title: Why are there no C4forests?
abstract: C4photosynthesis 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 C3trees can establish, their height advantage enables them to outcompete low stature C4vegetation. Had C4photosynthesis been able to evolve in an arborescent life form, forest cover (by C4trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C4trees. Physiological explanations associated with low light performance of C4photosynthesis are not supported, because C4shade-tolerant species exhibit similar performance as shade-tolerant C3species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C4trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C4lineages are more plausible. Most C4species are grasses and sedges, which lack meristems needed for arborescence, while most C4eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C4eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C4evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C4lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON M5S3B2, Canada. Electronic address: r.sage@utoronto.ca.
authors.forename: Rowan F
authors.initials: RF
authors.lastname: Sage
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON M5S3B2, Canada.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Sultmanis
mesh_terms: D002244:Carbon / Q000378:metabolism*; D017753:Ecosystem; D065928:Forests; D008954:Models, Biological; D010788:Photosynthesis; D014197:Trees / Q000502:physiology
chemical_list: D002244:Carbon
keywords: C(4) photosynthesis; Fire; Quantum yield; Savanna; Shade adaptation; Vascular cambium; Woodlands