|Veronica decora (here at Moke Creek, Otago) is cosexual; every plant produces both pollen and ovules, in this case in hermaphrodite flowers.|
|Female (left, cultivated) and male (right, Makara Peak) of Aciphylla squarrosa.|
Colin Webb, David Lloyd, and Lynda Delph asked this question back in 1999 (free download here). They calculated we have 83 seed-plant genera—23% of the genera in the flora—in which at least some species have separated sexes, and that's very high by international standards. Since then a few more genera have been added to the list, like Teucridium, Toronia, and Corynocarpus (free downloads here, here, and here), but taxonomic changes have reduced the number a bit too, so the estimate is still probably about right.
Given this unusual feature of the flora, it's perhaps not surprising that New Zealand has produced a number of internationally recognised researchers of plant reproductive biology, headed of course by David Lloyd (1937–2006). I've been aware for a while that nearly all of them did their research in the South Island, and we know that (roughly speaking) in New Zealand tree diversity decreases with increasing latitude, while alpine plant diversity increases. So it's not surprising perhaps that the newly-discovered instances of separate-sexed genera are mostly northern and two are trees. There might even be a few more waiting to be noticed; I've got my eye on another genus at the moment.
The variability of sexual systems in some genera, again often northern, still might not be fully described. Kohekohe (Dysoxylum spectabile) and tītoki (Alectryon excelsus) were listed by Webb et al. as dioecious, but they need close examination to check for gynodioecy. In kohekohe, a few fruits can be found on some male trees, and I've seen a few fruits on male tītoki, at least in cultivated trees.
|Kohekohe (Dysoxylum spectabile), fruits on a female tree.|
There were obviously two sorts of trees in this population: those that were covered in fruit and those that weren't. Clearly, last year's flowering had produced a bumper crop. The trees that weren't bearing fruit were also flowering, and every one that I could reach was a male; the flowers had usually eight stamens and masses of pollen.
|Tītoki, male flower.|
|Tītoki, female (or maybe hermaphrodite?) flower|
Also, for a male it's a good evolutionary strategy to have sex whenever you can (I always tell my students that biological truth—a consequence of Bateman's Principle—doesn't make it morally or socially acceptable though; we're talking about trees here, not sentient social animals that have to live in a society). Male flowers and pollen are cheap to produce, so even if most of them might be wasted it's a risk that's worth taking. This year, pollen from all those male flowers will be competing for the few flowers on that one female tree and next year I'd expect to see very few fruits available in this population. Unless, that is, some of those male trees can make some fruits, in other words if this population is gynodioecious.
|Tītoki, fruit. The black shiny seed is surrounded by a fleshy aril that attracts birds.|
Update, added 26 November 2012.
One of my undergraduate students read the blog above and immediately went to check trees in her mother's garden (in Waikato, North Island). She sent the photo below, which she correctly described as an inflorescence with mostly male but a few female flowers. That fits well with my observation that some male trees can set a few fruits, and suggests pretty strongly that tītoki is gynodioecious (we still need good population samples to be sure and to describe the system in detail).
|Tītoki inflorescence with male flowers and a few female ones (two are circled)|
You might ask how I can be confident the photo above shows a male with a few female flowers, rather than a female with a few male flowers. Populations with constant males and inconstant females (females that can produce some pollen) are called androdioecious. It's an exceedingly rare sexual system and seems to need some special conditions in order to evolve and persist. There are only a few well-documented examples. So, given that gynodioecy is vastly commoner, and also that most of the flowers on this branch are male while only a few are female, I think that gynodioecy is the most likely sexual system in tītoki.
Webb, C., Lloyd, D., & Delph, L. (1999). Gender dimorphism in indigenous New Zealand seed plants New Zealand Journal of Botany, 37 (1), 119-130 DOI: 10.1080/0028825X.1999.9512618
Lloyd, D. (1980). Sexual strategies in plants III. A quantitative method for describing the gender of plants New Zealand Journal of Botany, 18 (1), 103-108 DOI: 10.1080/0028825X.1980.10427235