By wind (known as anemochory)
Trees have developed a number of ways to use the wind to their advantage.
Some, such as hazel, release their pollen into the air from catkins, trusting the tiny grains to encounter female flowers in their path. This way, there's no need to invest precious energy reserves producing nectar to tempt in pollinating insects. But there's a downside - this risky strategy leaves a lot to chance. The solution? To produce a lot of pollen at once, and to do so before leaves emerge each spring which might get in the way.
Birches also use the wind for pollination, as well as to distribute their seeds. They produce large numbers of tiny, lightweight winged seeds called samaras which float and glide on air currents. Willows instead release fluffy seeds which use a parachute effect to ride the breeze.
Ash, maple and hornbeam seeds generate their own lift with their specially designed wings. These 'helicopter seeds' spin as they fall, creating a type of flight known as autorotation.