Plant 220

Taxodium distichum (L.) Rich. (Cupressaceae)


Swamp cypress

Taxodium distichum is a deciduous, coniferous tree whose natural range is the eastern USA, where it prospers, in riparian habitats, in lowland river flood plains and swamps. It is one of fourteen species, from among five genera of conifers, which lose their leaves in winter. Its leaf-dropping habit, associated autumn colour and flush of new fresh foliage in the spring, so anomalous among the gymnosperms, has made the swamp cypress a 'go to' ornamental tree species.

The introduction of Taxodium distichum to England is credited to John Tradescant the Younger in 1640. Tradescant made three trips to North America between 1637 and 1653, and alongside the swamp cypress he also introduced the tulip tree (Liriodendron tulipifera) and red maple (Acer rubrum). Tradescant eventually succeeded his father to become Charles I's head gardener.

Peculiar adaptations of the swamp cypress are the features known as 'knees'. These stalagmite-like, woody growths appear from the roots of the tree. The function of these structures has been the subject of much scientific discussion. The most widely accepted function is that they are pneumatophores that aid the tree in gaseous exchange in anaerobic environments. Other ideas have emphasised the starch-rich wood of the 'knees' which may be adaptations for storage. Yet another idea of 'knee' function is that they aid tree stability in saturated soils.

This tree not only tolerates waterlogged or flooded environments, it thrives forming extensive forests in the USA from New York through Florida and the southern states as far west as Texas. Consequently, it is considered of 'Least Concern' on the IUCN Red List.

Ancient swamp cypress forests, for example the Black River Preserve in North Carolina, still survive in the south-eastern USA, with older individual trees being between 500 and 1,600 years old; some trees may be up to 2,000 years old. Although as a species the swamp cypress has the status of 'Least Concern', old-growth stands, fragmented by human development, are vulnerable to destruction.

Swamp cypress is associated with Spanish moss (Tillandsia usneoides) an epiphyte which can cover the tree, adding to the bedraggled look as the tree loses its foliage during the winter months.

A fine specimen of swamp cypress can be seen at University of Oxford's Harcourt Arboretum in the meadow adjacent to the car park. The tree grows on the banks of the pond and flourishes in the wet soil and shows an orangey, russet autumn display.

Further reading

Martin CE and Francke SK 2015. Root aeration function of bald cypress knees (Taxodium distichum). International Journal of Plant Sciences 176: 170-173.

Perkins S 2017. How saving some of the Southeast's oldest trees might help scientists monitor climate change. Proceedings of the National Academy of Sciences 114: 6875-6876.

Luke Rowland