Root plasticity benefits a global invasive species in eutrophic coastal wetlands

Abstract Invasive species usually benefit more than native species from increases in nutrient availability. Phenotypic plasticity in above‐ground traits has often been used to explain the superiority of invasive species in response to increased nutrient availability. However, it remains largely unclear whether and how root trait plasticity predicts the ascendancy of invasive plants following nutrient enrichment. Using a 3‐year field nitrogen (N) enrichment experiment, we evaluated the effect of N enrichment on the root traits and growth performance of a globally invasive cordgrass, Spartina alterniflora , and a co‐occurring native plant, Phragmites australis . We found that S. alterniflora exhibited high plasticity in root traits in response to N enrichment, whereas the root traits of P. australis largely remained unresponsive to fertilization. Specifically, in response to N enrichment, S. alterniflora decreased soil exploitation with a lower length ratio of lateral to adventitious roots. However, the root traits of S. alterniflora shifted to be more acquisitive under N enrichment with higher root N concentrations and lower root tissue density. Moreover, there were a series of anatomical adjustments in the adventitious roots of S. alterniflora in response to N enrichment, such as a thicker diameter, lower living cortex‐to‐stele ratio and higher xylem‐to‐root area ratio. These plastic adjustments in root traits jointly resulted in fast nutrient uptake and transportation in S. alterniflora , boosting its above‐ground productivity under fertile N conditions. Our study demonstrated that greater plasticity in root traits enables a globally successful invasive species to take advantages of increased nutrient availability, potentially increasing its invasiveness. We suggest that consideration of root traits can improve our understanding of below‐ground mechanisms underlying invasion under global nutrient enrichment. Read the free Plain Language Summary for this article on the Journal blog.