ARTICLE : 2007 EVENING MEETING REPORTS

 Monday 16 April 2007 :   Karaka can be a weed.

Many of us have observed that karaka (Corynocarpus laevigatus) can be weed in the Wellington area.   It is a weed in that it naturalises where it has no history of occupation, to the detriment of the pre-existing vegetation.

In her illustrated talk, Jessica Costall demonstrated and quantified the extent of invasiveness of karaka out of its native geographic area.

Jessica, supported by her supervisor Gill Rapson, summarised the results of a study she did with three fellow Massey post-graduate students.   They compared 14 pairs of plots, with and without karaka, in the North Island south of New Plymouth.   Their field work also measured dispersal and recruitment patterns around parent trees.

The areas studied were mainly well-established groves of karaka planted by Maori as a source of food.   Whilst the extent of “weediness” varied, the study clearly demonstrated that :
•   Karaka seeds mostly fall near the parent but establishment improves outside the shade of the parent tree
•   Karaka seed can be widely dispersed by kereru / NZ pigeon
•   Karaka plots have fewer non-karaka native trees and woody seedlings
•   Karaka plots have more exotic species than the non-karaka plots
•   There is a management problem with karaka outside their native area in that they suppress other species.   This is likely to lead to a decrease in species diversity.

The wide discussion following the talk emphasised that birds do spread karaka over a wider area.   It was mentioned that, in the southern North Island, exotic bird species are as likely to be carriers as the kereru.

Shading seemed to be the main deterrent to the growth of other species.   One discussant observed that regeneration appeared to be discouraged in soil even after karaka had been removed.   The possibility was mooted that a chemical, associated fungi or bacteria, in the roots of karaka might deter other species.

A full report of the study can be found in the New Zealand Journal of Botany, 2006, Vol. 44: 5-22.

Rodney Lewington