Why is C4 photosynthesis so rare in trees?

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.

Repeated range expansion and niche shift in a volcanic hotspot archipelago: Radiation of C Hawaiian subgenus (Euphorbiaceae).

Woody perennial plants on islands have repeatedly evolved from herbaceous mainland ancestors. Although the majority of species in subgenus section (Euphorbiaceae) are small and herbaceous, a clade of 16 woody species diversified on the Hawaiian Islands. They are found in a broad range of habitats, including the only known C plants adapted to wet forest understories.

New and resurrected Hawaiian species of pilo (Coprosma, Rubiaceae) from the island of Maui.

Two species of Coprosma (Rubiaceae) J.R.Forst.

A new species of Cyanea (Campanulaceae, Lobelioideae), from the Ko'olau Mountains of O'ahu, Hawaiian Islands.

Cyaneakonahuanuiensis Sporck-Koehler, M. Waite, A.M.