Productivity and thallus toughness trade‐off relationship in marine macroalgae from the Japan Sea

Trade‐offs are considered key to understanding mechanisms supporting the coexistence of multiple plant species. Thus, understanding the mechanisms underlying trade‐offs is expected to contribute to conservation and management of macroalgal beds composed of diverse macroalgae of rocky shore ecosystems. To test the occurrence of trade‐offs between productivity and thallus toughness as well as pair‐wise thallus trait relationships that are expected to indirectly relate to any trade‐offs, traits and relationships for 13 species of macroalgae from the central area along the Japan Sea coast of Honshu, Japan were examined. In each species we examined for photosynthetic capacity per unit biomass (as Aₘₐₛₛ) and nitrogen (i.e., photosynthetic nitrogen‐use efficiency, PNUE), nitrogen content (as Nₘₐₛₛ), thallus mass per unit thallus area (as TMA) and force required to penetrate the thallus (as Fₚ, a common index of leaf toughness in land plants by punch test). A significant negative correlation indicating a trade‐off between productivity and thallus toughness was found between Aₘₐₛₛ or PNUE and Fₚ. Pair‐wise relationships that were expected to indirectly relate to the trade‐off were as follows. Aₘₐₛₛ was positively correlated with Nₘₐₛₛ. Thalli with high Nₘₐₛₛ extensively utilizing nitrogen in the photosynthetic parts, and consequently exhibiting elevated metabolic rates. Moreover, thalli with high Nₘₐₛₛ tended to be associated with low TMA, and Nₘₐₛₛ decreased with increasing TMA. A significant negative correlation was observed between TMA and Aₘₐₛₛ or PNUE because of the linkage of high Aₘₐₛₛ or PNUE with high Nₘₐₛₛ and high Nₘₐₛₛ associated with low TMA, while a significant positive correlation was observed between TMA and Fₚ. The two correlations indicate a physiological and structural trade‐off, which underlies the interdependency of thallus traits. Results of multivariate analyses also indicated that the thallus traits interdependently vary across a single axis based on the trade‐off.