Distribution of Carbon Uptake Capacity of Plant Functional Groups Across the Canopy Gradient in Old-Growth Tropical Wet Forest in Costa Rica

Because of the difficulties of accessing leaves within tree crowns, little is known about the photosynthetic capacity of different functional groups within tropical rain forest canopies. To address this deficiency, we measured photosynthetic capacity (A_max) in situ along vertical transects through old-growth forest canopy using a mobile walkup tower at the La Selva Biological Station in Costa Rica. We asked: What groups are responsible for most C-fixation and at what height in the canopy does most C-fixation occur? Photosynthesis (using a LI-COR Li-6400) and total leaf area were measured for all vascular plant species encountered within the tower footprint (4.6 m²). Plants were grouped into trees, palms, ferns, lianas, epiphytes, herbs, Pentaclethra macroloba (the dominant canopy tree), and vines. A_max values differed among functional groups. The ranking of A_max among the groups was trees > P. macroloba > palms > lianas > vines > epiphytes > herbs > ferns. Trees and P. macroloba had the highest photosynthetic rates, but the maximum rates occur at different heights. A_max of P. macroloba increases with canopy height to a maximum 10.3 \mumol m^-2 s^-1 at 17.5 m. A_max of trees increases with canopy height (r² = 0.77) and attains the highest A_max at 32.5 m (10.6 \mumol m^-2 s^-1). Palms and lianas presented similar patterns of A_max. However, lianas reach the canopy top whereas palms are shorter and were not observed above 27.5 m. The maximum photosynthetic rates for both groups were: lianas 9.2 \mumol m^-2 s^-1 at 27.5 m and palms 9.6 \mumol m^-2 s^-1 at 17.5 m. By scaling the functional group A_max values with their leaf area, we estimated that most of the photosynthetic capacity occurs between 17.5 m and 37.5 m and is attributed mainly to trees, followed by P. macroloba and then lianas.