Dave Kelly (University of Canterbury) recently talked to the Wellington Botanical Society about mast seeding.
Mast seeding is where individuals of a plant species synchronously produce unusually large seed crops every few years. There is often no regular cycle.
New Zealand is a world centre for mast seeding (and research on mast seeding). Some New Zealand plants are amongst the most variable mast seeders, with several orders of magnitude difference in seed production between years when they flower heavily and years when they barely bother.
The “mast” of mast seeding comes from a German word for “fatten”, with farmed pigs being fattened in oak forests producing a bumper crop of acorns.
In New Zealand, mast seeding causes conservation issues. When beech (Nothofagus) trees mast, their abundant seeds fuel an explosion in mice numbers. This in turn leads to an increase in stoats, which in turn exact heavier predation on native birds.
Dave offered several reasons why mast seeding occurs.
In Nothofagus beech trees, which are (probably) not self-fertile, it may be to increase the proportion of viable seed.
In Chionochloa grasses, it may be to satiate insect seed predators. Seed predators will not be able to eat all of the seeds in irregular years of super seed production if the populations of the predators have been kept low by years of low/no seed production.
How the plants achieve such synchronicity remains largely a mystery, but it appears to be based on temperature cues (with heavy flowering often following a warm summer).