Trees reproduce asexually, through cultivation, and sexually, through the exchange of pollen between male and female reproductive systems. A single tree can produce both male and female flowers, relying on adaptations such as different blooming times to prevent self-pollination. There's a remarkable range of pollination strategies, with each tree developing adaptations that help it reproduce more effectively. Adaptations that aid reproduction include the shape, color or smell of the flower, differences in the cone structures, and various methods of preventing self-fertilization.
Many trees, such as pine trees, rely on wind pollination. This is why the weather affects the levels of pollen in the air. When conditions are right and there is wind, many trees shed their pollen so it will be blown onto others. The pollen of these trees is small and designed to be easily carried on the wind because it might have to travel some distance to find another tree of the same species that's producing female flowers or cones. Each tree's method of collecting the wind-blown pollen is different as well. The female cones on a conifer, for instance, produce a sticky substance near the ovule so wind-borne pollen will stick when it finds a female cone.
Pollinators are a huge part of the reproduction of plants in general. Pollinators can be anything from hummingbirds to moths, bees, butterflies, wasps, beetles, flies or even bats. Not every pollinator can pollinate every tree, some specialize. To draw in pollinators, trees develop flower adaptations based on what kind of pollinator they want to draw in. Bees, moths, and butterflies all see differently, so the flowers used to attract them all look different. Sometimes there is a very close relationship between a pollinator and the tree or plant they pollinate. Some birds have beaks specially designed to crack open conifer cones, thus spreading the seeds. In fact, some plants only can be pollinated by specific species. Others draw in as many pollinators as they can, and because they draw so many it isn't as big a loss if the next flower is a different species that can't do anything with the pollen.
However pollination occurs, it isn't the end of the reproductive cycle. The seeds that pollination produces still have to be distributed. There are as many seed adaptations as there are pollination adaptations. Some seeds, as with fruiting trees and nut trees, are encased and dropped to the ground. They may sprout there, or they may be eaten by animals, which spread the seeds when they defecate. Many nuts are collected by squirrels and other such animals, who store them for later. Some of those seeds then grow into new trees. Other seeds are wind-borne, much like pollen, and have adaptations to help them glide in the air until they land on patch of ground and begin to grow.
Some species of tree can reproduce by cultivation of cuttings. Cuttings are stems taken from a tree and planted in soil. After a time, these stems begin to develop roots and row into another tree. This is a form of asexual reproduction and so the new tree will be an exact copy – or clone – of its parent, unless a mutation occurs. Using this method, you can create a whole field that consists of trees with identical characteristics.