Universal scaling relationships of marine and terrestrial photoautotrophs with differing developmental trajectories

Backgrounds and Aims: Partitioning of biomass to various organs is fundamental to growth and survival strategies of plant species, with broad-scale ecological implications from competition to carbon flux of ecosystems. Although partitioning patterns have been studied extensively in embryophytes, little is known about the variation and evolutionary drivers of biomass partitioning strategies in marine plants, such as seaweeds, that form marine forests analogous to those on land. Methods: We measured developmental variation in standing organ biomass for 18 species of kelp and used a previously published dataset on land plants to draw comparisons between kelps and embryophytes, with respect to developmental allometry, interspecific allometry and the relationship between the two measures. Key results: We report that kelps exhibit large variation in partitioning patterns, with some species differing by more than an order of magnitude in holdfast and stipe investment when controlling for size. The range of biomass partitioning patterns exhibited by kelps includes values not found among embryophytes, highlighting differences in selective pressures on land and in the ocean. However, as previously demonstrated, empirical values of interspecific scaling curves closely match those of land plants. This suggests that despite fundamentally different physical constraints, evolutionary scaling has converged on nearly identical interspecific exponents. We further demonstrate that allometric covariation exists between support organs (stipe and holdfast), highlighting the importance of physical forces in driving functional diversification of marine plants and supporting the notion that covariation of traits is a hallmark of botanical structure and function. Conclusions: The results of our study illuminate the variation and evolutionary patterns of biomass partitioning in marine primary producers. Moreover, these results highlight that universal evolutionary scaling relationships can emerge from fundamentally divergent developmental and biomechanical pathways.