The New Zealand Plant Conservation Network

I would like to know why karaka is taking over the forests in the Wellington region when it appears to be a small component of the canopy in northland forests. I understand that karaka is not native to the Wellington region but am perplexed as to why it is becoming more and more dominant. Would appreciate theories.

Hi Robyn, my observations are that it is invasive in Northland, but limited by insufficient dispersal of seed. Only kereru (kuku or kukupa in Te Tai Tokerau) are capable of eating the fruit, but observations of it feeding on karaka are rare. I hypothesise in my PhD thesis that karaka is not indigenous to NZ because of this lack of dispersal and it morphological and genetic similarity to a New Caledonian species C. disimilis. I did discuss this hypothesis briefly with Brian Molloy (who has looked at how karaka was transported in NZ) who strongly disagrees. I do plan on publishing my work which shows the difference in recruitment between other large fruited species and karaka on Northland offshore island groups with and without kukupa. The species is undoubtedly spread throughout NZ by people, and while this is mainly reasoned for the southern distribution, the same reasoning can be used for Northland populations.

Pt II. Why it is become more prevalent in Wellington than elsewhere is a bit of a mystery - it is likely to be either that there is little alternative fruit in the Wellington area so kereru feed on karaka more than in Northland, or there is an exotic disperser that has learned to feed on karaka (pigs or possums are the most likely), or that the highly disturbed forest may favour karaka regeneration as the seedlings require high light levels to mature.

Hopefully others have some thoughts they'd like to contribute to the debate?

HI Mike - I pretty much told Robyn the same re -not sure it is indigenous to NZ and spread by Maori - and selected for bigger fruits. The only time I saw kereru eating karaka fruit was when a drought resulted in fruits smaller than usual. Nevertheless quite a few karaka did arrive in my seedfall traps. The high light theory is certainly plausible

Thanks Astrid. I didn't measure seedfall (too difficult on offshore islands with short visit and rare trees) but measured seedlings & saplings in relation to adults. Karaka had a VERY different recruitment pattern to the other large fruited species on Lady Alice where there is a good population of kukupa. The pattern was almost exactly the same as on Three Kings with no kukupa. So offshore islands nothing moving karaka fruit, but your seed trapping did get quite a few. Maybe another disperser was involved? Possums? I know they strip the flesh from taraire but don't think they consume the seed. Mynah? Larger fruit than you would expect them to eat given gape size, but not impossible. Something screwy going on!

Plus mynah also present on Three Kings and Lady Alice

Hi Mike - possums completely destroy taraire fruit - it got so that I could time when possums would starting eating taraire on the size of the fruit - from about half-sized fruit on. Possum chewed karaka has a ragged look - stuff that has gone through a bird (presumably kereru) looks smooth. What I didn't do with the seed fall stuff was measure the size of all the kernels - counting them all was a big enough task. It could be that the clean karaka seeds were smaller - but can't confirm that any more

Thanks Mike and Astrid for your ideas. Mike I am looking forward to reading your PHD thesis. I do wonder if karaka has just moved through its lag phase. There could be a number of contributing factors such as relatively higher light levels in our "young bush" and the increase in kereru numbers in the region due to years of predator control, even if kereru can only eat small fruit. Should we be adding karaka to our list of native pest plants in Wellington along with karo and pohutukawa?

Hi Robyn, PhD not available yet online (too large). I have published a couple of papers from it (email me if you would like copies or they're on ResearchGate) and there are more to come shortly.

In regards to karaka in Wellington - I think there is a good case for controlling the species at high-value sites where they are not part of an Historic karaka grove. Should be easy enough to control by hand as they can be pulled out easily.

@ Astrid. Luckily the defleshing seems to occur when the fruit is viable. I have seen instances overseas where hungry frugivores strip the fruit from unripe fruit resulting in decreased regeneration in those species. As most fruit appears to be dropped following the flesh being stripped I doubt there is any effective dispersal in those instances