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Societal Impact Statement: C4 photosynthesis is an ultra‐efficient mode of photosynthesis found in some of our most productive crop species yet is notably rare in trees. Given C4 photosynthesis is associated with high yield in herbaceous species, especially under hot and dry conditions, C4 trees may seem an attractive prospect for biomass production and carbon sequestration in a rapidly changing climate. This may explain why some in the literature have optimistically linked C4 photosynthesis with the exceptionally fast‐growing tree Paulownia. However, this claim is lacking in evidence and represents an example of poor citation practices leading to the spread of misinformation. Summary: The rapid growth of trees in genus Paulownia (Paulowniaceae) has been attributed in the literature to their use of C4 photosynthesis, a complex trait that confers increased photosynthetic efficiency under certain environmental conditions. After careful examination of citations used to support the idea that Paulownia species use C4 photosynthesis, we find that there is no data underpinning this claim. Despite this, many investment schemes utilise information about the physiology of Paulownia, including photosynthetic type, to legitimise the use of Paulownia trees for financial investment and carbon offsetting.This study uses leaf physiology, anatomy and stable isotope data to determine whether or not three species in Paulownia (Paulowniatomentosa, Paulowniafortunei and Paulowniakawakamii) use C4 photosynthesis. These data are compared with existing data for C3 and C4 woody species in the literature.We show that the leaf physiology, anatomy and stable isotope phenotypes of the three Paulownia trees considered in the study are not consistent with those of C4 plants.Our findings highlight how inaccurate citation of scientific findings can contribute to the spread of misinformation beyond the scientific community, as some of those promoting investments in Paulownia plantations reference the photosynthetic superiority of Paulownia as a means to legitimise its use in carbon offsetting. [ABSTRACT FROM AUTHOR]

Since C4 photosynthesis was first discovered >50 years ago, researchers have sought to understand how this complex trait evolved from the ancestral C3 photosynthetic machinery on >60 occasions. Despite its repeated emergence across the plant kingdom, C4 photosynthesis is notably rare in trees, with true C4 trees only existing in Euphorbia. Here we consider aspects of the C4 trait that could limit but not preclude the evolution of a C4 tree, including reduced quantum yield, increased energetic demand, reduced adaptive plasticity, evolutionary constraints, and a new theory that the passive symplastic phloem loading mechanism observed in trees, combined with difficulties in maintaining sugar and water transport over a long pathlength, could make C4 photosynthesis largely incompatible with the tree lifeform. We conclude that the transition to a tree habit within C4 lineages as well as the emergence of C4 photosynthesis within pre-existing trees would both face a series of challenges that together explain the global rarity of C4 photosynthesis in trees. The C4 trees in Euphorbia are therefore exceptional in how they have circumvented every potential barrier to the rare C4 tree lifeform. [ABSTRACT FROM AUTHOR]