Shade tolerance as a key trait in invasion success of submerged macrophyte Cabomba caroliniana over Myriophyllum spicatum .

The synergy between climate change, eutrophication, and biological invasion is threatening for native submerged plants in many ways. The response of submerged plants to these changes is a key factor that determines the outcome of biological invasion. In order to explain the invasion successes, we investigated the combined effects of climate change and eutrophication-related environmental factors (temperature, light, and nutrients) on the trait responses of a native ( Myriophyllum spicatum ) and an alien ( Cabomba caroliniana ) submerged species. In a factorial design, we cultivated the two species in aquaria containing low (0.5 mg N L ^-1 , 0.05 mg P L ^-1 ) and high (2 mg N L ^-1 , 0.2 mg P L ^-1 ) nutrient concentrations, incubated at four light intensities (average 25, 67, 230, and 295 μmol m ^-2  s ^-1 PAR photon flux density) under two temperature levels (21.5 and 27.5 ± 0.5°C). We used four invasion-related functional traits (relative growth rate (RGR), specific leaf area (SLA), leaf dry matter content (LDMC), and nitrogen to carbon ratio (N:C molar ratio)) to measure the environmental response of the species. We calculated plasticity indexes to express the trait differences between species. Cabomba caroliniana showed significantly higher RGR and SLA than M. spicatum especially under low light intensity indicating that Cabomba is much more shade tolerant. Elevated temperature resulted in higher SLA and reduced LDMC for C. caroliniana indicating that Cabomba may have higher invasion success. Myriophyllum showed higher LDMC than C. caroliniana . Chemical analyses of the plant tissue revealed that although M. spicatum showed significantly higher N:C molar ratio, nonetheless, the daily nitrogen uptake of C. caroliniana was more than three times faster than that of M. spicatum . Results supported the idea that due to its higher shade tolerance and nitrogen uptake capacity, Cabomba likely has greater invasion success with increasing temperature combined with low light levels.
(© 2022 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.)