Measuring standardized functional leaf traits of aquatic carnivorous plants – challenges and opportunities

Aquatic carnivorous plants (ACP) are an important component of humic nutrient-poor freshwater environments. However, these habitats are facing multiple impacts that ultimately lead to habitat degradation and declining ACP populations. Functional traits, particularly those within the leaf economics spectrum, are a valuable tool for studying plant adaptation strategies and plasticity. Given their unique morphological structure, ACP are essentially excluded from functional comparisons, which potentially limits our knowledge about the ecological roles of these species compared to non-carnivorous ones. In this study, we developed a protocol for measuring the leaf functional traits of ACP (leaf fresh and dry weight, leaf area, leaf dry matter content, specific leaf area, leaf pigment content, leaf phosphorus, nitrogen and carbon contents), and carnivory-related traits (number of traps and investment in carnivory). We measured 15 traits in seven ACP species (Aldrovanda vesiculosa, Utricularia australis, U. bremii, U. intermedia, U. ochroleuca, U. stygia, U. vulgaris), grown in the outdoor collection of aquatic and wetland plants of the Institute of Botany CAS at Třeboň, the Czech Republic. We used the functional traits of other macrophyte groups/species (lemnids, Nuphar lutea, Ceratophyllum demersum), collected with a similar methodology, to assess the comparability of ACP traits. We identified the “functional unit”, a modular structure, including one leaf node, plus an internode, which performs the function of a leaf in ACP, and selected its position along the stem to reflect species-specific growth rates. We collected 714 new trait records for the target ACP. Based on a multivariate trait space representation (PCA), ACP were distinct from the other macrophyte groups/species, which highlights these species’ structural and physiological peculiarities. Nonetheless, ACP entirely overlapped the comparison data along the first PCA axis, and most of the traits lay within the ranges observed for other macrophyte groups/species, which demonstrates the comparability of the ACP traits measured by the new protocol. Applying this protocol in ecological studies could shed light on the adaptations of ACP to environmental variability, with important conservation implications.