Interspecific competition between a non-native metal-hyperaccumulating plant (Noccaea caerulescens, Brassicaceae) and a native congener across a soil-metal gradient

Adaptive traits are hypothesised to incur fitness trade-offs, and a classical example is metal-tolerant plants that exhibit reduced competitive ability when grown on low-metal substrates. In the present study, we examined whether metal-hyperaccumulating plants exhibit a similar trade-off, by assessing competition across a soil-metal gradient in the context of phytoremediation. We studied the cadmium- and zinc-hyperaccumulator Noccaea caerulescens (J. Presl & C. Presl) F.K. Mey., which has been introduced to potential remediation sites contaminated with those metals, and the nickel- and zinc-hyperaccumulator Noccaea fendleri subsp. glauca (A. Nelson) Al-Shehbaz & M. Koch, which is native to our study sites. We performed a greenhouse experiment with a response-surface design to quantify their competitive interactions on higher- and lower-metal substrates from each of three mine sites. Overall competitive effects between the species were weak, but we did find evidence of competition on the substrates that supported the highest rates of plant growth and reproduction. Abiotic factors were more limiting than competitive interactions for both species, and both performed better on substrates containing higher zinc, lower cadmium and lower copper concentrations. A complementary field trial supported these findings. Our results also showed that substrates outside of contaminated mine tailings can still contain sufficiently high zinc concentrations to support N. caerulescens, suggesting that the use of these plants for phytoremediation should be monitored to prevent the unintentional establishment of non-native species.