Effects of Canopy Position and Irradiance on the Leaf Physiology and Morphology of Pentaclethra Macroloba (Mimosaceae)

Pentaclethra macroloba (Willd.) Kuntze (Mimosaceae) is a dominant late‐successional tree species in the Atlantic lowland forests of Costa Rica. Leaves of P. macroloba from three heights in the forest canopy were compared with leaves of seedlings grown in controlled environment chambers under four different irradiance levels. Changes in leaf characteristics along the canopy gradient paralleled changes resulting from the light gradient under controlled conditions. The effect of light or canopy position on light‐saturated photosynthesis was small, with maximum photosynthesis increasing from 5 to 6.5 μmol m −‐2 s −‐1 from understory to canopy. Both chamber grown and field leaves showed large adjustments in photosynthetic efficiency at low light via reductions in dark respiration rates and increases in apparent quantum yields. Light saturation of all leaves occurred at or below 500 μmol m −‐2 s −‐1 . Leaf thickness, specific leaf weight, and stomatal density increased to a greater extent than saturated photosynthesis with higher irradiance during growth or height in the canopy. As a result, there was a poor correspondence between leaf thickness and light‐saturated photosynthesis on an area basis. It is concluded that Pentaclethra macroloba possesses the characteristics of a typical shade‐tolerant species.