Riparian plants - stability

Final trial results - series of downloadable posters. Added 19 May 2006.

Some riders on the use of results from this research. Added 17 February 2005.

Introduction

In the settlement of New Zealand, much native riparian vegetation was cleared for agriculture and urban development. As a consequence the ecosystem services provided by riparian vegetation have been lost or seriously compromised. 

Services such as stabilising stream banks, filtering runoff, shading and protection of fish habitats, enhancing aesthetics and controlling downstream flooding, are vital for healthy streams and coastal waterways.

Of these services, bank stabilisation is often regarded as being of high importance because, without it, many of the other services may be limited by erosion and its consequences

In New Zealand, and for that matter in other parts of the world, there is currently a strong lobby to improve indigenous biodiversity.  This is reflected in a wide range of initiatives, policies, and plans at every level of governance but perhaps is more strongly reflected in a ground-swell of community action to revegetate "their place".

However, in New Zealand at least, all these activities are set against a backdrop of poor information about the stabilising characteristics of plants used in these restoration efforts.

In general, information on the below-ground characteristics of plants is pretty lean worldwide. This is largely due to the costly and time-consuming effort required to examine the root systems of plants, particularly those of trees.

Current Research

We have examined 12 native plant colonisers for their ability to stabilise stream banks. These plants are part of the early succession as well as being some of the key species used in stream -side restoration projects. 

(Click on the pdf icon to download the posters)

Common Name	Botanical Name	Results - posters
Karamu	Coprosma robusta	151 KB
Ribbonwood	Plagianthus regius	117 KB
Kowhai	Sophora tetraptera	135 KB
Lemonwood	Pittosporum eugenoides	134 KB
Kohuhu	Pittosporum tenuifolium	136 KB
Lacebark	Hoheria populnea	147 KB
Mapou	Myrsine australis	157 KB
Fivefinger	Pseudopanax arboreus	136 KB
Cabbage tree	Cordyline australis	146 KB
Rewarewa	Knightia excelsa	141 KB
Manuka	Leptospermum scoparium	Incomplete results
Tutu	Coriaria arborea	184 KB

We have had a field trial (located in Gisborne) and collected various bits of information about these plants.

To obtain growth data for older-aged plants we established a trial plot (60 m × 15 m) on flat alluvial land near Gisborne, East Coast, North Island. The topsoil is a black, very friable sandy loam with a weakly developed structure of fine granules, with a layer of loose sand beneath. The soil is free draining to a depth of 2 m and requires irrigation in summer.

Before planting, the plot was sprayed, ploughed and covered with weed mat. Twelve trees of each species have been planted randomly in blocks, 10 of which will be extracted each year for the next 3 years according to age. The 2- and 3-year-old blocks are planted at 1m spacing and the 4- and 5-year-old blocks at 1.5-2.0 m spacing. The trees were irrigated for the first three months after planting.

We selected 10 specimens of each species at each age and look at a range of variables which includes: canopy spread, root depth, lateral root spread, above and below ground biomass, tree height and so on.

4-year-old data are on this poster Performance of native riparian plants - how different are they?.

(23/11/04) Selection of final results at 5-year-old in this updated poster from 2003 Performance of native riparian plants - how different are they?

Data below are for 5-year-old plants. See the table above to download the summary results in poster form for each of the species.

Results

Root depth

The mean maximum root depths sorted on Year 5 data, show there is considerable overlap both between species. Mean root depth varied from 20 to 45 cm. Cabbage tree on the extreme right of the graph exhibited the greatest root depth at around 45 cm, after 5 years. Nursery conditioning is a factor in the relative consistency of results both within and between species in the first few years of growth.

Root spread

Lateral root spread data, like root depth, shows variation between species. Root spread in Year 5 ranged from 100–300 cm. Tutu, lemonwood, and ribbonwood had twice the root spread of some other species.

Above and below ground biomass

The general pattern is much the same as for root depth and root spread with a lot of overlap but one or two top performers which are significantly different from the others at the 95% confidence level. Cabbage tree and tutu were the top performers at year 5.

Ribbonwood	Fivefinger

Summary

To summarise these preliminary results, we find that for an individual plant species, its "performance" relative to the other species changes according to the parameter in question.

While most of the trial species exhibited similar values for each parameter for the first 2 years of growth, ribbonwood outperformed all the species for two parameters – above- and below-ground biomass - and was second to tutu in lateral root spread.  The relative position of these species changed as the plants grew and at age 5 cabbage tree was a top performer for a number of growth attributes.

Root studies of plants less than 2 years old, are commonly affected by factors in their early life in the wild or in the nursery such as wrenching, root training and constriction in planter bags.  It is likely that these factors may have contributed to some of the the early differences seen in the data, rather than the results being truly representative of an inter-species difference.

The trial was completed in 2004 when the plants were 5 years old.

