Differences in leaf traits of Spartina alterniflora between native and invaded habitats: Implication for evolution of alien species competitive ability increase

Abstract

The enemy release and pre-adaptation of alien species often drive their contemporary evolution across latitudinal gradients within its invasive and native ranges. Leaf functional traits constitute an important part of plant functional traits, reflecting their resource acquisition and usage. Thus, comparing the leaf functional traits (growth and defense traits) and latitude patterns of invasive and native plants could provide leading indicators of contemporary evolutionary and adaptative abilities that explain the success of invasive plants. Spartina alterniflora, a coastal wetland invasive plant native to the Atlantic coast and the Gulf of Mexico, has invaded China along the coast wetland (19–40 °N), similar to the United States (27–43 °N). To understand the leaf trait differences and variation across latitudes among continents of S. alterniflora within its invasive and native ranges and determine which factors drive leaf trait variation across latitudes, we investigated the relationship between leaf traits, related to growth (leaf area and specific leaf area), and defensive traits (leaf thickness and leaf toughness) at 10 invaded locations and 15 native locations. We identified correlations between leaf traits and abiotic conditions such as annual mean temperature, tide range, and soil salinity. Compared with native S. alterniflora, Chinese costal S. alterniflora has a larger leaf area and specific leaf area, less leaf thickness, and lower leaf toughness, suggesting that biosynthetic resources are transferred from defense to competitive traits, and indirectly supporting the Evolution of Increased Competitive Ability (EICA) hypothesis. The leaf area, specific leaf area, thickness, and toughness of S. alterniflora from both ranges had similar quadratic latitudinal relationships. Some of these relationships were stronger in the invasive range than the native range, indicating that the leaf traits of invasive S. alterniflora experienced pre-adaptation and had high phenotypic plasticity. Tide range, soil salinity, and especially temperature formed latitudinal gradients in different geographic regions. By analyzing indicators of leaf functional traits across geographical ranges, this study shows that the escape from natural enemies plays an important role in the successful invasion of S. alterniflora, while pre-adaptation and high phenotypic plasticity promote its rapid expansion along the latitudinal gradients.