Sunday, 9 February 2014

Fruiting karaka

Karaka (Corynocarpus laevigatus) is a small New Zealand tree with large fleshy fruits.  It's interesting for a number of reasons.  Its fruits were an important food resource for Māori but the kernels had to be treated to remove the toxins they contain.  It's become a weed in some parts of the world. Some botanists consider it a weed within New Zealand too, when it becomes invasive outside its presumed native range or habitats.

Right now, karaka trees are fruiting heavily.  But not all of them.  Some trees are covered in fruit and others have none or very few.  Some years ago, I wondered if this meant they had separate sexes, and was able to show that this is the explanation (Garnock-Jones et al. 2007).  Male trees do produce a few fruits, so the sexual system in karaka is best described as gynodioecy (some plants strictly female; others are inconstant males).

Here are the two trees that started this research off, photographed this month in Kelburn.
Karaka trees in fruit, Kelburn, Wellington, 2014

Here are the same two trees about 10 years ago.
Karaka trees in fruit, Kelburn, Wellington, 1998 (from Garnock-Jones et al., 2007)

On the female tree, the panicles fruit heavily, with many of the flowers (but by no means all) developing fruits.
Fruits on a female karaka tree

On males, usually a single fruit develops on each of a few panicles.
Fruits on a male karaka tree
Karaka flowers are small and white, but if you look closely you can tell the male from the female flowers.  The male flowers are actually about twice the diameter of females, open more widely, and have pollen in their anthers.  The male flowers in the photo have pollen on the stigmas, but only very few of them will produce fruits.
Karaka flowers.  On a female tree (left); male tree (right)

Reference.

Garnock-Jones PJ, Brockie RE, FitzJohn RG 2007.  Gynodioecy, sexual dimorphism and erratic fruiting in Corynocarpus laevigatus (Corynocarpaceae).  Australian Journal of Botany 55: 803–808.


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