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Reproduction Main article: Plant reproduction Form, leaves and reproductive structures of queen sago Cycas circinalis Tree forms are found in a wide range of plants and their reproductive strategies are substantially the same as shrub or herbaceous plant forms. Many trees are wind pollinated which may be an evolutionary adaptation to take advantage of increased wind speeds high above the ground, particularly in the case of those that produce pollen before the leaves emerge.[64] A vast quantity of pollen is produced because of the low likelihood of any particular grain landing on an appropriate female flower. Wind-pollinated flowers of broad-leaved trees are characterised by a lack of showy parts, no scent and a copious production of pollen, often with separate male and female flowers, or separate male and female trees. The male flowers may be high up in the tree, often in the form of dangling catkins. The female flowers may be lower down the tree. The pollen of pine trees contains air sacs which give it buoyancy and it has been known to travel as far as 800 kilometres (500 mi).[65] Tree pollen can cause allergies. A prime example would be hay fever, which can be caused by pollen.

The earliest tree-like organisms were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period.

Trees are an important part of the terrestrial ecosystem,[86] providing essential habitat for a community of organisms. Epiphytic plants such as ferns, some mosses, liverworts, orchids and some species of parasitic plants (e.g., mistletoe) hang from branches; these along with arboreal lichens, algae, and fungi provide micro-habitats for themselves and for other organisms, including animals. Leaves, flowers and fruits are seasonally available. On the ground underneath trees there is shade, and often there is undergrowth, leaf litter, fallen branches and/or decaying wood that provide other habitat. Trees stabilise the soil, prevent rapid run-off of rain water, help prevent desertification, have a role in climate control and help in the maintenance of biodiversity and ecosystem balance.[87] Many species of tree support their own specialised invertebrates.