PARENT SESSION
Organized Oral Session 31: Structure and function of tropical rainforest canopies
Organizer(s): MG Ryan and DA Clark
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 516 A, Level 5, Palais des congrès de Montréal

Photosynthetic characteristics within the canopy of old-growth tropical rain forest in Costa Rica; relationships with height, light, and plant functional type.

Ordoñez, Harlyn^*,1, Oberbauer, Steven ², Ryan, Michael ³, Clark, David⁴, Clark, Deborah⁴, Olivas, Paulo¹, ¹ Organization for Tropical Studies La Selva Biological Station, Puerto Viejo, Sarapiqui, Costa Rica² Florida International University, Miami, FL³ USDA Forest Service Rocky Mountain Research Station, Ft. Collins, CO⁴ University of Missouri-St. Louis, St. Louis, MO

ABSTRACT- The logistical challenges of accessing leaves within tall tree crowns have limited the data available on the photosynthetic characteristics within tropical rain forest canopies. To address this deficiency, we measured in situ photosynthetic capacity (A_max), and CO₂ and light responses along vertical transects through old-growth forest canopy using a mobile walkup tower at the La Selva Biological Station in Costa Rica. We asked: How do photosynthetic characteristics change with height and light level in the canopy, and how do photosynthetic characteristics of the functional groups that comprise the canopy differ? Photosynthesis characteristics (using a LI-COR Li-6400) and total leaf area were measured for all vascular plant species encountered within each 1.9 m vertical section of 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, maximum electron transport capacity (J_max), and CO₂ compensation points increased with canopy height. A_max increased linearly with the fraction of light reaching leaves estimated from the amount of leaf area above. 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 among the different groups occurred at different heights. A_max of P. macroloba increases with canopy height to a maximum 10.3 mol m^-2 s^-1 at 17.5 m. 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 mol m^-2 s^-1 at 27.5 m and palms 9.6 mol m^-2 s^-1at 17.5 m.

Key words: canopy tower, la selva