PARENT SESSION
Organized Oral Session 22: Comparative ecology of tropical trees: Linking physiology to dynamics and distribution
Organizer(s): SC Thomas and KE Harms
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal
Dominance of legumes in the Neotropics: Symbiotic nitrogen fixation contributes to high photosynthetic rates.
Santiago, Louis*,1, Qunital-Tun, Fausto2, Andrade, Jose2, Dawson, Todd1, 1 University of California, Berkeley, CA, USA2 CICY, Mérida, Yucatán, México
ABSTRACT- Legumes are members of a select set of plant families with the potential to fix atmospheric nitrogen (N). They are also the most dominant family in tropical America and Africa. Our objective was to determine the number of legume species that do in fact fix N along a successional gradient (5-10, 10-20, 20-30 years) in dry tropical forest in the Yucatan region, Mexico. Further, we wanted to ascertain whether N-fixation is associated with any physiological advantages that could help explain the abundance of these legumes in this tropical forest region. N-fixing status was determined by root excavation to verify the presence of nodules and with measurements of N fixation activity using the acetylene reduction assay. The proportion of N derived from biological fixation was estimated using leaf N isotopic composition. We also estimated the proportion of foliar N within each successional stand that was derived from biological N fixation. We found that 21 out of 23 legume species were nodulated and these species had significantly higher leaf N concentration and photosynthetic rates than non-fixing species, suggesting that N-fixation contributes to high potential growth rates. The percent of N derived from biological fixation for individual species varied from less than 1% to 78% across all successional stages. Percent stand foliar N derived from biological fixation was 9, 15, and 18, in the earliest, middle and later successional stages, respectively. Overall, the results support the hypothesis that N fixation contributes to the dominance of legumes in tropical forest by increasing plant N content which in turn appears to enhance photosynthetic rates and potential growth rates.
Key words: tropical forest, photosynthesis, nitrogen fixation, Yucatan, Mexico