Résumé
The earliest forests composed of canopy trees exceeding 2 m in height date from the Middle Devonian. The best example is provided by the paleosol and associated fossils of Givetian age recorded at Riverside Quarry, Gilboa, New York. The canopy trees in these early forests belong to the Pseudosporochnales which, above ground, consist of a trunk topped by a small crown of short-lived branches. If the advent of trees and forests is believed to have deeply changed the composition of the atmosphere during the Devonian, this contribution has never been quantied. As a contribution to resolve this challenge, we present here an attempt to calculate the above-ground biomass and carbon content stored in a Middle Devonian forest structured like that reported at Riverside Quarry.The first step consists of modeling of a 3-m high Pseudosporochnus tree using the AmapSim software and its computer simulator. The carbon content of the tree and of its components is calculated at any time during growth using the mean carbon density of two Carboniferous plants of comparable structure, Psaronius and Medullosa. The carbon content of a fully grown tree ranges between 837 and 1,300 g C. At 40% of development and beyond, most of the carbon is stored in the trunk. However, when considering the cumulative amount of carbon of the tree, most of the carbon synthethized during its life has gone to the construction of the short-lived branches.Given some assumptions on the density and age of the trees, the carbon content of a Pseudosporochnus forest ranges between 4.3 and 15.5 t C/ha. These values are relatively low compared to those of extant forests, the closest analogs in terms of productivity being either thickets of young trees or environmentally constrained forests. The accuracy of the model is discussed and any conclusion about an adaptation of pseudosporochnalean forests to constrained environments must be taken with great care. Yet, it is interesting to note that previous authors reporting forests around Gilboa suggested that they inhabited either stressed or disturbed environments.