A process-based ecosystem model (Paleo-BGC) to simulate the dynamic response of Late Carboniferous plants to elevated O₂ and aridification

Abstract

Ecosystem process models provide unique insight into terrestrial ecosystems by employing a modern understanding of ecophysiological processes within a dynamic environmental framework. We apply this framework to deep-time ecosystems made up of extinct plants by constructing plant functional types using fossil remains and simulating—as close as possible—the in vivo response of extinct taxa to their paleoclimatic environment. To accomplish this, foliar characteristics including maximum stomatal conductance, distance from leaf vein to stomata, and cuticular carbon and nitrogen were input as model parameters derived from measurements of well-preserved Pennsylvanian-age fossil leaves. With these inputs, we modeled a terrestrial tropical forest ecosystem dominated by “iconic” plant types of the Pennsylvanian (∼323–299 Ma) including arborescent lycopsids, medullosans, cordaitaleans, and tree ferns using a modified version of the process model BIOME-BGC, which we refer to as Paleo-BGC. Modeled carbon and water—and, for the first time, nitrogen—budgets of a tropical ecosystem from Euramerica driven by daily meteorology are simulated using the Global Circulation Model GENESIS 3.0. Key findings are:

1. lycopsids have the lowest daily leaf water potential, soil water content, surface runoff, and degree of nitrogen leaching indicating an intensive water use strategy compared to medullosans, cordaitaleans, and tree ferns that have increasingly lower simulated water use, greater surface, and nitrogen loss in this order;

2. modeled vegetation response to aridification, which was caused by reduced precipitation and intensified through the close of the Carboniferous and into the Permian shows that lycopsids and medullosans have the lowest tolerance for precipitation decrease compared to cordaitaleans and tree ferns, consistent with the paleobotanical record of occurrence of floral turnovers through the Middle Pennsylvanian through earliest Permian;

3. elevated atmospheric pO₂, hypothesized as characteristic for the latter half of the Pennsylvanian and early Permian (∼299–272 Ma), caused higher atmospheric pressure reducing plant transpiration, higher surface water runoff rates, and increased nitrogen export for all plant types simulated, manifested most strongly in the lycopsid dominated ecosystems—with overall only a small reduction in net daily assimilation (≈1 μmol CO₂ m⁻² s⁻¹).

Both aridification and elevated atmospheric oxygen reduced transpiration, increased water retention in soils, with higher surface runoff. With more discharge, enhanced and higher short-term surface soil loss and silicate weathering would have been possible in broad regions of the paleotropics during the late Carboniferous and early Permian. These results are only obtainable by integrating multiple, fossil-derived measurements into the simulation framework of an ecosystem model that utilizes daily meteorology.