All epiphytes photosynthesize, but certain life stages of several taxa are heterotrophic – for example, gametophytes of arboreal Lycopodium, Ophioglossum, Psilotum, and Tmesipteris. Similarly, orchid seedlings remain achlorophyllous for weeks to months, subsisting on substrates provided by symbiotic fungi (Figs. 4.10, 4.12). Flow of fungal metabolites into adult orchids may also occur, but claims for epiparasitism in canopy-based Orchidaceae (e.g., Ruinen 1953; Johansson 1977) need confirmation using labeled nutrients. As for dwarf mistletoes, utilization of host substrates has been great enough to allow considerable leaf reduction; most of the vegetative body is endophytic.

The question now is how autotrophy operates in canopy-adapted vegetation. This chapter will examine (1) photosynthetic pathways among epiphytes, two of which are certain, the third equivocal; (2) accommodation of carbon balance to mineral scarcity and shade; (3) segregation of co-occuring populations along light gradients; (4) photosynthetic phenomena peculiar to certain specialized taxa; (5) ancestral habitats; and (6) the economics of epiphyte foliage versus that of phorophytes. The interrelationship between photosynthesis and water balance is covered more thoroughly in Chapter 3.

Photosynthetic pathways

The reductive pentose phosphate (C3) pathway

Machinery for trapping radiant energy, perfected in plants long before land was colonized, required tailoring as the terrestrial flora developed. Light intensity and quality, supplies of moisture, nitrogen (N), and presumably other key nutritive elements, influenced selection during the subsequent radiation, up to and including colonization of tree crowns.