Relative size at the onset of maturity in rain forest trees : A comparative analysis of 37 Malaysian species
Relative size at onset of maturity (RSOM), defined as the size at reproductive onset divided by asymptotic maximal size, has been found to be remarkably constant among species in many animal taxa that display indeterminate growth. The present study extends this general approach to plants, examining 37 species of Malaysian rain forest trees. Most of these species exhibit pronounced reproductive size thresholds; however, in contrast to previous studies, values of RSOM vary substantially among closely related species. The relationship among species between height at reproductive onset and asymptotic maximal height deviates significantly from linearity: larger tree species display larger reproductive size thresholds on both an absolute and a relative basis. Larger tree species also generally display more distinct size thresholds, as quantified by the exponent parameter of a modified logistic regression function describing probability of reproduction as a function of size, than do smaller tree species. Statistically significant values of RSOM (on a height basis) vary among species from 0.20 to 0.75, with a mean of 0.42. Differences in RSOM among genera were not significant; however, there was some evidence that species associated with riparian areas have higher average values for RSOM than non-riparian species. Compared with other organisms that have been examined, tropical tree species exhibit a remarkable range of values for RSOM. Such variation has important implications to theories explaining canopy stratification, life-history variation, and niche differentiation in tropical forests.