A new trial was established in May 2006 with the aim of examining some species of podocarps and some non-woody species used in riparian restoration. The same site and methodology will be used.

Relevant publications

Marden, M.; Rowan, D. and Phillips, C.J.  2005:  Stabilising characteristics of New Zealand indigenous riparian colonising plants.  Plant and Soil 278: 95-105.

Czernin, A. and Phillips, C.J. 2006. Below-ground morphology of Cordyline australis (New Zealand cabbage tree) and its suitability for riverbank stabilisation. NZ Journal of Botany 43: 851-864.

Landcare Research 2003. Researchers measure growth of native youngsters.  Discovery, Issue 6, 2003, p5.       http://www.landcareresearch.co.nz/publications/newsletters/discovery/DiscoveryIssue6.pdf 

Ekanayake, J.C.; Phillips, C.J. 2002. Slope stability thresholds for vegetated hillslopes: a composite model. Canadian Geotechnical Journal 39: 849-862.

Marden, M.; Phillips,C.J.; 2002. How do native riparian plants grow? Conservation Quorum, Issue 26, 14-15.

Phillips, C.J. 2001. Soil stabilising characteristics of native riparian vegetation in New Zealand. Paper presented at NZARM Annual Conference, Hamilton, 19-21 September 2001.

Phillips, C.J.; Marden, M.; Rowan,D.; Ekanayake, J.C. 2001. Stabilising characteristics of native riparian vegetation in New Zealand. In: Proceedings of 3rd Australian Stream Management Conference, Brisbane August 2001, 507-512.

Phillips, C.J.; Ekanayake, J.C.; Marden, M., Watson, A. 2000. Stabilising-Parameters of Vegetation: A Critical Look Down-Under. In: Proceedings of Landscape 2000, Leura, NSW, Australia, 16-20 October 2000. (Conference proceedings on CD).

Marden, M.; Phillips, C.J. 2000. Using native plants to provide stability to streambanks. Conservation Quorum Issue 21, Winter 2000, p6.

Ekanayake, J. C.; Phillips, C. J. 1999. A model for determining thresholds for initiation of shallow landslides under unsaturated conditions in the E. Coast region, New Zealand. Journal of Hydrology(NZ), 38(1): 1-28.

Ekanayake, J.C.; Phillips, C.J. 1999. A method for stability analysis of vegetated hillslopes: an energy approach. Canadian Geotechnical Journal 36: 1172-1184.

Watson, A.J.; Marden, M.; Phillips, C.J. 1999. Root strength, growth and rates of decay: root reinforcement changes of two tree species and their contribution to slope stability. Plant and Soil 217: 39-47.

Ekanayake, J.C.; Phillips, C.J.; Marden, M. 1999. A comparison of methods for stability analysis of vegetated slopes. In: First Asia-Pacific Conference and Trade Exhibition on ground and water bioengineering for erosion control and slope stabilisation. April 19-21, 1999, Manilla, Philippines.

Phillips, C.J.; Ekanayake, J.C.; Marden, M. 1999: Can planting pattern make a difference to slope stability? Conservation Quorum. 17: 14.

Rider on the use of these results

This trial was carried out in Gisborne in the North Island.  At the time the trial commenced, advice was sought from plant ecologists in the North Island on appropriate species to study and the work was not part of the Motueka ICM research programme.  

Shannel Courtney from DOC has kindly highlighted that not all the trialled plants we studied may be suitable for use in the Motueka area - his comments are below:

"I noted that there were a few species studied that, while suitable for Gisborne, were not in fact native to the Motueka catchment, namely: Hoheria populnea (native to NI only) Sophora tetrptera (NI only) and Knightia excelsa (NI and Sounds). Tasman District readers, who are looking for appropriate native plants to restore riparian zones in the Motueka catchment, need to be aware that these three species are not appropriate for their area. 

In fact, the hoheria acts very weedily where it is cultivated and actually excludes native plants. Thorps Bush in Motueka township is a great example of this. There is also the problem of hybridisation of NI kowhai with the native S. microphylla. I am in the process of producing various native species lists for habitat restoration throughout the Tasman and Golden Bays partly funded by TDC. There are currently 24 lists for the various ecosystems including the "Motueka-Riwaka Plains and Valleys Ecosystem" which is relevant for riparian restoration of this area. I make a point of promoting ecosourcing and only using species which I know are native to the ecosystem so that there is ecological integrity to any restoration project. Copies of this list are free from TDC."

Given these comments it is probably advisable, no matter where you are in New Zealand, that you check with your local DOC office, Regional Council or city council or seek assistance from agencies such as the ecological restoration network http://www.bush.org.nz/ .

Nevertheless, it is important to realise that data on the below-ground characteristics of our native plants is rare and very hard to come by!

Page last updated 24 July 2006