The partitioning of plant biomass among specific organs is fundamental to growth and survival strategies and can have broad scale implications in plant ecology. In particular, the proportion of biomass present in photosynthetic versus non-photosynthetic tissue can influence carbon assimilation rates, metabolism and relative growth rates of plants, often having significant downstream effects on community production and dynamics. Although partitioning patterns have been studied extensively in seed plants, marine plants (and especially multicellular algae) have received little attention despite their important role in nearshore communities. In this study, we investigate the interspecific and developmental allometry in kelps (Laminariales), which evolved multicellularity independently of land plants. Because kelps possess analogous organs (holdfast, stipe and blades) to the roots, stem and leaves of embryophytes, they provide an important test of well-documented patterns seen in land plants – particularly because they lack internal vascular systems for water transport. Consequently, we hypothesize that biomass partitioning strategies of kelps are likely to reflect resistance to mechanical failure from water motion rather than fractal-like geometries that maximize photosynthetic area while providing water to photosynthetic organs. Here, we test whether species differ in their allometric strategies, and investigate which factors might influence them. We also assess whether developmental allometries match interspecific allometries, or whether increases in the maximum size of species are associated with concomitant changes in species-specific developmental strategies.
We report that kelp species exhibit large variation in biomass partitioning, with some species displaying allometries that favour blade growth whereas others strongly favor holdfast and stipe investment. Although interspecific allometries of kelps closely matched those of embryophytes, the range of developmental strategies exhibited by kelps include allometric exponents not seen in the vast literature on land plants. Allometric growth of holdfasts and stipes were closely aligned across all species, suggesting strong allometric covariation. Stipe and holdfast investment, however, were decoupled when considering scalar coefficients - the absolute mass of each organ produced during early development, suggesting that evolution may act on early development when dissociation of stipe and holdfast absolute biomass is favourable. Developmental allometries of kelps also differed from evolutionary patterns with maximum size and habitat both influencing the allometric exponents. The results of this study have important implications for understanding the evolutionary ecology of habitat-forming marine plants, but also shed light on how environmental forces alone can influence plant development and partitioning.