Workshop Report: Poplar and willow planting for Land Overlay 3A, Gisborne, East Coast region
Background
Until relatively recently ECFP funded afforestation as the primary erosion control treatment. In closely planted forest stands, the roles of canopy closure and root occupancy are paramount to the success of this afforestation to control soil erosion.
The processes of evapotranspiration and root reinforcement, as mechanisms through which slope stability is substantially restored to currently eroding sites and maintained on already planted sites, have been well researched (Pearce et al. 1987, Watson et al. 1995, 1999, Zhang et al. 1993, Marden et al. 2005).
The success of these processes in stabilising many existing gully, earthflow and slump erosion features, and in ameliorating against the initiation of new erosion during storm events, particularly once canopy closure has been attained (between years 6–8), has been demonstrated (O’Loughlin 1984, Phillips et al. 1990, Marden et al. 1991, 1992, 2005). In addition, there is considerable scientific evidence in support of the concept that for large erosion features the greater the proportion of the watershed treated surrounding individual erosion features, the more likely the treatment will succeed (Marden et al. 2005). This is less of an issue where erosion features are small and watersheds are large.
As afforestation generally includes planting to natural ridge lines, there is a well-founded expectation that afforestation projects on the “worst of the worst” eroding land, in most instances, will achieve a successful outcome. However, it has to be acknowledged that, when dealing with erosion features typically found within Land Overlay 3A, afforestation also has limitations. Often, local site conditions (for example, geology, groundwater, fault crushing) and size of the feature dictate whether reforestation will prove to be successful or not (Marden et al. 2005).
Since the 1998 ECFP review, other treatment options for controlling erosion on farmland have become eligible for funding through the ECFP. These include reversion of indigenous scrub and the use of poplar and willow poles and/or wands. Research data in support of the effectiveness of indigenous reversion is scant (Marden & Rowan, 1993, Bergin et al. 1993, 1995, Rowe et al. 1999) but reversion has been shown to be effective at an early age on account of the very dense nature of closed-canopy stands and the presence of a dense fine-root network with a higher root tensile strength than Pinus radiata (Ekanayake et al. 1997, Watson et al. 1999).
Post-storm damage assessments of poplar and willow plantings show that, where implemented appropriately, poplar and willow plantings substantially reduced physical damage to hill country grazing lands, by between 50 and 80 percent, even during exceptional storms such as Cyclone Bola in 1988 (Hicks 1992). They also show that damage reduction was minimal on hill slopes where plantings were absent, inadequate or not maintained (Hicks 1992). However, we must take into account that these findings are of early plantings of clones that differ from those used today, many of which were decimated by poplar rust, and these plantings were undertaken prior to a time when forest woodlots were just beginning to be accepted and used as an alternative soil conservation practice on farms. This seemingly poor performance, at times, is also symptomatic of the fact that there has never been any assistance or mandatory requirement to maintain these early plantings, combined with what was probably some poor siting of planted material, some inappropriate stocking rates and a narrow range of early species available for planting.
Experience has shown that many of the larger erosion features likely to be encountered in the Land Overlay 3A are beyond the scope of conventional pastoral-based erosion control measures and need a forest cover, often in association with gully planting, for stability. Where smaller linear gullies, slumps and earthflows are identified as controllable by pole/wand treatment, the challenge is to reach agreement on planting and management strategies that will deliver a successful erosion control outcome for land classes identified in Gisborne District as Land Overlay 3A.
This reinforces the importance of getting it right with planting specifications, that is, ensuring that trees establish on all the unstable parts of the slope. Elements in this are good siting, appropriate stocking rates, planting the right sort of tree for the type and severity of erosion in question, and targeting soil conservation works not just at controlling existing erosion but at areas of potential erosion (Hicks 1995). It is clear that unless erosion control measures are implemented in a relatively short space of time, at a scale appropriate to the problem, and then maintained, money is simply being wasted (Trotter 1989).
In addition, there is the consideration that the ECFP and the “District Plan Rule Variation” target land classes identified at property scale as the “worst of the worst” eroding land in Gisborne District.
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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