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C-4 photosynthesis is absent from the arborescent life form, with the exception of seven Hawaiian Euphorbia species and a few desert shrubs that become arborescent with age. As a consequence, wherever C-3 trees can establish, their height advantage enables them to outcompete low stature C-4 vegetation. Had C-4 photosynthesis been able to evolve in an arborescent life form, forest cover (by C-4 trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C-4 trees. Physiological explanations associated with low light performance of C-4 photosynthesis are not supported, because C-4 shade-tolerant species exhibit similar performance as shade-tolerant C-3 species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C-4 trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C-4 lineages are more plausible. Most C-4 species are grasses and sedges, which lack meristems needed for arborescence, while most C-4 eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C-4 eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C-4 evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C-4 lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands. (C) 2016 Elsevier GmbH. All rights reserved.