Cypress Knees – Are they just for tacky table lamps?

Cypress Knees – Are they just for tacky table lamps?

a Carolina Paddler Article

By Mitch Lloyd

∞ Everybody thinks they know what purpose is served by a cypress knee.  After all, the internet has taught us that if you say it often and long enough, it must be true. My curiosity has revealed that other strange people like me have been wondering for centuries about the reason for knees to exist.  Everything’s got a purpose, right?

Harvard University has Arnold Arboretum, which publishes a respected scientific journal called Arnoldia.  It is here that Christopher H. Briand published a definitive review in the December 1, 2021 issue (Vol 60, Issue 4). Mr. Briand reviewed long-standing theories and discussed their debunking, so everything I am about to share, I stole from him and a few other earlier authors, most of whom were cited by Mr. Arnold.

Cypress trees have long horizontal roots that spread out across the sandy swamp bottoms. A massive root system is needed to keep these swamp dwellers from blowing over in storms. From these horizontal root structures, vertical roots grow down into the soil and cypress knees grow up and into the air. Good so far but then we learn that there are cypress trees that grow on dry soil and they don’t have knees.  Also, knees don’t grow on cypresses in deep water.

The most common theory you will hear is that cypress knees supply air or gases to the trees (think oxygen absorption or carbon dioxide) release, normal tree aspiration), that the trees need the knees to “breathe”, especially in watery environs.  Seems reasonable until you go back to the fact that dry soil cypress or trees in deep water don’t have knees, wouldn’t these trees need to “breathe” as well? Also, some researchers from Duke University conducted experiments in 1952 (after the invention of oxygen sensors) and determined that no measurable oxygen absorption was occurring through knees.  So, when you hear this theory, scoff loudly.

The next most popular hokum being spouted is the theory of mechanical support, but how could a piece of wood sticking up in the air provide any kind of bracing to a tree?  Well, it was observed that vertical roots penetrated the soil beneath the knees.  It was a good theory until other scientists observed there were vertical roots that didn’t have knees above them and there were knees that didn’t have roots below them, so no correlation there. We have already scoffed loudly at another theory so we can apply a stern Harrumph here, or a Humbug! or Pish Tosh!

A couple of throw-away theories were thrown away almost immediately, one being organs of reproduction (a knee doesn’t become a new tree) and methane emission (that’s the job of cows).  A mere condescending sniff is all that is required if you hear these postulates.

There is one theory that has not been disproved yet and it is my favorite, the one to which I adhere: nutrient acquisition.

As children, we were taught in our science classes that trees have growth rings that vary with wet/dry cycles; in wet years the trees will be happy and grow a lot and have big fat tree rings. It is the simple concept of hydration/dehydration.  So, nobody thought that swamp trees, being wet all the time, would show climatic changes of wet and dry, but cypress trees do. The theory of hydration/dehydration doesn’t work here.  If not water, then how about food?

Our Piedmont rivers flow from the mountains and hills and bring with them vast amounts of minerals and nutrients in the form of silt. When rivers flood, fresh layers of silt invigorate the soil with new fertilizer, which is why humans always farmed near rivers, so that the soil would not get depleted.  All that natural erosion of the land feeds our ecosystem.

Our coastal plains rivers do not feed from the Piedmont or further inland; they don’t get the yellow mud coming down packed with nutrients.  When our black water rivers flood, they just wet the land with a natural tea, not much nutrients there for anything.

The primary nutrient source for our black water coastal plains rivers is organic debris: leaf litter and detritus that washes from the land in heavy rainfall.  On a good summer’s day as you wade in our coastal streams you can see tiny bits of leaves tumbling across the sandy bottom, this is the food for the swamp dwellers. In dry periods, the water levels are low and many trees are out of the water (although the soil beneath is still wet) so no organic mulch can get to the trees.  In dry periods, organics are not being washed into the streams from the higher grounds. The river flow is reduced so there is not much current to push the debris along the bottom to feed the river.

Then it rains and organics start flushing in and flushing by and the water level comes up to deliver to the dryer trees. How do you as a tree get your share and beat out that sorry ash behind you? The best current theory is cypress knees, sticking up in the water, serve as food traps, catching the leaf litter and larger flotsam as it floats by, making it stop here at my roots rather than wash on down.  Rain means more food for me and food equals growth. Cypress trees show cyclic climatic growth patterns, not based on hydration/dehydration, but rather abundance vs starvation.

Yes, cypress trees grow in ponds and lakes that don’t have big flows and they grow in mighty rivers like the Mississippi that have huge nutrient loads, but the theory still holds in my mind. Cypress knees are obstructions that can capture organic nutrients for the benefit of the parent tree.  It’s a good theory and it hasn’t been disproved yet.

Next time you trip over a cypress knee while fighting your way through a portage, remember all those cheap horror movies you saw as a kid and try not to become forest food for the cypress trees.