Premise of the study: Wood specific gravity (WSG) mediates an interspecific trade-off between growth and mortality and is a key measure for estimating carbon stocks. Most studies use species mean values to represent WSG, despite variation at different levels of biological organization. We examined sources of variation in WSG across four nested scales (segments within cores, cores within trees, trees within species, and species), compared the pattern of radial variation in WSG among species differing in growth strategies, and investigated the effect of WSG radial variation on aboveground biomass estimates. Methods: We took two perpendicular cores from six individuals each of 20 tropical tree species representing a broad range of mean WSGs and growth-mortality strategies in a lowland tropical moist forest in Panama. Cores were divided into 1-cm segments, and WSG was determined for each segment. Key results: The bulk of the total variance in WSG was dominated by interspecies variation (88%), whereas variation due to measurement error, segments within cores, and cores within trees (8%) was minimal. Radial variation in WSG, defined as change in WSG with increasing distance from the pith, was significant in 17 of the 20 species and included significant monotonic increases in 6 species and nonmonotonic patterns in 11 species. Radial variation in WSG resulted in a small but significant bias in aboveground biomass estimates. Conclusions: Radial variation in WSG is related to a species' growth strategy and, though minimal compared with interspecific variation in WSG, can cause a downward bias when not incorporated into aboveground biomass estimates.
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