It is amazing how few people realize that all of our native broad-leaved trees here in Pennsylvania are flowering plants. While some have showy flowers that attract bees and other insect pollinators, some are wind pollinated and have less conspicuous flowers.
Apple blossoms (two photos below are hard) to miss and the bees love them. These are not native trees but are quite common.
Pin Cherries grow clusters of small cherries that will develop from the flowers shown below. Black cherry forms longer strings of flowers (racemes). Cherries and apples are related, both members of the rose family.
Sassafras has small yellow flowers that blossom before the leaves sprout.
Birches grow male catkins that shed pollen into the wind to fertilize female catkins. Black (left) and Gray Birch (right) catkins are shown below.
Red oaks are flowering now and have tassel-like strings of green flowers. These and the birch catkins are litter the ground when pollination is complete.
Meanwhile, the Red Maple flowers have been pollinated and are turning into sumaras, or winged seeds that will eventually twirl to the ground like helicopters.
Watch the trees in your yard, a park, or a nearby forest and look for their flowers, seeds and fruits.