Drought Dominates over Warming in Structuring Growth and Defense in Invasive Solidago canadensis vs. Native Solidago decurrens
Plants are increasingly exposed to simultaneous warming, drought, and herbivory, yet how invasive and native species regulate carbon allocation under these interacting stressors remains poorly understood. We conducted a full-factorial greenhouse experiment manipulating temperature (+3 °C), soil water availability, and herbivory to compare carbon allocation responses in the invasive Solidago canadensis and its native congener S. decurrens. Drought emerged as the dominant driver of carbon reallocation, shifting investment away from biomass production and photosynthetic function toward belowground allocation, osmotic adjustment, defense, and structural reinforcement. Herbivory imposed additional carbon demands and modified drought-induced responses, particularly by constraining phenolic accumulation under combined stress, whereas warming had comparatively weaker and largely context-dependent effects. Our results showed that the two species differed in how carbon was distributed under stress: S. canadensis maintained greater biomass, chlorophyll, soluble sugars, and hemicellulose, whereas S. decurrens showed stronger investment in structural components. Structural equation models further revealed contrasting pathways linking physiological, defensive, and structural traits to biomass, supporting species-specific strategies of carbon allocation under multiple stressors. These findings suggest that the greater carbon-allocation flexibility observed in S. canadensis may contribute to maintaining performance under increasingly variable environmental conditions. However, because S. canadensis was raised directly from field-collected seeds without a refresher generation, potential maternal environmental effects cannot be completely excluded. Overall, our findings identify coordinated redistribution of carbon among competing functional pools as a potential mechanism underlying contrasting stress responses in these two congeneric species.