Judy Simpson, of Karori, asks :-
In our garden we have a totara which is nearly 90 years old. Last summer there was a mast flowering with a huge crop of ripe drupes. Most years there are a few ripe drupes, sometimes it seems none. There is another totara tree, I think male, about one hundred metres away from our tree. Some years there are lots of seedlings from the tree in our garden but other years we don’t notice any. In a nutshell my question is; Can the female “flowers” of the totara ripen into fleshy drupes even if no pollen from a male tree has fertilised them? Can they ripen but not be fertile and so they don’t produce seedlings?
Janice Lord, a botanist at the University of Otago, responded.
Like most New Zealand conifers, Totara "masts", which means it has heavy fruiting years interspersed with years with little or no fruit. This strategy is thought to minimise losses to specialised seed predators (mainly insects) by "predator satiation", i.e. flooding the market with more seeds than can be eaten.
For this strategy to work there has to be "off" years in which the seed predators have little or no food so their numbers are reduced. In the case of wind-pollinated, fleshy-fruited plants like Totara, it is also thought that masting improves the efficiency of pollination because there are more pollen grains in the air at any one time. Masting may also benefit fruit dispersal by attracting more dispersers from a wider area.
The fact that you see some fruit apparently forming, but then failing to germinate, is related to the specific way in which conifers go about seed production.
Conifers, along with cycads, ginkgo and a few other species, are known as "gymnosperms" and they differ from flowering plants (angiosperms) in the timing of investment in developing seeds. In flowering plants, very few resources are put into the ovules (these are located in the ovary at the base of the flower and each contains an egg cell) prior to pollination. It is only after pollination that the endosperm in the seed starts to develop and the seed enlarges.
In gymnosperms, a large amount of resources is invested in the structures associated with the ovule (there is no ovary) before pollination, for example in the Australian cycad Macrozamia, each of these structures (technically known as mega-gametophytes) can grow to nearly 80% of full seed size even if it hasn’t been pollinated. Sterile, half-developed fruit, is more likely to be seen in years were there is little pollen available.
The increase efficiency of the angiosperm system is thought to be a major reason why they dominate in most habitats around the world.