Extending the leaf economics spectrum: Freshwater plant communities exhibit realm‐specific leaf adaptations

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Abstract
The leaf economics spectrum bridges plant demography with terrestrial biodiversity gradients, community ecology and ecosystem functions, providing a framework for the mechanisms underlying ecosystem change. However, our understanding of whether this framework applies to aquatic plant communities and how their biogeography is shaped by their unique adaptations to life in water is limited. Moreover, it remains unclear whether leaf trait expression of aquatic plants responds to changes in environmental conditions differently depending on their life form (i.e. emergent, submerged). Here, we compiled a dataset of community composition for 6953 lake and river plant communities across Europe. We appended leaf trait data and environmental data to each community. Then, we calculated the community mean traits for emergent and submerged plant communities to determine their position along the leaf economics spectrum and how they deviate from terrestrial expectations. We used Gaussian multivariate mixed linear regression models to investigate life form‐specific variation in trait‐environment relationships. We find that emergent communities exhibit trait–environment relationships comparable to terrestrial predictions, with high bicarbonate and phosphorus concentrations, and warm temperatures selecting for acquisitive leaf traits. In contrast, acquisitive submerged communities are selected for by high bicarbonate and low temperature, and not by phosphorus availability. We attribute these differences to freshwater‐specific adaptations which influence the leaf economics of submerged communities more profoundly than emergent communities, since submerged plants grow in particularly challenging conditions with respect to light and gas availability. These findings suggest that aquatic plants have adaptations to their unique environments that require trade‐offs between leaf form and function to facilitate underwater metabolism, which pushes the boundaries of the leaf economics spectrum. Synthesis . Our ability to apply existing knowledge of leaf economics based on terrestrial plants to the aquatic realm is limited, especially with respect to the influence of temperature on plant growth, which is key for predicting the impacts of climate change on vegetation globally. Our findings highlight the importance of a realm‐specific approach for linking functional traits to freshwater ecosystem functioning and biodiversity, as the distinct responses of freshwater plants to their environment are not captured by terrestrial‐derived frameworks.

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