Nutrient addition amplifies salinity-dependent differences in competitive ability of invasive and native vines

Abstract

Invasive plants may have greater competitive ability than native species and greater invasion success in fertile habitats. However, they may not have high tolerance of abiotic stress and enriched resources may not increase invasion success in stressful habitats such as coastal areas with high soil salinity. We conducted a greenhouse experiment using the invasive vine Ipomoea cairica, which is expanding into coastal areas in South China, and the widespread native vine Merremia hederacea. Pots received two invasive vines, two native vines, or one of each. They were assigned nutrient (water vs. Hoagland’s solution) and salinity stress (water vs. 4 g L−1 NaCl) treatments in a factorial design. Salinity decreased plant biomass and nutrient increased plant biomass, especially without salinity. Ipomoea cairica were larger or similar to natives in monoculture. Without salinity, M. hederacea was larger than I. cairica in mixed culture, especially with nutrients. In saline conditions, I. cairica had greater competitive ability, which was increased by nutrient. Ipomoea cairica had higher root:shoot and nitrogen concentration in non-saline conditions, while M. hederacea had higher root: shoot and SLA in saline conditions. Our results indicate that competitive abilities of the two species depend on salinity and that nutrient addition only magnified the differences between them. This suggests that nutrient addition may increase invasion success but only in habitats where conditions were already suitable for invasion. The mismatch between the competition results in monoculture and mixed culture suggests caution in inferring the effects of environmental factors on competition based on measures of individual performance.