Some species of trees (but not all) have female and male flowers on separate plant individuals (dioecious). That means that a male and a female have to be relatively close in proximity to pollinate each other and produce viable seeds. Males produce the pollen, and females have the ovaries that produce seeds.
An example species is the Kentucky Coffeetree (Gymnocladus dioicus). My neighbor has several Kentucky Coffeetrees that produce seed pods that drop in my backyard and driveway. These seedpods used to be eaten by giant megafauna like the Woolly Mammoth and Giant Ground Sloth, who used to help disperse the seeds around the environment. Since those animals are extinct, no wild animals really utilize the seed pods anymore, and they're just kind of left on their own. However, the tree seems to germinate somewhat readily on its own, but doesn't spread very far from the parent tree.
I like to gather the seeds and germinate them indoors during the winter time. I usually get a 60-75% germination rate, and they grow just fine indoors for a month or two before it's warm enough to plant them outside in a pot or in the ground. The tree is native to the mid-west / eastern US, but doesn't support too many insects or wildlife (less than 10 Lepidoptera), so I don't plant too many of them. They make great street trees and are tolerant of heat and pollution.
