Knees

December 28, 2022

When I examined the odd looking trees at the reservoir earlier this week, I thought at first that they were some sort of deciduous (not conifer) tree.  They were about 30 yards (27.5 m) away and they appeared to have leaves still hanging from the branches.  When I got closer these turned out to be the fern-like leaves that had died but not yet dropped from the trees.  The trees were both standing in water and situated closely along the shore, where they would have been inundated as the lake level rose.  Around each of the trees were a series of knobby protrusions sticking up through the soil.  These gave the appearance of knees supporting the large trees.

When I looked online, I found these knees are not exclusive to the Bald Cypress (Taxodium distichum).  This distinctive structure forms above the roots of various species of the subfamily Taxodioideae, of which the bald cypress is one.  Their function is unknown, but they are generally seen on trees growing in swamps.  Current hypotheses state they might help to aerate the tree’s roots, create a barrier to catch sediment and reduce erosion, assist in anchoring the tree in the soft and muddy soil, or any combination of these.  The 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 the 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.  This was thought to act as a specialized respiratory root in certain aquatic plants that grows upward and protrudes above the water or mud into the air (pneumatophores).  Mangroves (Avicennia marina) are another tree with a similar adaptation. 

There is little actual evidence for the assertion that knees provide oxygen to the trees root system.  It has been noted that swamp-dwelling specimens whose knees are removed continue to thrive and lab tests demonstrate the knees are not effective at depleting oxygen in a sealed chamber.  Despite the fact there is no expert consensus on their role, the belief that they are pneumatophores is repeated without comment in several introductory botany textbooks.  Another more likely function is they act as a structural buttressed support for stabilization.  Lowland or swamp-grown cypresses found in flooded or flood-prone areas tend to be buttressed and have knees, while cypresses grown on higher ground may grow with very little taper.  This was the case with the cypress grown in my back yard at the camp.  I was in a dry and even slightly elevated section of the yard.  It was over twenty feet high (6.5 m) and had been there for a while.  There were no knees associated with the tree.  Despite there being no evidence, this theory has continued to be passed through the scientific community.

THOUGHTS:  Cypress developed during the Mesozoic era when the continental crust was concentrated in a single huge landmass, the supercontinent Pangea.  This was a time of ferns and fern-like trees buttressed by knees and was often depicted as swampy grounds of early illustrations.   This era is also associated with the Jurassic Park dinosaurs of the movie franchise.  I grew up enthralled by these dinosaurs and their habitats.  This love was passed on to my son, and he in turn passed it on to his two children.  Knowledge and culture are passed from one generation to the next through a combination of oral tradition, observation, and imitation among family members.  This results in the shared traditions that form the basis of our actions.  That emphasizes how important it is to pass on love and compassion rather than fear and hate.  Act for all.  Change is coming and it starts with you.

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