Differential Responses of Invasive Trees Ailanthus altissima Mill. Swingle and Fraxinus americana L. to Soil Phosphorus Availability

The availability of phosphorus in the soil has a key role in plant physiological processes, particularly in the adaptive responses of invasive species. This study examined how contrasting soil phosphorus concentrations (low: 9 mg/kg and adequate: 27 mg/kg) influence biomass production, lignin and extractive content, P concentration in leaves, and root exudate composition in two invasive species, Ailanthus altissima and Fraxinus americana. Seedlings were grown in rhizoboxes filled with alkaline soils of two types. Adequate phosphorus concentration increased both aboveground and root biomass in the examined species, while low phosphorus significantly reduced biomass, especially in the aboveground parts, which were 3 to 4 times smaller compared to plants grown under adequate conditions. Low phosphorus concentration increased lignin and extractive content in the stem. Root exudate analysis revealed that low phosphorus availability enhanced the secretion of malate in both species. Ailanthus altissima exhibited higher malate concentrations in root exudates compared to Fraxinus americana under both phosphorus conditions. Ailanthus altissima is more competitive than Fraxinus americana on low-phosphorus alkaline soils. The results reveal how Ailanthus altissima and Fraxinus americana adapt to varying soil phosphorus levels, aiding the development of strategies to manage these invasive species and preserve ecosystem stability.