Differences in the dry-mass cost of sapling vertical growth among 56 woody species co-occurring in a Bornean tropical rain forest

1. Above-ground structure was analysed in saplings of 56 sympatric species in a Bornean rain forest with consideration of the phylogenetic background to elucidate interspecific variation in the dry-mass cost and its ecological consequences. 2. The extension cost (total above-ground mass) in 1.5-m tall saplings varied eightfold among the 56 species. However, no significant differences in extension cost were observed among saplings of three crown types (branched, monoaxial simple-leaved, and monoaxial compound-leaved), although the monoaxial crown type has been considered an adaptation for achieving rapid height growth. The lack of differences arose because the advantages of monoaxial saplings in structural cost for displaying a given leaf area were unexpectedly small. 3. Understory species had a significantly higher extension cost than canopy species because of their thicker, and consequently, heavier trunks. This trend was common to the three crown types; thus, the higher extension cost was not caused by the prevalence of a specific crown type in understory species. 4. For all 56 species combined, the trade-off between height growth efficiency and light-interception-enhancing morphology was prominent. This structural trade-off, which makes efficient vertical growth incompatible with survival in the understory, potentially enables the stable coexistence of these species in a temporally heterogeneous light environment.