Taxodium TAXONOMIC clusterf*ck in Central Texas
For years, the “Cypress” (not a true cypress, which are Western North American trees in the genus Hesperocyparis; and Cupressus in Eurasia) trees in central Texas have been referred to as Taxodium distichum, which they are explicitly NOT due to a number of shared morphological as well as ecological trademarks possessed by the Mexican species, Taxodium mucronatum.
The Flora of North America section covering the genus Taxodium - which was written in 1985 - for whatever reason decided that these trees are Taxodium distichum, which is a tree of Eastern Swamps.
I generally hate lumpers (as opposed to “splitters”) because in lumping species together, taxonomists can sometimes obscure blatant morphological and ecological differences.
In the case of Taxodium mucronatum vs. Taxodium distichum, to pretend that millions of years of evolution in drastically different habitats has had no discernible effect on the traits of the two species is silly.
First, let's discuss the habitat differences…
Taxodium mucronatum in Mexico (and the Rio Grande Valley of Texas) occupies streams and creeks (sometimes dry) in seasonally arid climates. This habitat makes Taxodium mucronatum much more likely to experience drought stress than Taxodium distichum (which almost never experiences drought stress).
Taxodium distichum in Louisiana, Mississippi (and on up in to Southern Illinois) occupies boggy swamps that NEVER fully dry out. Likely because of it's perpetually wet habitat and very soft, water-logged soil Taxodium distichum almost always forms “knees” if it's roots are submerged. It was once thought these “pneumatophores” were an adaptation to helping the trees roots acquire oxygen. This seems pretty unlikely, and it much more reasonable to assume that the adapti benefit of these “knees” is that they aid in stabilization.
Why is it likely that the central Texas Taxodium species is Taxodium mucronatum? First, San Antonio and Austin Texas are much more ecologically and climatically in line - comparatively speaking - with the habitat of South Texas and Eastern Mexico, which are decidedly NOT swamps.
Second, the WILD (not planted, as many Taxodium distich have been planted in places like San Marcos, Texas) Taxodium that occurs in Central Texas in places like Austin, San Antonio, San Marcos, Seguin and even East to Uvalde never form “knees” but have roots that instead form rebar-like cages over the stream banks. These trees also have a much different habit from Taxodium distichum when full grown. Taxodium distichum tends to get taller than and stay much narrow from Taxodium mucronatum, which tends to be a much stouter bastard and spreads out (take El Arbol del Tule in Oaxaca, for example).
Also, as one would expect given the differences of habitat, Taxodium mucronatum also displays a greater drought tolerance (though still not much) in cultivation then Taxodium distichum. Having grown them both in South Texas, which is remarkably drier than the swamps of Eastern Texas and Louisiana, I can confirm this.
Why does this matter if not for some petty squabble among taxonomists? When we ignore the vast differences between two species in both their evolution and ecology, we can disturb ecosystems by planting the wrong tree in restoration projects, meaning the project fails when an ecologically inappropriate (read: not adapted to drought) species later fails during exposure to an ecological condition (ie drought) to which it has not much experienced in its evolutionary history. Taxodium mucronatum has had a few million years of selection pressures in the hot-as-balls and sometimes VERY dry habitat of Central Texas - an ecosystem and set of conditions in which Taxodium distichum has explicitly not (nevermind the possible differences in adaptation to PH regarding the acidity of an Eastern Swamp versus the limestone rocky-stream-bank substrate of Central Texas.
Both species are wonderful plants and exceptional trees, remnants of the Jurassic like many members of their family (Cupressaceae), but ecology and evolution matter, so let's not skip over those stories for the sake of being taxonomically lazy.
