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
Low carbon use efficiency in tropical forests: Potential causes and implications.
Chambers, Jeffrey1, 2, Tribuzy, Edgard2, 3, Teixeira, Liliane2, Higuchi, Niro2, 1 Tulane University, New Orleans, USA2 INPA, Manaus, Brazil3 ESALQ - USP, Piracicaba, Brazil
ABSTRACT- The fraction of carbon assimilated in photosynthesis that is ultimately used to produce new tissues is often referred to as carbon use efficiency (CUE). CUE is an important ecophysiological parameter in describing differences in basic functioning among ecosystems. We have carried out extensive field studies of productivity, autotrophic respiration, and photosynthesis on canopy trees in the Central Amazon to better understand factors controlling CUE variability. First, a study comparing autotrophic respiration with net primary production demonstrated that only about 30% of carbon assimilated in photosynthesis was used to construct new tissues, with the remaining 70% being respired back to the atmosphere as autotrophic respiration, differing considerably from the relatively constant CUE of about 0.5 found for temperate forests. Expanding these studies to include an experimentally logged site, showed an increase in woody tissue CUE for surviving trees experiencing elevated growth rates, in disagreement with many ecological models assuming a constant CUE. Comparing woody tissue respiration studies at the logged site and in nearby undisturbed forests demonstrated that woody tissue carbon dioxide efflux for trees of equivalent size and growth rate was about 50% less in logged than primary forest. Finally, a study of net photosynthetic response of canopy leaves to changes in both temperature and carbon dioxide concentration demonstrated that high leaf temperatures often resulted in post-midday conditions of low carbon assimilation and high leaf respiration rates. Extremely low post-midday stomatal conductance often resulted in leaf temperatures reaching 40-50 degrees C, significantly above our quantified forest-wide photosynthetic optimum leaf temperature of 30.9 degrees C. A number of potential causes for variability in CUE are plausible and will be discussed, including low nutrient availability, carbon over-supply, temperature, and changes in other ecophysiological parameters.
Key words: tropical forest, photosynthesis, respiration