Potential effectiveness of low-density plantings of manuka (Leptospermum scoparium) as an erosion mitigation strategy in steeplands, northern Hawke’s Bay, New Zealand

Background: Government-funded schemes introduced to encourage new afforestation (exotic and indigenous) include the establishment of low-density plantings (less than 1100 stems ha–1) of manuka (Leptospermum scoparium Forster et Forster f.) on steeplands with severe erosion susceptibility and considered marginal for pastoral production. There is, however, little quantitative data to establish when (i.e., how many years after planting) these plantings likely afford effective mitigation against the initiation of shallow landslides.Methods: Permanent sample plots [PSPs] were established within manuka plantings established in northern Hawke’s Bay, New Zealand, where above-ground growth metrics were recorded for plantings on three landform units including interfluve ridges, colluvial slopes, and slopes affected by landslides. The root systems of a small sub-set of manuka were hand-excavated and whole plants destructively sampled for analysis of differences in allometric relationships by year, between landforms, and for comparison of the above-ground components of trees within PSPs.Results: Six years after planting, interfluves and colluvial slopes were understocked by between 10 and 25%, and by up to 45% on sites previously affected by landslides. Root collar diameter (RCD) explained between 92 and 99% of the variation in manuka biomass. The overall above- and below-ground metrics for manuka excavated from interfluves were significantly greater (P<0.05) than those excavated from landslides. Irrespective of landform type, the greatest proportion of the mean total root length and biomass was found in the top 0–0.5 m of the soil profile, within a 0.5-m radius of the stump, and importantly, no roots were found below 1 m depth. The timing (years after planting) to attain canopy closure and root occupancy, if stands of m?nuka were to remain fully stocked, varies between landforms and would likely occur between 6.5 and 9 years after planting. However, variable rates in planting density, and of plant mortality, resulting in under-stocking would significantly delay this timing, particularly on landslide-affected slopes.Conclusions: Manuka planted at densities below 1100 stems ha–1 are unlikely to provide effective erosion mitigation on steep land until significant root mass develops below the depth of the shear plane at which most landslides occur. Increasing the planting density, reducing early seedling mortality by better management of weed competition, and/or their replacement (blanking) would probably improve the erosion mitigation effectiveness of future low-density manuka plantings.