Photo: Eric Victor Gitter (flickr)
Flowers on trees may contain both male and female reproductive organs within one flower or have separate male and female flowers on the same tree. Additionally, there are trees that produce flowers of just one sex.
Since trees are well and truly rooted to their spot, they must rely on other agents to help them cross pollinate, or they must self pollinate. If a tree can self pollinate, because it has both male and female organs, it is referred to as a hermaphrodite.
If a tree needs specific agents to pollinate it, the flowers it produces will be designed to attract such agents. For example, large showy flowers attract animals. However, insects are the most common pollinators and they are attracted by nectar-rich flowers with sticky pollen grains that adhere to the insects and can then be later transferred to other flowers. Flowers pollinated by birds are usually tubular in shape and frequently have pollen on stamens that protrude and therefore brush against the feathers of birds.
In cold northern temperate regions, where there are fewer insects and birds than there are in the tropics, many trees are pollinated by the wind. For example, oak alder birch and hazel trees have drooping catkins loaded with pollen. When the wind blows the trees may be happy but those people who suffer from allergies to tree pollen are not so thrilled.