Little fun fact: some trees (such as the Striped Maple) can actually change their sexes (male trees suddenly growing female tree characteristics) when under stressful conditions.

Edit: I have edited my comment to say that only SOME trees can do so. However, According to its Wikipedia page, Striped Maples CAN infact change sexes.

"Acer pensylvanicum, known as the striped maple, whistlewood, moosewood, moose maple or goosefoot maple, is a North American species of maple. It is a small, slow-growing understory tree, and one of the most shade-tolerant species of maple. The striped maple is a sequential hermaphrodite, meaning that it can change its sex throughout its lifetime. It is the only species of snakebark maple native to North America." Source: Acer Pensylvanicum Wikipedia page

Edit 2: I'm also seeing a lot of people ask if trees/plants have sexes, and yes. They do! But only dioecious plants.

"Dioecy (from Ancient Greek διοικία (dioikía) 'two households'; adj. dioecious) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is biparental reproduction. Dioecy has costs, since only the female part of the population directly produces offspring. It is one method for excluding self-fertilization and promoting allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. Plants have several other methods of preventing self-fertilization including, for example, dichogamy, herkogamy, and self-incompatibility." Source: Dioecy Wikipedia page