Arborescent lycopsid productivity and lifespan: Constraining the possibilities

Abstract

One of the most enigmatic components of early terrestrial vegetation was the arborescent lycopsids. Because of the sheer abundance of their biomass in many wetland environments of the Late Paleozoic, they may have been an important variable in the global carbon cycle and climate. However, their unusual structure has invited extraordinary interpretations regarding their biology. One idea that has persisted in the literature for over forty years is that these trees had extremely short lifespans, on the order of ten years. Such an accelerated lifecycle would require growth rates twenty times higher than modern angiosperm trees (and at least 60 times higher than modern lycopsids). Here, we evaluate the morphology and anatomy of lycopsid trees-including aerenchyma, phloem, leaf base distributions, leaf structure, rootlet anatomy, and the demography of the preserved fossils-with comparison to modern plants with some similarity of overall form, most notably the palms. The environmental context of lycopsid trees also is considered in the light of the vegetation of modern water-saturated substrates. It is concluded that such rapid growth would violate all known physiological mechanisms. One hypothetical mechanism that had been proposed to provide for increased carbon fixation, a unique photosynthetic pathway, could not have been viable in these plants and there is no accounting for the increases in nitrogen and phosphorous uptake that would be necessary to sustain enormous rates of carbon fixation. Of the various aspects of lycopsid anatomy and ecology that might militate against this conclusion that productivity was not high, no line of evidence requires a uniquely rapid growth rate for the arborescent lycopsids and several lines of evidence seem to prohibit it. Thus, we conclude that the lifespans of arborescent lycopsids most likely were measured in centuries rather than years. These trees should not be expected to have been unique outliers with physiological function completely distinct from all other tracheophytes. Furthermore, they require no special consideration in the evaluation of Paleozoic biogeochemical cycling. Finally, the conclusion that lycopsid lifespans were an order of magnitude longer than previous expectations invites reconsideration of many other aspects of their ontogeny, physiology, and structure.