Mast flowering a hot topic

HAVEN'T we had a fine display of pohutukawa flowers this summer? Northern rata has had similar heavy flowering. In recent weeks you might have compared the flowering of planted trees of pohutukawa and rata together at the foot of the steps up Durie Hill by the City Bridge; rata on the left, pohutukawa on the right.

Flowering of pohutukawa and rata and many other plants seems to have been later in summer this year than is "usual". This raises some questions. Firstly, how can we measure flowering time? In other words, can we quantify what constitutes "normal'" flowering time, compared with early or late flowering? Secondly, how can we measure the degree of flowering, eg, "heavy" flowering versus "light" flowering? What causes heavy or light flowering in different years?

As a further example, did you notice that harakeke (swamp flax, Phormium tenax) hardly flowered at all this summer, whereas wharariki (mountain flax, Phormium cookianum) flowered "normally"? That's ignoring some hybrid garden flaxes that seem to be ambivalent about their flowering time. A similar confusion exists for pohutukawa, in which the widely grown Kermadec pohutukawa can have some flowers almost any day of the year; hybrids between that and mainland pohutukawa flower irregularly.

The term "masting" has been used a lot in the media recently, especially in relation to the flowering and seeding of New Zealand beech trees. Masting is the intermittent production of large flower or seed crops by a population of perennial plants. In mast years, abundant beech seed leads to bumper populations of mice, which lead to high numbers of mouse (and native bird) predators, rats and stoats.

Among the native plants that mast are beeches, pohutukawa, rata, harakeke and snow tussocks. In these species, the whole population has synchronised flowering and, furthermore, this often happens simultaneously across the whole of New Zealand. Over the recent holiday period we saw no flowers on harakeke in the southern North Island, nor in Marlborough, Canterbury or Westland. This is not a mast year for harakeke.

Studies have shown that masting is caused by higher than usual temperatures the summer before seed fall. For example, flower buds of pohutukawa and snow tussocks are set before winter. Plants cannot anticipate the future, for example, seeing abundant cabbage tree (ti kouka) flowers in spring does not tell us it will be a hot summer - the flower buds were initiated the previous autumn. It is thought the timing and intensity of masting may be sensitive to global climate change. More frequent hot seasons may trigger more frequent masting, with widespread effects on plant and animal communities.

Answers to my questions at the start, about the amount and timing of flowering, require a large database covering many years. The New Zealand Plant Conservation Network (NZPCN) has a website to which anyone can contribute sightings of flowering and fruiting of plants http://www.nzpcn.org.nz/. You can record specific details of where and when, with a more generalised measure of the degree of flowering or fruiting.

- Colin Ogle is a retired ecologist from the Department of Conservation. He is now working as a volunteer in various local conservation and plant-related projects.