Below‐ground morphology ofCordyline australis(New Zealand cabbage tree) and its suitability for river bank stabilisation

Abstract

Observations and measurements on the below-ground characteristics of the New Zealand cabbage tree, Cordyline australis, from river bank environments near Christchurch, New Zealand, revealed a unique structure of peg-like rhizomes and fine spaghetti-like roots. By age 25 years, root depths reached 1.75-2.00 m, root spread reached 3.00 m, and below-ground root biomass, including the rhizome, exceeded 50 kg or 38% of the total tree biomass. Fine C. australis roots of diameters 0.6-3.8 mm had mean tensile strengths in the range of 26.7-17.5 MPa, 30% less than those of most willow (Salix) species. The pullout resistance of five c. 8-year-old self-seeded Cordyline australis trees ranged between 5.57 and 14.2 kN. In terms of the parameters assessed against published information for willows, it appears that C. australis falls short on both growth rate and tree anchoring parameters (tensile strength and resistance to pullout) for use as a river bank protection plant in all rivers. However, when grown with other native riparian colonising plants such as flax (Phormium spp.), river bank protection may be comparable, especially in low-order streams with silty soils and lower hydrodynamic forces.

Establishing native plants in a weedy riparian environment

“…This does not agree with previous research indicating C. australis was not optimal for riparian planting (Czernin & Phillips 2005), although weed control was not a component of the earlier study. It could therefore be considered the best candidate for planting at this site with weed control, although the fine-edged crown margins of this species, together with shorter crowns that do not expand as rapidly with maturity as other species (such as P. tenuifolium and C. robusta), decrease its ability to shade out competing vegetation over the longer term.…”

Observations of “coarse” root development in young trees of nine exotic species from a New Zealand plot trial

“…Measurement methods of both above-and belowground metrics followed well-established procedures used in earlier studies (Watson et al 1999;Czernin & Phillips 2005;Marden et al 2005;Phillips et al 2014). Various metrics or traits have also been used to help assess species performance in terms of erosion-control effectiveness (Stokes et al 2009;Phillips et al 2011).…”

“…The complete root system of each plant was photographed before the plant was destructively partitioned into 0.5 m radial and depth segments (see Czernin & Phillips 2005). Roots within each radial segment were further partitioned into diameter size-classes (<1 mm (fibrous), 1-2, >2-5, >5-10, >10-20, >20-50, >50-100 mm) (Watson & O'Loughlin 1990).…”

Observations of root growth of young poplar and willow planting types

“…Root system extraction and measurement methods followed well-established procedures (e.g. Watson et al 1999;Czernin and Phillips 2005;Marden et al 2005b). Root systems were extracted using an air spade (a high pressure device to remove soil from around the roots) and by hand.…”

“…Root spread was taken as the average of the maximum lateral root spread diameter measured in two directions (N-S and E-W). The root system of each plant was photographed before being partitioned into 0.5 m radial and depth segments (Czernin and Phillips 2005;Marden et al 2005b). Belowground components were partitioned into root bole (stump), tap, lateral and sinker roots.…”