There is a struggle going on in the New Zealand forest, and it’s a battle for ultimate (plant) domination. Kānuka (Kunzea ericoides) and mānuka (Leptospermum scoparium) were the original plant species that colonised forest sites cleared by natural disturbances in New Zealand. This has changed since the introduction of many “shrubby weed species” to a situation where many cleared sites are now colonised by gorse (Ulex europaeus), broom (Cytisus scoparius), tree lupin (Lupinus arboreus) and many other adventive plant species. The nearest kānuka and mānuka are often some distance away and don’t get a chance to establish.
Jon Sullivan (Lincoln University), Peter Williams (Landcare Research) and Susan Timmins (Department of Conservation), researched three different hypotheses that they obtained from the New Zealand literature on the relationship between the naturalised shrub gorse and the native shrub kānuka.
1. Kānuka stands have a different plant species composition and greater plant species richness than gorse stands at comparable successional stages.
2. Differences between gorse and kānuka stands do not lessen over time.
3. Several native plant taxa are absent from or less common in gorse than in kānuka stands.
The research was conducted in environmentally similar sites throughout the Nelson and Wellington regions. They selected a mix of young and old gorse and kānuka sites. At these sites they recorded the presence of all native and naturalised woody species, Department of Conservation weeds, ferns, orchids and a selection of herbaceous plants; they also recorded environmental variables. The results were published in a 2007 issue of the New Zealand Journal of Ecology.
They showed that there are many differences in the final vegetation composition of a site that establishes under gorse dominated vegetation compared to a site that establishes under kānuka dominated vegetation. For example, gorse sites tend to be absent of, or have fewer beech trees (Nothofagus spp.), orchids (e.g. Pterostylis spp.), small leaved shrubs (e.g. Coprosma, Leptecophylla, Leucopogon) and the shrub daisy Olearia rani than kānuka sites. The gorse sites also displayed lower species richness (number of species) and a higher incidence of naturalised species than the kānuka sites. These factors tended to be persistent at the older sites, suggesting that these effects are long term and therefore inferring that gorse is not a substitute for kānuka when the desire is to return the site to a “natural state.”
There are many explanations for the differences between gorse and kānuka sites. Some of the differences may be explained by biological factors such as the nitrogen fixing ability of gorse and the effect this may have on below ground micro-organisms and soil invertebrates, or this could be due to different physical site characteristics like light penetration, soil temperature or soil moisture levels. Other factors may be different bird feeding preferences and subsequent seed dispersal between gorse and kānuka sites, or the documented increase in naturalised plants invading gorse sites
This study has shown that there is a difference in plant composition between gorse and kānuka sites and therefore, as good as any forest regeneration is, there needs to be more protection for areas of kānuka in landscapes where it is scarce. Also, in such landscapes, there will be benefits for biodiversity of planting kānuka back into areas dominated by gorse if the aim is to initiate native forest regeneration. There is also a need for further research into the main effects driving the differences found in this research paper. There is still much to learn about the ecology of gorse vs. kānuka.
Glossary
Adventive a plant or animal found in an environment where that it is not native to.
DoC weeds List of weed species actively managed on Department of Conservation reserves.
Succession the series of changes that create a fully-fledged plant community, e.g. from the colonization of bare rock to the establishment of a forest
Taxa A taxonomic group i.e. a plant species, genus or family.