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.[66]
Main article: Pollination syndrome
Seeds
Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.[67] The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to disperse their offspring.
The single extant species of Ginkgophyta (Ginkgo biloba) has fleshy seeds produced at the ends of short branches on female trees,[68] and Gnetum, a tropical and subtropical group of gymnosperms produce seeds at the tip of a shoot axis.[69] The seeds of conifers, the largest group of Gymnosperms, are enclosed in a cone and most species have seeds that are light and papery that can be blown considerable distances once free from the cone.[70] Sometimes the seed remains in the cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of seeds of the jack pine, and also enriches the forest floor with wood ash and removes competing vegetation.[71] Similarly, a number of Angiosperms including Acacia cyclops and Acacia mangium have seeds that germinate better after exposure to high temperatures
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.[66]
Main article: Pollination syndrome
Seeds
Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.[67] The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to disperse their offspring.
The single extant species of Ginkgophyta (Ginkgo biloba) has fleshy seeds produced at the ends of short branches on female trees,[68] and Gnetum, a tropical and subtropical group of gymnosperms produce seeds at the tip of a shoot axis.[69] The seeds of conifers, the largest group of Gymnosperms, are enclosed in a cone and most species have seeds that are light and papery that can be blown considerable distances once free from the cone.[70] Sometimes the seed remains in the cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of seeds of the jack pine, and also enriches the forest floor with wood ash and removes competing vegetation.[71] Similarly, a number of Angiosperms including Acacia cyclops and Acacia mangium have seeds that germinate better after exposure to high temperatures
