In many plant genera the same pollination method (or combination of methods) is found in every species. Not so in Acer.
The vast majority of Acer species are insect pollinated. The flowers are strongly scented due to the presence of large amounts of nectar. This is made available to attract insects to crawl around in the flowers and, accidentally, get covered in pollen before flying off to get their next sugar fix – hopefully at another tree of the same species where the pollen will be delivered.
This is a service that the tree has to ‘pay’ for by investing in producing nectar but the chance of any particular pollen grain transferring to another suitable tree is relatively high.
Norway maple (Acer platanoides), including red/purple-leaved varieties, are planted on campus. Their yellow-green flowers are highly attractive to suitable-sized pollinators.
Also on campus, at the side of the car park in front of the Harborne Building, is an Acer that has opted for wind pollination. It is also different from the majority of Acer species in having compound leaves.
Ash-leaved maple (Acer negundo) is a native of North America. It has separate male and female trees. The tree here is male. Each flower is suspended at the end of an extremely long pedicel (flower-stalk) and consists only of suspended stamens. There is no sweet scent as no nectar is produced.
The tree has not invested resources into nectar production, nor has it produced any petals or sepals for its flowers although the long stalks require extra materials. The tree’s pollen will be picked up by the wind and blown away.
In wind-pollinated species the chance of an individual pollen grain arriving on the sigma of a female flower of the same species is extremely low. For this reason, large quantities of pollen are usually produced to allow for the huge percentage of pollen grains that are wasted. So insect-pollinated species invest resources in nectar while wind-pollinated species devote resources to extensive pollen production.
In other words: you pays your money and takes your choice!