![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
(no subject)
Sep. 9th, 2021 09:00 amWhen the "cities plant male trees because they don't want to deal with cleaning up fruit, and this makes allergies worse" meme made its rounds a while back, I didn't think much of it, but now that I've read this thread, I went and refreshed my memory about tree sex.
The meme is wrong, but the thread linked above swerves too far the other way and ends up wrong as well.
Actually, species that have separate sexes make up about 1/5 of the trees in the Eastern US. When planting trees of those species, cities may choose to go with male trees only, but probably don't bother unless the fruit/seeds are large and messy but the trees themselves are desirable (selecting for the gender of tree adds to the cost, and cities are loath to spend more money when they can avoid it by just picking a different, non-messy species or not bothering to screen their plantings for sex).
Things to remember:
1. Plant sex is complicated.
2. Dioecious is a fancy greek term for species that have separate organisms for male and female reproductive functions. Nearly all animals we're familiar with are dioecious. Among plants, dioecy is relatively rare. The "urban male trees are responsible for your allergies" meme is talking about dioecious species, which are a minority among trees (1/5 in the Eastern US, 1/20 worldwide).
3. The only pollen that's an issue for those with allergies is pollen from wind-pollinated species, which would be many trees, most grasses, and various herbs (Flowers that look pretty are not wind pollinated). The pollen from male dioecious trees that have been deliberately planted because they are male and will not litter the ground with fruit is a drop in the bucket of the overall pollen load.
Nerdy tree sex tidbits:
4. Non-dioecious plants cover a lot of variety. They can produce one flower that performs both sexual functions (cosexual). They can have separate pollen and seed producing parts on the same plant (monoecious). Or, just to be different, they can have some combination of cosexual and monoecious bits on the same or separate plants (polygamous).
5. monoecious and polygamous trees cover the gamut from having sexual phases of life (eg, making pollen when young and seeds when mature), to switching from making pollen to making seeds and back again every year or every few years, to being boring and producing both each year.
The meme is wrong, but the thread linked above swerves too far the other way and ends up wrong as well.
Actually, species that have separate sexes make up about 1/5 of the trees in the Eastern US. When planting trees of those species, cities may choose to go with male trees only, but probably don't bother unless the fruit/seeds are large and messy but the trees themselves are desirable (selecting for the gender of tree adds to the cost, and cities are loath to spend more money when they can avoid it by just picking a different, non-messy species or not bothering to screen their plantings for sex).
Things to remember:
1. Plant sex is complicated.
2. Dioecious is a fancy greek term for species that have separate organisms for male and female reproductive functions. Nearly all animals we're familiar with are dioecious. Among plants, dioecy is relatively rare. The "urban male trees are responsible for your allergies" meme is talking about dioecious species, which are a minority among trees (1/5 in the Eastern US, 1/20 worldwide).
3. The only pollen that's an issue for those with allergies is pollen from wind-pollinated species, which would be many trees, most grasses, and various herbs (Flowers that look pretty are not wind pollinated). The pollen from male dioecious trees that have been deliberately planted because they are male and will not litter the ground with fruit is a drop in the bucket of the overall pollen load.
Nerdy tree sex tidbits:
4. Non-dioecious plants cover a lot of variety. They can produce one flower that performs both sexual functions (cosexual). They can have separate pollen and seed producing parts on the same plant (monoecious). Or, just to be different, they can have some combination of cosexual and monoecious bits on the same or separate plants (polygamous).
5. monoecious and polygamous trees cover the gamut from having sexual phases of life (eg, making pollen when young and seeds when mature), to switching from making pollen to making seeds and back again every year or every few years, to being boring and producing both each year.
