Adaptive nutrient responses vary with growing environment but not altitudinal origin in an invasive plant

Invasive plants often thrive in nutrient-rich environments because of their superior ability to capture and efficiently exploit nutrients. This fitness advantage is commonly explained by invasive species being more plastic than their non-invasive counterparts. However, the extent to which individual traits vary in their plastic responses to nutrient availability-and how these responses translate into fitness gains-remains poorly understood. We conducted a nutrient addition experiment in invasive Erythranthe guttata to determine how plasticity to nutrient levels varied among traits, among populations, and with growing conditions. Populations from both upland and lowland New Zealand were grown under "normal" and excessive soil nutrient levels, in an upland and a lowland common garden. We found no evidence of evolution in plastic responses between altitudinal groups. Common garden (growing environment) had a small but significant maladaptive interaction with nutrient responses; in the upland garden, plants growing under excess nutrients showed stunted growth, in contrast to the expected adaptive plastic response of larger leaves under high soil nutrients. The strength of nutrient responses among traits corresponded to their importance in the selection analysis, suggesting an adaptive plastic response. Additionally, in contrast to recent findings for other species, we found no relaxation of seed size and number trade-offs with nutrient addition, which suggests that this is not a consistent driver of invasive success. Overall, our findings increase our understanding of how invasive plants exploit high resource conditions through adaptive plasticity at the trait level, despite limitations in challenging environments.