that these should be planted at the recommended higher stocking rates as indicated in Table 1 because of their availability, lower cost and general lower resistance to summer dry.
Table 1: Indicative planting spacing recommended for earthflows, linear gullies and slumps to achieve a successful erosion control outcome within Land Overlay 3A and for adjacent eroded areas
| Feature | Spacing (m) | Planting Location | |||
|---|---|---|---|---|---|
| Poles | Thinning | Wands | Thinning | ||
| Moderate earthflow | 7-10m | 10-12m @ 10-20 years |
5-7m | 10-12m @10-20 years |
unstable part of flow |
| 10‑12m plus blanking | none | ||||
| Severe earthflow | 7-10m | none | 5-7m | none | unstable part of flow |
| Moderate linear gully | 6-8m | 10-12m @ 10-20 years |
2-4m | 10-12m @ 10-20 years |
watercourse |
| 7-10m | 5-7m | gully walls | |||
| 10-12m plus blanking | none | ||||
| Severe linear gully | 6-8m | none | 2-4m | none | watercourse |
| 7-10m | 5-7m | gully walls | |||
| Moderate slump | 7m | 10-12m @ 10-20 years |
5m | 10-12m @ 10-20 years |
unstable part of slump |
| Severe slump | 5m | none | 5m | none | unstable part of slump |
Note
Planting spacing will vary with topography and erosion severity within a given planted area. (Table contents supplied by D Hicks, based on published field survey data from Hawley & Diamond 1988, Thompson & Luckman 1993, Hicks 1992, Miller et al. 1995, McElwee & Knowles 2000 and modified in response to comments on the draft working report regarding the spacing and waste of resources if required to thin pole plantings.)
There was strong disagreement on the idea of over-planting as a means of ensuring an adequate number of trees survived. Over-planting results in the excessive and unnecessary use of poles at a time when there is a dire shortage of planting material. Instead, blanking was seen as a more cost effective and efficient means of accounting for losses and of avoiding thinning costs at a later date. Counter to this argument, and raised by only one respondent, is the viewpoint that over or heavy stocking is the best way to effect early stabilisation and could well have application to the most severely eroding earthflows and slumps particularly within Land Overlay 3A. Conversely, the risk of under-planting poles on such sites could result in inadequate protection and financial and resource wastage.
In preparing and presenting details of stocking rates, as part of property scale “work plans”, it would be helpful to both practitioners and landowners to have ortho photo maps with delineated planting zones and their required stocking rates.
The recommended planting spacings or stocking rates are indicative only and should be used as a guide for sites where a prior agreement has been reached that poles and/or wands are an appropriate treatment option. Listed below are suggested additional factors and considerations that will need to be taken into account before reaching such a decision.
Whether or not an erosion feature is classified as severe or moderate is dependent on its activity and size at the time it is assessed. This assessment will likely occur when developing each farm plan but will require reassessment at the time of planting. Cognisance should also be given to the potential erosion status of each site as shown in the Land Use Capability Classification of the Gisborne–East Coast Region (Jessen et al. 1999), but taking into consideration the time-dependent nature of causative factors. This is particularly important for all large-scale earthflows, gullies, slips and slumps where consideration of the potential for failure could greatly influence not only the choice of treatment option but also the area required to be treated to achieve a successful erosion control outcome.
Where applicable, that is linear gullies shallower than five metres, gully planting will require watercourse planting with willows in addition to gully wall planting with willows or poplars. The timing of watercourse planting relative to gully wall planting will require careful consideration. However, willow and poplar plantings in gullies in crushed geologies deeper than five metres, particularly those in bentonite or crushed argillite, are likely to have severe establishment constraints. Some will succeed, but many will fail. For these features, reforestation with pines or natural reversion along with poles or stakes in the watercourses is the preferred treatment option.
Pole planting on slumps will require higher concentrations on the most active parts of the slump, and lower concentrations on less active parts. Experience has shown that many of the larger and deep-seated slumps likely to be encountered in Land Overlay 3A are beyond the scope of conventional pastoral-based erosion control measures and need afforestation with pines, often in association with gully planting.
Unlike past erosion control efforts, largely funded through local authorities, there will only be one chance to secure funding through the ECFP, so all concerned with future erosion control efforts must do their utmost to get it right first time.
As the goal of the ECFP and the Sustainable Hill Country (SHC) Regulation is to achieve long-term erosion control, a number of external risks should be taken into account when considering the most appropriate erosion control option. The probability of failed erosion control efforts, particularly for a severely eroding feature, could be high in the event of increased storminess, droughts and fire risk (climate change effects) and this is of concern. In addition, there are risks associated with the very narrow genetic pool of Pinus radiata and willows. Thus the inclusion of a reversion option and a provision for combinations of effective erosion control options to be used are essential if this goal is to be realised.
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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