It's all in the timing

Forest and Bird, Issue 280, 1 May 1996, Page 8

It's all in the timing

MANY FOREST and Bird members will have noticed how Vv

native fruits and flowers can be found throughout the year in New Zealand forests. But have you noticed how closely they dovetail? Astrid Dijkgraaf is a PhD student researching the timing of fruiting and flowering (phenology) of native tree species. Her work is partly funded by the J.S. Watson Trust administered by Forest and Bird. The key plants of her study are those with large fruit (bigger than a centimetre in diameter) or with large nectar producing flowers. Astrid has been able to show that the phenology of these species dovetails very closely but hardly ever overlaps. Although such a tight sequence of events initially seemed somewhat surprising, it makes perfect sense from an evolutionary viewpoint. A tree with a crop of ripe fruit that needs to be distributed by birds would benefit if it were the only species with fruit at a particular time. All the birds would have to forage for this species and thus the fruit would be dispersed most widely. Thus tree species within a forest would evolve to produce fruit at a time when not many other species have ripe fruit, leading to staggered fruiting throughout the year.

For the birds, it is much easier and saves on flight costs, to live in an area where food is available all year round. By staggering fruit production throughout the year the trees assist in maintaining a good local population of birds to disperse fruit. Thus both plants and animals benefit from this arrangement. The same concept applies in relation to birds and flower pollination. The introduction of exotic animals into the equation has severely disrupted this tight pattern. Possums, for example, tend to eat the flowers of some species, thus preventing or severely reducing fruit set, or eat the juvenile or immature fruits before they are ripe enough to be eaten by native animals. Some species, such as kohekohe, do not produce any flowers at all because there isn’t sufficient leaf area left after possum browsing to generate the energy required. This creates huge gaps in the food available to native birds, and will reduce their overall success and survival rates. Another intriguing aspect is the (sometimes large) geographic differences in the timing of fruiting and flowering. Astrid’s two original research sites, Wenderholm, north of Auckland, and Whitford, south of Auckland, are only 100 kilometres apart yet differ in fruiting or flowering by up to a month. Astrid is now gathering information on how the phenology of certain plants changes up and down the country and how this relates to the animals that depend on those plants.