When and why do trees begin to decrease their resource allocation to apical growth? The importance of the...

Trees reduce their allocation to apical growth compared with that to radial growth during later life stages. This has often been attributed to hydraulic and mechanical limitations; however, a growth-reproduction tradeoff might also play an important role. To test whether the height-diameter (H-D) relationship in trees changes with the timing of the onset of reproduction, we analyzed the H-D relationship of 13 deciduous broad-leaved tree species in a Japanese old-growth temperate forest using linear and segmented-linear regression models. These models showed a better fit than common continuous models (simple allometry and saturating curve). For 11 out of 13 species having break points on the H-D lines, we assessed whether the height at the breakpoint (BPH) was related to the height at the onset of reproduction determined by multiyear observatory records on several trees. Although BPH estimates for these 11 species were highly variable, most were within 3.1 m of the stable flowering height (staFLH), or the height at which trees achieve the ability to flower regardless of their growth conditions. The interspecies mean value of the difference between staFLH and BPH was only 45 cm, which suggested a causal relationship between these parameters. BPHs of nine out of 11 species were within the canopy layer and, for the two remaining species, were within the subcanopy layer. These results suggest that several species in this forest begin to reduce their allocation to apical growth around the canopy layer at the onset of stable reproduction.