Summary: A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo‐ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait‐based ecology, to evaluate which well‐established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait‐based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi‐quantitative ranking of 26 paleo‐functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo‐ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo‐demography (life history), and Earth system history can be derived through the application of paleo‐functional traits to fossil plants.

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 Computer-assisted studies of natural history that consider extinct plant function contribute to the understanding of how Paleozoic glacial cycles controlled the distribution of forest cover and continental surface erosion. Simulated plant water balance supports widespread vegetation during the late Paleozoic ice age (LPIA). However, physiological inference suggests that plant freezing limited the geographic distribution of vegetation. Assuming LPIA plants had limited freeze tolerance, we found that increased surface runoff could have contributed to late Paleozoic climate change. Modeling that combines deep time climate reconstructions and paleobotanical data improves understanding of past Earth systems, which can help project future change.

Magnetic switchbacks are large amplitude deflections of the magnetic field within the solar wind. They are Alfvénic in character and so are associated with a spike in velocity and a generally small variation in local plasma density. Early orbits of Parker Solar Probe revealed that the solar wind near the Sun is dominated by these structures, and therefore, they may be playing an important role in the energy budget and acceleration of the young solar wind. In this review, we present an overview of different mechanisms that have been proposed for how switchbacks could be formed. We group the mechanisms by whether they predominantly act in the low solar atmosphere or within the solar wind (in situ). We focus on mechanisms that can create reversals of the ambient magnetic field direction and, thus, account for the most extreme perturbations. The general consensus is that mechanisms in the lower solar atmosphere do not form such reversals on their own but provide the seed perturbations, flows, or particle beams necessary for in situ mechanisms to create switchbacks within the solar wind. Switchback observations thus likely contain an imprint of the coronal source of the seed perturbation or flow, which is evolved further locally by one of several plausible in situ mechanisms. We discuss the strengths and weaknesses of each mechanism and outline future observational and theoretical tests that could help differentiate between them.

Aim: Plants are hypothesised to have changed in function, biogeography and environmental impact throughout the Phanerozoic. The fossil record preserves large‐scale shifts in water use traits with evolution. We test how time‐appropriate plants modified their environment differently based on their water use traits and where they survived during Earth's penultimate icehouse using a computer model. Location: Global Pangea. Time Period: The Late Palaeozoic Ice Age (LPIA, 330–290 Ma). Major Taxa Studied: Iconic plant groups of the LPIA—marratialean tree ferns, arborescent lycopsids, medullosans and early diverging conifers. Methods: We utilise fossil‐preserved water‐use traits to inform a fundamental biophysical trade‐off in simulations using the palaeo‐ecosystem model Paleo ‐BGCv2.0. Simulations were driven by atmospheric conditions from an Earth System Model (CESMv1.2) for the Carboniferous (~300 Ma) to test hypotheses regarding LPIA vegetation–environment interactions. We use these simulations to infer arborescent vegetation cover and frame simulation differences so that they may be compared to two large‐scale, well‐documented community turnovers late in the LPIA, using end‐member CO ₂ concentrations and plant types to evaluate potential feedbacks to water availability. Results: Simulated forest cover varied widely in global terrestrial extent, from less than 5% to greater than 65% cover. This difference was the result of fossil‐informed water‐use traits corresponding to two broad categories of LPIA plants: wet‐ and dry‐adapted. The addition of stem physiology in this work restricts wet‐adapted plants to high‐precipitation regions. Expanded biogeography interacted with the prolonged water use of dry‐adapted plants to reduce site water balance. The transition to dry‐adapted plant types with constant CO ₂ reduced runoff up to 36%, as compared to halving CO ₂ , which reduced runoff up to 95%. Main Conclusions: Dry‐adapted plants likely created environments that promoted their success during the LPIA, whereby continued water use under dry conditions further dried soils and limited the abundance of drought‐sensitive plants, thereby constituting an eco‐evolutionary feedback. The representation of interacting plant traits in this palaeo‐ecosystem modelling framework, increasingly representing whole plants, opens pathways to studying ecosystems dominated by extinct, non‐analogue plants. Expanding application of the fossil record may improve our understanding of vegetation impacts on the terrestrial environment across deep time.