The role of wood density and stem support costs in the growth and mortality of tropical trees

The rapid growth rates of light-demanding tree species have been attributed in part to their low-density, low-cost stems. We evaluated the influence of light and biomass support costs on growth rates of trees 8-25 cm in diameter at breast height (d.b.h.) among 21 species differing in wood density in two aseasonal rain forests. Measurements of crown width, tree height, d.b.h. and wood density (rho) were used to estimate the stem biomass (M-s) of a standard-sized tree (17 m tall and 16 m(2) in crown area), i.e. the cost in stem biomass of supporting a given sized crown at a given height. The species showed a three-fold range in support cost, which was highly correlated with wood density (M-s proportional to rho(0.77), r(2) = 0.72 for the log-transformed relationship). This relationship is due to the high interspecific variation in wood density and the fact that the stem diameter of the standard-sized tree increased only slightly with decreasing wood density, i.e. light-wooded species did not compensate for their lighter, weaker wood by substantially increasing stem thickness. Mean growth rate per species showed a 10-fold range and increased with the fraction of trees at least partly in gaps (gap fraction), the reciprocal of support cost (1/M-s), and the reciprocal of wood density (1/rho). The relationship between mean growth rate and 1/M-s was particularly strong when one outlier was excluded (r(2) = 0.88) and among the Dipterocarpaceae (r(2) = 0.89). Log(mortality rate), as determined for all trees per species >= 1 cm d.b.h., increased linearly with 1/M-s, 1/rho and gap fraction. These results suggest an important role for wood density and support costs in the classic tradeoff between rapid growth and increased risks of damage and death.