The independent effects of nutrient enrichment and pulsed nutrient delivery on a common wetland invader and its native conspecific

Human activities often lead natural systems to be nutrient enriched, with anthropogenically derived nutrients commonly delivered in discrete pulses. Both nutrient enrichment and nutrient pulses can impact plant performance and phenotypic plasticity, especially in invasive species, but quantifying their independent effects remains challenging. To explore the effects of nutrient enrichment and nutrient pulse magnitude, we established a common garden experiment using the North American wetland invader Phragmites australis and its native conspecific Phragmites australis subsp. americanus (five source populations each). We exposed plants to three levels of nutrient enrichment that were delivered either in small or large-magnitude pulses, examining productivity and plasticity responses over a single growing season. Productivity and biomass allocation differed by lineage, with invasive Phragmites producing 73% more biomass and 66% more culms, but with the native growing 31% taller and allocating more of its biomass belowground. Contrary to expectations, both lineages responded similarly to nutrient enrichment and were similarly plastic in their traits. Nutrient enrichment, rather than nutrient pulses, led to large productivity gains and trait plasticity magnitudes. However, total biomass and leaf-level traits (specific leaf area and chlorophyll concentration) were responsive to variation in nutrient pulse magnitudes. By decoupling the effects of nutrient enrichment from nutrient pulses, our study demonstrates the independent effects of these two key factors for plant performance and, by extension, invasion success. We report trait-based similarities between two lineages of Phragmites that play contrasting ecological roles in North American wetlands, and we highlight the potentially detrimental effects of nutrient pulses.