Influence of sunflecks on the δ 13 C of Adenocaulon bicolor plants occurring in contrasting forest understory microsites.

Leaf characteristics and carbon isotope ratios (δ13C) of Adenocaulon bicolor were examined in the understory of a redwood forest along a gradient of microsites that differed in the amount of direct (sunfleck) photon flux density. Comparisons were made between plants that had been shaded from sunflecks with shadow bands but still received diffuse light, and adjacent plants that received both sunflecks and diffuse light. The δ13C of the shaded plants were 1.2‰ lower than predicted from the intercellular CO2 pressure (pi), probably because of recycling of respired CO2 in the understory. Plants receiving sunflecks had higher δ13C values because assimilation during sunflecks occurred at a lower pi than assimilation in diffuse light. The amount that their δ13C was higher was positively correlated with predicted direct photon flux density received by a plant. Leaf weight per unit area increased with increasing PFD. Although plants receiving sunflecks had greater leaf weights per unit area and photosynthetic capacities than those under shadow bands, there was no apparent acclimation of photosynthetic capacity to the differences in PFD among the microsites.