Cypress knee
A cypress knee is a distinctive structure forming above the roots of a cypress tree of any of various species of the subfamily Taxodioideae, such as the bald cypress. 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]
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:
Also the unrelated Sonneratia caseolaris (Lythraceae) has knees very similar to those of Taxodium distichum.[6]
See also
[edit]- Cypress dome – Swamp dominated by pond or bald cypress
References
[edit]- ^ Meanly, Brooke (1972). Swamps, River Bottoms and Canebrakes. Barre, Massachusetts: Barre Publishers. p. 63 photo with caption.
- ^ Kernell, Judith L.; Levy, Gerald F. (December 1990). "Relationship of Bald Cypress Knee Height to Water Depth". Castanea. 55 (4): 219-220 including photo.
- ^ Morrison, H. Robert (1985). America's Wild Woodlands. National Geographic Society. p. 114. ISBN 9780870445422.
- ^ a b c d Arnold Arboretum.; Arboretum, Arnold; Arboretum, Arnold (2000). Arnoldia. Vol. 60. Jamaica Plain, Mass: Arnold Arboretum, Harvard University.
- ^ 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.
- ^ Tomlinson, P.B.; Zimmerman, Martin H. . (1978). Tropical Trees as Living Systems. Cambridge, England: Cambridge Univ. Press. p. 328.
External links
[edit]- Christopher H. Briand. "Cypress Knees: An Enduring Enigma Archived 2010-08-01 at the Wayback Machine." Arnoldia. 2000-2001.Vol. 60(4). p. 19-20, 21-25. An extensive review of the published literature concerning cypress knees.