14 October 2013

Can natives species make a comeback in Christchurch's urban woodlands?

This blog post was written by postgraduate student Denise Ford as part of the course, Research Methods in Ecology (Ecol608). Denise revisits a Lincoln University research area that measures the re-emergence of indigenous forest  in Christchurch published in 2004.

Can Christchurch move away from the image of an "English garden city" to a city which reflects its indigenous natural heritage?  Research on the regeneration of native species within urban Christchurch has shown definite signs that indigenous forest can reappear in the city. This move then, may well depend on changes to the cultural and aesthetic values held by many Christchurch citizens.

Christchurch Botanical Gardens and the Avon River (by Steel Wool)
The city of Christchurch was a planned English settlement and, in the 150 plus years since the arrival of the first settlers, it has developed into a typical western urban centre. The need for familiarity, nostalgia and shelter on an exposed Canterbury site meant that much of the urban habitat became reflections of English country gardens and parks. The indigenous flora was replaced by exotic species. Trees from Europe, North America, Australia, Asia and Africa now dominate the woodlands of the city. Some pre-colonial plants survived in small pockets in and around the city but in a much reduced form.
Riccarton Bush (by Steel Wool))
In 1993 the Christchurch City Council recognized the high ecological value of many of these remnant patches and designated them as ecological heritage sites. The Kahikatea (Dacrycarpus dacrydioides) forest at Riccarton Bush is one site identified. The bush is a small remnant of podocarp forest which has survived Polynesian settlement and 150 years of urbanisation. In the last decade over a million indigenous trees, shrubs, tussocks and wetland plants have been planted in the city, not without controversy from some sectors of the community. The Christchurch City Council is actively involved in the protection and restoration of remnant areas, Travis Wetland being one example. The ongoing restoration of such sites and the inclusion of native plants in public and private land along with earlier plantings of native podocarps form a seed source from which dispersal and regeneration can occur.

Glenn Stewart and his colleagues,  Maria Ignatieva, Colin Meurk and Richard Earl, examined the exotic and indigenous shrub and tree components of urban Christchurch and how it affected the food and habitat of native birds. Their investigations of records kept by the Christchurch City Council showed that over 80% of the 50,000 plus trees planted in the cities parks and streets by local government were exotic. Most of these exotic trees are dry fruit bearing and have no or little value as a high energy food for native wildlife. In contrast to public areas, suburban gardens contained a greater proportion of native woody plants.  The majority of native plants in the city are those that produce fleshy fruits and nectar which are important food sources for native birds, lizards and invertebrates.
Regeneration of native seedlings
 (Coprosma robusta)
 ( by Elisa Ruiz used with permission)
Glen and his colleagues were also interested in the regeneration rates of exotic and indigenous vegetation within the city. They sampled parks and suburban gardens, finding a significantly higher number of indigenous seedlings than exotic woody species. This difference may be explained by seed dispersal in which wind and birds play an important role.

Many wind dispersed exotics in the city have poor dispersal ability, whereas native species, such as lowland ribbonwood (Plagianthus regius) and akeake (Dodonaea viscosa), have light seeds that can travel long distances. Seedlings are quite often found under trees where birds have perched and passed the fruit they have eaten. Only 26% of exotic species have edible fruit compared with 78% of indigenous species.

With an available seed source and the regeneration ability of indigenous species there are positive indications that the re-emergence of an indigenous-dominant woodland forest is possible. Of course sociological factors will also come into play; many citizens are comfortable with the English nature of our city and are reluctant to see a dominance of indigenous species.

This study is very pertinent to Christchurch in 2013 given the abandonment of land in the residential red zone following a series of major earthquakes in 2010 and 2011.
Map of the Residential Red Zone. The land in red has been deemed unsuitable for rebuilding.
Sourced: http://www.fix-it.co.nz/index.asp?s1=news&s3=Christchurch%20articles

In September 2012 a survey of indigenous and exotic seedling regeneration was conducted in nine suburbs (a total of 90 properties) in the red zone. The results of this survey indicated that seedling regeneration was following the preliminary seedling study done by Glenn and his colleagues in 2004. The number of indigenous seedlings found was considerably higher that the number of exotic seedlings. The two most numerous seedlings were indigenous; Kohukohu Pittosporum tenuifolium, and Cabbage trees (Cordyline australis) With the abandonment of properties, gardening practices such as weeding and mowing no longer occur. This 'neglect' has allowed, in some cases, the proliferation of indigenous species.

Is this an opportunity, with control of aggressive exotics, to see the transformation of a Christchurch urban woodland "to a new kind of indigenous-dominant forest biotype" as envisaged by authors of the 2004 paper? The red zone opportunity will not only benefit the indigenous plant biodiversity of the city but will allow our native wildlife to make a comeback. I am looking forward to a city that represents the  natural fauna and flora of the area and the cultural sense of being that it will bring.

Check out Glenn's blog Natural disasters and the nature of cities regarding the Canterbury earthquakes and subsequent vegetation dynamics in the residential "red zone".

This blog is based on the journal article:
Stewart, G. H., Ignatieva, M. E., Meurk, C. D., & Earl, R. D. (2004). The re-emergence of indigenous forest in an urban environment Christchurch, New Zealand. Urban Forestry & Urban Greening, 2, 149-158.

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