Tree Talk – looking at colour and pollination

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David English explores an intriguing feature of our native plants: Have you ever noticed that only a few native plants have colourful flowers? The bright yellow colour of the kowhai herald spring and the pohutukawa’s crimson flowers are the harbinger of Christmas.

A tui foraging for nectar from colourful kowhai flowers

The crimson pohutukawa attracts the tui too but also shows that Christmas is coming!

But the majority most of our native plant flowers are rather inconspicuous. Most have flowers which are small, clustered and typically green or white.  Why is this?

The answer is not clear but it is generally accepted that the most likely reason is that in the early evolution of our native flora there was an absence of long tongued bees, or other suitable insect pollinators, which are attracted by bright flower colours. Indeed it is interesting to note that our brightly coloured flowers such as kowhai and pohutukawa are pollinated by birds and not insects. Another interesting fact is that a number of our native plants with inconspicuous flowers have very strong scents and are pollinated by moths, which are predominantly active at night and attracted to flowers by their perfume.

But why do we have a greater occurrence of inconspicuous flowers than in Australia and other countries in Europe? Part of the answer is that New Zealand has been isolated from other land masses for a very long time and has developed what JW Dawson has called the ‘isolated island syndrome’ (See Forest Vines to Snow Tussocks: The story of New Zealand Plants).

Coprosma grandifolia male flowers

Coprosma grandifolia female flowers

 Photos: Bushman’s Friend Tony Foster

Coprosma grandifolia is a New Zealand example of both wind pollination and dioecism. The male flowers, as is generally the case with wind-pollinated flowers, are small and inconspicuous, but have disproportionately large dangling stamens with large anthers. The hanging stamens move readily with the wind, which shakes out the large quantities of pollen necessary for this rather wasteful method of pollination. The female flowers have long sticky stigma which catch the wind borne pollen.

The family to which Coprosma belongs (the Rubiaceae) is mostly insect-pollinated with often showy flowers. The NZ species of Coprosma are unusual in the family in being both dioecious and wind-pollinated.

The geological history of our land and its isolation has led to the absence of long-tongued bees. But what has this geographic isolation meant for plants? By being isolated, our native plants exhibit a wide variety of growth habits (e.g. Coprosma has some species which are small trees with large leaves, others are densely-branched small-leaved shrubs, and a few are mat-forming) and a high rate of hybridism (e.g. Red, Hard and Mountain Beech). They have also become predominantly wind-pollinated with a high proportion of dioecism, where plants have male and female flowers on separate plants (as opposed to monoecism where plants have male and female flowers on the same plant). Dioecious plants make up 12% of our native flora compared to less than 2% in Europe.

Colourful berries of C. grandifolia are eaten voraciously by birds Photo: Bushman’s Friend Tony Foster

There is an apparent conundrum here in that New Zealand has a high proportion of inconspicuous wind-pollinated flowers as well as dioecism. Surely, plants colonising an isolated land mass would benefit by having flowers of both sexes on the same plant (monoecious) or, even better, being hermaphrodite thus (flowers with viable female and male parts)? The close proximity of flowers of both sexes surely would making reproduction easier. That this isn’t so is explained by genetic material being exchanged (outcrossing) between different plants by wind pollination, thus providing for greater variability and so better suiting plants to successfully evolve and invade new habitats.

And how did these plants invade new habitats? Many of New Zealand’s wind-pollinated plants have their seeds widely distributed because they have brightly coloured fruits, containing seeds, which are eaten by birds and then dispersed in their droppings.
The link between our native birds and the formation of the New Zealand flora, and the preservation of our plant heritage is clear for all to see. Not only are birds integrally involved in pollination and seed distribution of many native plants, but birds also depend on native plants for their food. Our history has shown that breaking this link by destroying forests and hunting birds has caused the extinction of many birds and plants while placing many more species in serious jeopardy in the future.

If you are interested in reading more, I suggest:
J W Dawson (1998): Forest Vines to Snow Tussocks: The Story of New Zealand Plants (VUP) Available online; http://nzetc.victoria.ac.nz/tm/scholarly/tei-DawFore-t1-body-d1.html

Tony Foster (2008): Plant Heritage New Zealand (Penguin Raupo) A brilliantly useful book available by downloading from http://www.bushmansfriend.co.nz/plant-heritage-new-zealand-tony-foster-xidp222126.html

The Bushman’s Friend website (http://www.bushmansfriend.co.nz/) is a wealth of information about NZ plants.

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