Hydraulic conductances of angiosperms versus conifers: similar transport sufficiency at the whole-plant level

Bond's "slow seedling" hypothesis proposes that, because of slow growth rat es caused by an inefficient transport system and low leaf photosynthetic ca pacity, gymnosperm seedlings are weak competitors with angiosperms in produ ctive habitats. We measured component (shoot, leaf, and root) and whole-pla nt hydraulic conductances of sapling-sized tropical plants growing on nitro gen-poor white sand in Borneo. After accounting for size effects, there wer e no significant differences in conductances between evergreen angiosperms (nine species) and conifers (three species). Plant successional status or t ranspiration rate seemed more important than soil fertility in determining hydraulic conductance-colonizers had significantly higher whole-plant condu ctance than late-succession species. Contrary to prediction, leaf hydraulic conductance (normalized by projected leaf area) was unrelated to complexit y of venation in conifers and angiosperms, but was highly correlated with w hole-plant conductance. Analyses of published data showed that leafless bra nches of temperate deciduous angiosperms had higher leaf-area normalized hy draulic conductivity than conifers, but there was no significant difference in adult, whole-plant conductance between these taxa. Thus, at the branch level, conifers with narrow tracheids have less efficient transport than an giosperms with wider vessels, but variations in other resistance components and hydraulic architecture (e.g., sapwood/leaf area ratio) ultimately equa lize the sufficiency of water transport to leaves of conifers and angiosper ms. Although failing to support one of the proposed mechanisms, our finding s did not refute the "slow seedling" hypothesis per se.