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

Landscape-scale structure of leaf area distribution in a tropical rain forest.

Clark, David^{*,1, 2}, Olivas, Paulo¹, Oberbauer, Steven³, Clark, Deborah^{1, 2}, Ryan, Michael⁴, ¹ La Selva Biological Station, Puerto Viejo, Heredia, Costa Rica² University of Missouri - St. Louis, St. Louis, Missouri, USA³ Florida International University, Miami, Florida, USA⁴ USDA/FS Rocky Mountain Research Station, Ft. Collins, Colorado, USA

ABSTRACT- Current estimates of tropical rain forest (TRF) carbon stocks and fluxes are poorly constrained due to a variety of technical issues. The TOWERS project is using multiple approaches develop independent estimates of GPP, NPP, and NEE over an old-growth upland TRF landscape at the La Selva Biological Station, Costa Rica. A core element of the project is measuring the vertical and horizontal distribution of plant biomass using a random design stratified by soil nutrients and slope conditions. A GIS was used to map 515 ha area of old growth into 10 x 10 blocks of high, medium, or low conditions of slope and total soil P, resulting in a 9 x 9 matrix with approximately 5700 blocks in each cell. Within each of the 9 categories five sites were selected using random coordinates. At each of these sites a walk-up canopy tower was erected. Within the 1.86 x 2.45 m footprint all plant biomass was harvested, stratified by tower section (height above ground) and plant functional group. Mean landscape-level Leaf Area Index (LAI) was 5.99 + 0.33 (1 SEM). Trees (55%), palms (22%) and lianas (13%) accounted for 90% of total LAI, while herbaceous epiphytes and climbers, understory herbs and ferns accounted for the remainder. The first canopy LAI layer, the site of highest energy absorption and gas exchange, was conservatively estimated to average 2.79 m in depth. At the tower footprint scale (ca. 5 m²), LAI was not related to soil P or slope, and forest height explained only 12% of the variation in total LAI. The weak relation of forest height to total LAI is partially explained by the rarity of low canopy sites in a random sample. In a sample of additional sites selected to span canopy heights from 0 - 20 m, total LAI was highly correlated with canopy height (r²=0.70), reaching the landscape average LAI at 17 m. We discuss the implications of these results for analysis of TRF landscape canopy structure, modeling of carbon uptake and interpretation of high resolution remote sensing data.

Key words: carbon cycling, tropical rain forest, landscape sampling, Leaf Area Index