Stabilising Characteristics of New Zealand Indigenous Riparian Colonising Plants

Abstract

This paper presents selected results on the above-and below-ground growth performance of twelve indigenous woody species commonly found growing naturally in unstable riparian slope and/or bank environments throughout New Zealand. This study was needed because little information exists on the effectiveness of New ZealandÕs indigenous riparian plant species for slope and stream bank stabilisation. By examining the growth performance of selected riparian species during the first 5 years following establishment, we provide valuable insights into the likely strengths and limitations of individual species at maturity and, therefore, into their overall potential usefulness, singularly and/or as mixed plantings, for future riparian stabilisation projects. For all species, their root systems are typically shallow and confined to the uppermost 31 cm of soil. Root spread (mean maximum diameter) increased with increasing age with interspecies differences, by age 5 years, ranging from between~1 and 2.5 m. At age 5 years the mean root biomass, for all species combined, was 1.2 kg/plant, and averaged~23% of total plant biomass. Changes in the allocation of biomass for root and shoot growth appear to be species and age dependent. The results of this study indicate that most have above-and below-ground growth attributes well suited to colonising steep and unstable riparian slopes where shallow soil failure is prevalent and/or where stream banks are rocky with skeletal soils. All form part of the early plant succession. Once established, and in the absence of grazing, they are relatively fast growing. The effectiveness of riparian restoration programmes using indigenous species, though potentially high for low-order stream, will be limited by their relatively shallowrooted habit for bank stabilisation on larger rivers without the prior installation of structural protection works.

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.…”

“…Marden et al 2005b). Temperatures over summer average 23°C and over winter average 12°C and mean annual rainfall is 1000 mm.…”

Establishing native plants in a weedy riparian environment

“…Although survival rates for P. totara were greater, the mean growth increments and absolute increases in volume for P. tenuifolium and P. regius across the trial were substantially greater, making them better candidates for planting at locations similar to the trial site. A previous riparian trial examining the growth of C. australis, P. tenuifolium and P. regius along with several other native species also found these species generally performed well in terms of root depth and spread, further supporting the use of these three species (Marden et al 2005). The growth rates P. totara were much less than those obtained in other studies of seedling development in this species (e.g.…”

“…Until recently, the majority of species used in riparian revegetation plantings were introduced plants, particularly willows and poplars (van Krayenoord & Hathaway 1986), but now interest is shifting to the use of New Zealand indigenous plants (Porteous 1983;Pollock 1986;Phillips 2005;Marden et al 2005). Regional governments, charged with the protection and maintenance of air, soil and water values, are actively promoting the planting of native species alongside waterways (e.g.…”

Successful translocation of the threatened New Zealand root-holoparasite Dactylanthus taylorii (Mystropetalaceae)

“…Close proximity to a specific host stem has been shown to determine availability of roots from that tree for another root holoparasite Thonningia sanguina Vahl (Balanophoraceae s.str.) (Olanya and Eilu 2009), however, at least some infections in our plots may have been on roots of a different host species nearby rather than the targeted one (Marden et al 2005). Assuming that such infections are in the minority and no other site-specific factors have influenced the outcome unduly, results indicate that the wide host range of dactylanthus (Ecroyd 1996;Moore 1940) does not translate to an equal preference for different hosts.…”