Infodoc : Sciences, Techniques et Culture

Differences in distribution among plant species across environmental gradients are often assumed to be due to tradeoffs in organismal traits, individual performance, or both. In this thesis, I investigated how tradeoffs among neotropical tree species might lead to differences in their distirbution in contrasting environments, linking the three levels at which tradeoffs might occur : organismal traits, individual performance and population structure. I first conducted a survey of traits for seeds and seedlings of 202 tree species at Paracou, French Guiana, and found that taxa with larger seeds generally had seedlings with hypogeal cotyledons and taproots. While larger-seeded taxa were associated with larger seedlings, they also had lower maximum relative growth rates and lower specific[...]

Differences in distribution among plant species across environmental gradients are often assumed to be due to tradeoffs in organismal traits, individual performance, or both. In this thesis, I investigated how tradeoffs among neotropical tree species might lead to differences in their distirbution in contrasting environments, linking the three levels at which tradeoffs might occur : organismal traits, individual performance and population structure. I first conducted a survey of traits for seeds and seedlings of 202 tree species at Paracou, French Guiana, and found that taxa with larger seeds generally had seedlings with hypogeal cotyledons and taproots. While larger-seeded taxa were associated with larger seedlings, they also had lower maximum relative growth rates and lower specific leaf area. I then studied differences among a total of 31 species differing in seed size and seedling morphology, examining performance response to multiple environmental variables at fine spatial scales. At the seed-seedlings stage, no tradeoffs occured for survival or growth across a wide range of conditions; larger seeds performed better in all environments. In contrast, species-environment interactions for the relative growth rates of older seedlings resulted in a shift in the performance hierarchy between soil types and light environments. However, this tradeoff did not reflect the tradeoffs in trait observed among seedlings. Established seedlings more than one year in age did not exhibit tradeoffs in growth. Moreover, traits related to initial performance, such as seed size, were not associated with growth rates over a two-year period. The lack of tradeoffs I found for individual performance was consistent with the few differences in distribution of juveniles that I observed in a 1.5 hectare permanent plot census. For all 15 species studied, juveniles coexisted across environmental gradients without partitioning microhabitats. These results suggest that competitive hierarchies during regeneration are not strong forces influencing seedling community structure in neotropical forests. The early structuring of these communities may largely determined by life history tradeoffs related to seed size, independent of environment. In contrast, subsequent growth of juveniles appears to be uncoupled from initial because species-environment interactions occur that are governed by morphological tradeoffs unrelated to seed size.