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The distribution of forest cover alters Earth surface mass and energy exchange and is controlled by physiology, which determines plant environmental limits. Ancient plant physiology, therefore, likely affected vegetation-climate feedbacks. We combine climate modeling and ecosystem-process modeling to simulate arboreal vegetation in the late Paleozoic ice age. Using GENESIS V3 global climate model simulations, varying pCO2, pO2, and ice extent for the Pennsylvanian, and fossil-derived leaf C:N, maximum stomatal conductance, and specific conductivity for several major Carboniferous plant groups, we simulated global ecosystem processes at a 2° resolution with Paleo-BGC. Based on leaf water constraints, Pangaea could have supported widespread arboreal plant growth and forest cover. However, these models do not account for the impacts of freezing on plants. According to our interpretation, freezing would have affected plants in 59% of unglaciated land during peak glacial periods and 73% during interglacials, when more high-latitude land was unglaciated. Comparing forest cover, minimum temperatures, and paleo-locations of Pennsylvanian-aged plant fossils from the Paleobiology Database supports restriction of forest extent due to freezing. Many genera were limited to unglaciated land where temperatures remained above −4 °C. Freeze-intolerance of Pennsylvanian arboreal vegetation had the potential to alter surface runoff, silicate weathering, CO2 levels, and climate forcing. As a bounding case, we assume total plant mortality at −4 °C and estimate that contracting forest cover increased net global surface runoff by up to 6.1%. Repeated freezing likely influenced freeze- and drought-tolerance evolution in lineages like the coniferophytes, which became increasingly dominant in the Permian and early Mesozoic.

Significance Massive carbon (C) release with abrupt warming has occurred repeatedly during greenhouse states, and these events have driven episodes of ocean deoxygenation and extinction. Records from these paleo events, coupled with biogeochemical modeling, provide clear evidence that with continued warming, the modern oceans will experience substantial deoxygenation. There are, however, few constraints from the geologic record on the effects of rapid warming under icehouse conditions. We document a C-cycle perturbation that occurred under an Earth system state experiencing recurrent glaciation. A suite of proxies suggests increased seafloor anoxia during this event in step with abrupt increase in CO 2 partial pressure and a biodiversity nadir. Warming-mediated increases in marine anoxia may be more pronounced in a glaciated versus unglaciated climate state.