Costs of height gain in rainforest saplings: main-stem scaling, functional traits and strategy variation across 75 species

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Abstract
Height gain plays an important role in plant life-history strategies and species coexistence. Here main-stem costs of height gain of saplings across species within a rainforest community are compared. Scaling relationships of height to diameter at the sapling stage were compared among 75 woody rainforest plant species in subtropical eastern Australia using standardized major axis regression. Main-stem costs of height gain were then related to other functional traits that reflect aspects of species ecological strategies. Slopes (β) for the height–diameter (H–D) scaling relationship were close to 1·3, in line with previous reports and with theory. Main-stem volume to achieve 5 m in height varied substantially between species, including between species within groups based on adult height and successional status. The variation was largely independent of other species traits, being uncorrelated with mature plant height (Hmax) and with leaf size, and weakly negatively correlated with wood density and seed size. The relationship between volume to reach 5 m and wood density was too weak to be regarded as a trade-off. Estimated main-stem dry mass to achieve 5 m height varied almost three-fold across species, with wood density and stem volume contributing roughly equally to the variation. The wide range in economy of sapling height gain reported here is presumed to be associated with a trade-off between faster growth and higher mortality rates. It is suggested that wide diameters would have a stronger effect in preventing main-stem breakage in the short term, while high wood density would have a stronger effect in sustaining stem strength over time.