Tree growth and death in a tropical gallery forest in Brazil: understanding the relationships among size, growth, and survivorship for understory and canopy dominant species

Background/Question/Methods

Measuring functional traits and defining functional groups of trees are fundamental approaches to understanding forest dynamics. Comparing the population dynamics of different functional groups is an important step in that direction. Most studies have used a broad approach to assess functional groups in tropical forests, relating species traits to the different stages and conditions along forest succession. Fewer studies have examined the different strategies that allow tree species to be successful in mature forest patches. We studied the relationships between size, growth and survival for two functional groups of trees, the persistent canopy and understory dominant tree species in a largely undisturbed tropical gallery forest in Southeastern Brazil. In 28 plots of 10 × 30 m we tagged, identified, and measured the diameter of all trees with diameter at ground level ≥ 5 cm in 1993/1994, 1998 and 2004. We choose the three dominant canopy species (Protium spruceanum (Benth.) Engler, Copaifera langsdorffii Desf. and Pera glabrata (Schtt.) Bailllon) and two dominant understory species (Ixora brevifolia Benth. Arg. and Trichilia emarginata (Turcz.) C.DC.) for the comparisons. We assessed the relationship between previous growth rates and mortality, variation in growth and mortality rates among size classes, and temporal correlations in growth rates.

Results/Conclusions

Trees (whole community) with null or negative growth had higher rates of mortality, although this effect was not significant for individual species. Growth patterns were consistent across consecutive evaluation periods for canopy species, but not for understory species. Trees of canopy species with higher or lower growth in the first period tended to maintain higher or lower rates, respectively, in the second one. The differences between the two groups persisted even when we restricted comparisons to trees within in the same size class. Canopy species had higher and more variable growth rates than understory species, which we attribute to greater access to the light when reaching the canopy and to a wider range of light condition experienced during growth. All canopy species present accelerated growth as they became larger. That also happened for T. emarginata, but not for I. brevifolia. Mortality rates were higher for the smallest trees for the community as a whole and for P. spruceanum. Canopy species appear to have higher diameter growth rates than understory species, even when they are growing in the same understory environment. Additionally, individual trees of canopy species seem to have a more consistent growth across time than trees of understory species.