Annals of Botany

Gas exchange of the understorey epiphyte Aspasia principissa was studied in fluctuating light conditions both in the laboratory and in the field, testing the hypothesis that vascular epiphytes differ from most terrestrial understorey plants in showing a higher priority for water conservation. Consequently, a slow response of stomatal conductance to sudden increases in incident photon flux density (PFD) was expected, as was a fast loss of induction after such a light fleck. Results were only partly consistent with these expectations. Full induction of photosynthesis was indeed very slow and was not reached before, respectively, 40 and 60 min of saturating PFD in the field and the laboratory. In contrast, kinetics of induction loss were comparable with those of most terrestrial species studied to date. The overall impact of light flecks on in situ carbon gain again fulfilled expectations, being rather limited: the observed carbon gain was only approx. 66 % of the potential carbon gain estimated from a square-wave response model. It is concluded that in the drought-prone epiphytic habitat of a moist lowland forest, water conservation takes priority over carbon gain, which severely limits the use of light flecks for CO₂ fixation in vascular epiphytes.