Tree functional hyperdominance in a hyperdiverse forest in Central Amazon or Why a tropical forest may not be irreducibly complex

Background/Question/Methods

It has long being held that before understanding community assembly we must know how species interact with each other. This pairwise approach led us to believe that species-rich systems such as tropical forests would be intractably complex. In terms of complexity, however, it should matter more how ecologically different species are from each other (functional diversity), rather than just the number of interacting species. Regarding life-forms, e.g., forests’ biomass is highly concentrated on trees, what indicates the importance of light limitation.

Given the clarification potential of such classification schemes, and in hopes of glimpsing important drivers of forest structure, we here address the question of how functionally diverse is one of the world’s most taxonomically diverse forests.

In continuous and pristine forests to the north of Manaus, Brazil (BDFFP), we sampled >1000 adult trees (≥10 cm dbh) of 157 species. These were the most dominant tree species in four 1-ha plots of a contiguous 9-ha plot. For each species, we measured seven leaf traits and the height of the biggest individual in the area, and compiled wood density, dispersal mode, and seed size from literature. We then performed a cluster analysis to didactically assign species into distinct functional groups (FG).

Results/Conclusions

The most striking result is that most species (∼80%) were assigned to only two FG, which matches the simplistic classification of trees into canopy and understory trees. These two groups showed a great dominance in terms of basal area (∼70%) and were also the only groups that co-occurred in every 400 m²-subplot. The total number of FGs, however, was considerably high (11).

Despite the great diversity of species, most of the trees in this forest showed either a canopy or an understory tree “syndrome”. This suggests very simple assembly rules based on tree maximum height and shade-tolerance, and again points to light as the key driver of forest structure. The high number of other less represented FG, however, may show that other strategies are also possible, such as palms and giant emergent trees. Some of these rare groups may have specific regeneration requirements, and may have been underrepresented because the studied forest was mainly composed of mature stands. In conclusion, the canopy-understory conformation seems to be the most stable combination, in accordance with niche segregation theories. However, other mechanisms, such as negative density-dependence or neutrality, would be necessary to explain coexistence of so many species belonging to the same FGs.