Friday, August 6, 2010

PS 108-139: Small tropical trees decrease investment in height growth with increasing biomass growth

DanaŽ M.A. Rozendaal1, Heinjo J. During1, Frank J. Sterck2, Daan Asscheman3, Jeroen Wiegeraad1, and Pieter A. Zuidema1. (1) Utrecht University, (2) Wageningen University, (3) University of Amsterdam

Background/Question/Methods

Biomass growth of juvenile tropical trees in the understory varies greatly among individuals. Trees growing in high light attain higher biomass growth compared to shaded individuals. At first sight, one would expect that high biomass growth leads to high growth in both diameter and height. Here we test this hypothesis using a technique new to tropical forest ecology: historical volume growth reconstruction. Tree-ring analysis was applied using stem discs collected along the entire height of a tree for 21-27 juveniles per species of 1-15 cm diameter at breast height. This approach allowed the reconstruction of lifetime growth in diameter, height and stem volume. We then used stem volume growth as a proxy for biomass growth, and related this to height growth in various tree size categories.

Results/Conclusions

Biomass growth varied strongly among individual trees of the same height for all species, probably as a consequence of differences in growth conditions among trees. These differences persisted over height classes; thus among-tree variation in growth conditions remained similar over a long timespan. Height growth showed similar, large variation among trees, but showed less persistence over time. Thus, ranking in biomass growth among trees remained similar over time, whereas shifts in investment in height and diameter growth occurred. Remarkably, for small trees of all species a negative correlation between height growth and biomass growth was found. Thus, fast-growing trees invested relatively less in height growth compared to slow-growing trees. This could mean that small juvenile trees in favorable (light) conditions invest relatively more in stability and crown development than in height growth and/or that individuals growing in low light invest relatively more in height growth. In contrast, for trees of larger size height growth increased with biomass growth.