Bald cypresses and their knobbly knees

While I'm in an investigative mood, do Bald Cypresses breath through their knees? This was another question posed to me by our Melbourne Gardens Senior Curator of Horticulture, Peter Symes. Above is a cypress knee in Melbourne, and next up a collection (a 'knobble' perhaps) of knees at the South China Botanical Garden in Guangzhou (taken during a visit in 2009).

It's a common perception and two researchers at the University of Kansas, Craig Martin and Sarah Francke, believe so. In one rather simple experiment they have overturned years of alternate theory and folklore, to show that the knobbly 'aerial' roots of Taxodium distichum are, like those of mangroves, pneumatophores (literally, air stalks).

In nature, the Bald Cypress produces a lot of these woody projections from its roots, and while short and stumpy in the Melbourne Gardens they can be up to 'several metres' tall in the wild. The function of these cypress knees has been much debated although it's fair to say that until recently it has been assumed they have something to do with getting oxygen to roots in a sometimes waterlogged environment - i.e. a swamp.

While this sounds perfectly reasonable, there is actually little evidence to support the supposition or to discount alternative proposals. For example the knees might stabilise the tree in some way. They they might provide access to nutrients, perhaps by encouraging litter to accumulate and break down near the tree. Or could they be a storage organ, a place to hold excess carbohydrates for a less rainy day in the swamp.

Martin and Francke devised a rather simple experiment, measuring oxygen in the submerged (underground) roots when the knees were above water, and then when the knees were submerged. If there was less when the knees were entirely covered by water this, to their mind, proved that these knees were indeed air stalks (pneumatophores).

Happily, they found that oxygen was 'much higher' (about three times) in the roots when the knee was above water and therefore able to diffuse oxygen down into the soggy roots. Than ran various controls and concluded that there is now 'unequivocally strong evidence that cypress knees do indeed allow the diffusion of oxygen to the submerged attached root'.

Like any good scientist, they do qualify their result saying that the cypress knees may have evolved in response to things other than a need for oxygen by submerged roots. They are confident though that aeration was at least one major factor in their evolution.

You can see these knees in what we appropriately call the Taxodium Lawn, between the William Tell Rest House and The Terrace cafe, on Ornamental Lake at Melbourne Gardens. I'm not sure how much the knees are trimmed during mowing but it can't make that job particularly pleasant for our horticulturists.

There are other Taxodiums around the lake, mostly, I think, Taxodium mucronatum, the Montezuma Cypress. This species is less pyramidal in shape and tends to have weeping branches. It also lacks knees. Although it makes up for this by keeping its leaves in winter, unlike the Bald Cypress, named after this deciduous trait (in fact it's sometimes called more explicitly, the Deciduous Swamp Cypress). Both species come from swampy places in southern North America and Central America.

The taxonomy of the specimens in horticulture seems to be a bit of mess (according to Roger Spencer's Horticultural Flora of South-eastern Australia) but generally the Bald Cypress has longer leaves (about 10-15 mm), no leaves in winter and nice set of pneumatophores.

Images: Apart from the knobble photographed at South China Botanical Garden, all images are of the specimen of Bald Cypress on Taxodium Lawn, Melbourne Gardens, taken in March 2017.