What good are knees on a tree? Baldcypress famously has these odd structures that stick out above the water line. Botanist Francois André Michaux saw knees on baldcypress trees and, in 1819, said “No cause can be assigned for their existence.” Nearly 200 years later, what do we know about the function of this strange structure? We’ll get to the evidence shortly, but first lets take a closer look at this remarkable tree.
Baldcypress, Taxodium distichum, is in the cypress family, the Cupressaceae. This family includes some of the most ancient lineages of trees, dating back to the late Jurassic, about 150 million years ago (mya). The cypress family includes the world’s tallest tree, coast redwood (Sequoia sempervirens); the world’s largest tree in volume giant sequoia (Sequoiadendron gigenteum); and some of the longest lived trees. Many members of the family are relicts, species that were once widespread but are now confined to very small ranges, often with unusual climate or soil conditions. Because the family is so ancient, the distribution of modern species are best explained as a result of plate tectonics, especially the breakup of the supercontinent Pangaea 100 mya.
The modern members of the cypress family are mostly restricted to harsh or special environments where they don’t have to compete with the more efficient hardwood (angiosperm) trees. Baldcypress is found in the swamps of the Atlantic and Gulf Coastal Plains, or on soils that are waterlogged at least part of the year. In that habitat, it is a slow-growing tree that can live thousands of years.
One of the biggest challenges to life in a swamp is the variable water level. If the water is too deep, the tree will die from oxygen stress. If the soil dries out, drought stress and competition from hardwoods like red maple may eliminate the baldcypress. In between is where we find baldcypress growing the best. And it is here that we find the two characteristics of baldcypress – the buttressed lower stem and the knees sticking out of the water.
Lots of trees make buttresses, especially in the tropics. Buttressed stems and roots are a response to very poor soils that are either low in nutrients or low in oxygen. As a result, the root system remains near the surface of the soil. The buttress allows the tree to stand up to the stresses of gravity and wind without having a deep root system.
But the knees are still a problem because almost no other tree has them. We see occasional looped roots and other odd growths in other trees, but nothing like the knees.
Early botanists were quite certain that they knew what the knees were for and they gave the knees a fancy name “pneumatophores,” meaning air roots. They thought that the purpose of knees was to allow the root system, deep in the anaerobic swamp water, access to air. In the absence of any evidence, this seemed reasonable. You still see many descriptions of knees that assert that they play a gas exchange role.
In spite of the certainty of early botanists, there is no evidence that knees play any role in gas exchange. Plant organs that are adapted for gas exchange have several important features – lenticels and aerenchyma. Lenticels are areas of the bark or epidermis that have a lot of air space, allowing oxygen and carbon dioxide to exchange through the bark. Aerenchyma consists of special cells with a lot of air space in between the cells. The problem is that baldcypress knees have no lenticels or aerenchyma.
A few scientists have measured gas exchange between baldcypress knees and the air. Paul Kramer, the renowned plant physiologist at Duke, and his students found no evidence of significant gas exchange and concluded that knees do not play a significant role in bringing oxygen to the root system.
Knees could play a structural role, making the root system stronger in the face of high wind, but there is no evidence for this either. It would be quite difficult to measure the mechanics of a tree with and without knees. Since trees with and without knees grow in slightly different habitats – with knees in shallow water, without knees on dry land or deep water – it’s hard to see how one could design a proper experiment.
And there we are, exactly where Michaux left us nearly 200 years ago. We do not know what baldcypress knees are for. Big trees are hard to study, and field work in swamps is difficult and expensive. There are not a lot of research funds to allow scientists to pursue esoteric subjects like the function of knees. I suspect that we may be using the same explanation as Michaux for a long time to come.