Carbon forms, nutrients and water velocity filter hydrophyte and riverbank species differently: A trait‐based study

Questions The majority of theories of trait‐based plant community assembly have been developed and tested predominantly in terrestrial ecosystems. Studies investigating the functional trait composition of aquatic plant communities and their relation to environmental determinants remain scarce. Macrophytes are essential components of aquatic ecosystems, and a more detailed knowledge of their trait‐based assembly is crucial for their management. We identified how plant functional traits respond to environmental gradients in streams and rivers. Location Danube River Catchment, Hungary. Methods We studied the processes governing community assembly along major environmental gradients related to carbon‐ and nutrient‐limiting factors as well as physical strain. We used six continuous traits (leaf area, specific leaf area, leaf dry matter content, seed weight, seed shape, woodiness) and calculated community‐weighted mean and standardised effect size of functional diversity for each community. We then used stepwise regression analyses for each trait along the environmental gradients to test which environmental factors explain the changes in community‐weighted mean and functional diversity. All analyses were conducted for aquatic (hydato‐helophyte) and riverbank species separately. Results We found that the effect of environmental filtering significantly increased toward higher pH, indicating the response of functional traits to carbon limitation. Our results showed trait convergence among riverbank species in rivers with higher productivity. Larger functional diversity (i.e., trait divergence) among hydato‐helophyte species suggests an increase in the diversity of resource acquisition strategies under higher productivity. Conclusions Here, we have shown that the functional trait distribution of aquatic and riverbank plant communities responds to major environmental drivers related to nutrient and carbon availability. The understanding of how community assembly mechanisms varied along environmental gradients might be useful when proposing future management and restoration plans and actions towards the conservation of the aquatic vegetation in streams and rivers. In highland and lowland streams and rivers, we studied the trait‐based community assembly of aquatic plants along environmental gradients related to carbon, nutrient and physical properties. We found that the functional trait distribution of aquatic and riverbank plant communities responds to major environme