Tropical forests harbor a high diversity of woody plants storing large amounts of carbon. The density of wood varies widely among tropical plant species, and is consistently associated with species’ survival rate, suggesting that it is a trait with ecological significance.
However, the relationship of wood density to wood growth is not straightforward. Wood growth rates of individual trees have been inferred from measurements of tree trunk diameter growth, because diameter is easy to measure. Prior analyses of hundreds of thousands of measurements taken from across the tropics have shown a negative relationship between tree diameter growth and wood density. However, these studies have not addressed the relationship of wood density with wood mass growth, because each unit of diameter growth in a species with high wood density contains more biomass than that of a species with low wood density. The objective of this study was to quantify the relationship of wood biomass growth with wood density, using existing large datasets of tree diameter growth rates and wood density. We did this by analytically deriving expected relationships of wood density with diameter growth, under the assumption that wood biomass growth is constant with wood density, and an alternative assumption that wood biomass growth scales with crown area. We then compared the derived parameters to observed parameters calculated directly from measurements of tree diameter growth and wood density taken from two well-studied tropical forests in Panama and Malaysia.
Results/Conclusions
The derivation showed that the relationship of wood growth to wood density is a power function, suggesting that future analyses of wood density and tree growth should be modeled as log-log rather than log-linear or linear relationships as done previously. Wood biomass growth was constant with wood density in mature trees at both sites. At Barro Colorado Island, Panama, small trees with high wood density showed slower mass growth, which may be a result of disproportionate representation in low-light environments. The results provide evidence from more than five hundred tree species from two tropical forests that variation in diameter growth with wood density is more closely related to patterns in wood mass allocation, rather than wood mass growth. Thus, in addition to their higher survival rates, species with high wood density have equal biomass growth rates as species with low wood density. These results support forest demographic model assumptions of equal wood biomass growth rates with varying wood density.