Cypress knee

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Knees with very little taper Bald Cypress Taxodium distichum Knees High DoF.JPG
Knees with very little taper
A bald cypress exhibiting tapered knees CypressKnees.jpg
A bald cypress exhibiting tapered knees

A cypress knee is a distinctive structure forming above the roots of cypress trees of some of the species of the subfamily Taxodioideae, such as the baldcypress. Their function is unknown, but they are generally seen on trees growing in swamps. Cypress knees can grow to considerable size. The largest on record, along the Cache River in northeastern Arkansas, is 3 m (9.8 ft) high and 0.9 m (2 ft 11 in) diameter at water level. [1] while the tallest, along the Suannee River in North Florida, is 4.3 m (14 ft) in height. [2] Some current hypotheses state that they might help to aerate the tree's roots, [3] create a barrier to catch sediment and reduce erosion, assist in anchoring the tree in the soft and muddy soil, or any combination thereof. [4]

Contents

Knees are woody projections sent above the normal water level, roughly vertically from the roots, with a near-right-angle bend taking them vertically upward through water. One early assumption of their function was that they provided oxygen to the roots that grow in the low-dissolved-oxygen (DO) waters typical of a swamp, acting as pneumatophores: mangroves have similar adaptations. [4] There is little actual evidence for this assertion; in fact, swamp-dwelling specimens whose knees are removed continue to thrive, and laboratory tests demonstrate that the knees are not effective at depleting oxygen in a sealed chamber. [5] Even though there is no expert consensus on their role, the supposition that they are pneumatophores is repeated without comment in several introductory botany textbooks. [4]

Another more likely function is that of structural buttressed support and stabilization. [4] Lowland or swamp-grown cypresses found in flooded or flood-prone areas tend to be buttressed and "kneed", as opposed to cypresses grown on higher ground, which may grow with very little taper.

Trees that develop these "knees" include:

Although the genus Cryptomeria (sugi) is in the same subfamily, there are no reports of it producing knees.

Also the unrelated Sonneratia caseolaris (Lythraceae) has knees very similar to those of Taxodium distichum. [8]

See also

References

  1. ↑ Meanly, Brooke (1972). Swamps, River Bottoms and Canebrakes. Barre, Massachusetts: Barre Publishers. p. 63 photo with caption.
  2. ↑ Kernell, Judith L.; Levy, Gerald F. (December 1990). "Relationship of Bald Cypress Knee Height to Water Depth". Castanea. 55 (4): 219-220 including photo.
  3. ↑ Morrison, H. Robert (1985). America's Wild Woodlands. National Geographic Society. p.  114. ISBN   9780870445422.
  4. 1 2 3 4 Arnold Arboretum.; Arboretum, Arnold; Arboretum, Arnold (2000). Arnoldia. Vol. 60. Jamaica Plain, Mass: Arnold Arboretum, Harvard University.
  5. ↑ Kramer, Paul J.; Riley, Walter S.; Bannister, Thomas T. (1952-01-01). "Gas Exchange of Cypress Knees" . Ecology. 33 (1): 117–121. Bibcode:1952Ecol...33..117K. doi:10.2307/1931260. JSTOR   1931260.
  6. ↑ Henry, Augustine; McIntyre, Marion (1924). "The Swamp Cypresses, Glyptostrobus of China and Taxodium of America, with Notes on Allied Genera". Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science. 37. Royal Irish Academy: 90–vi. ISSN   0035-8983. JSTOR   20490319 . Retrieved 2025-08-02.
  7. ↑ "Taxodium mucronatum (ahuehuete) description". The Gymnosperm Database. 2025-02-10. Retrieved 2025-08-02.
  8. ↑ Tomlinson, P.B.; Zimmerman, Martin H. . (1978). Tropical Trees as Living Systems. Cambridge, England: Cambridge Univ. Press. p. 328.
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